Thread: System Load analyze

System Load analyze

From
Peter Bauer
Date:
Hi all,

i have a system here with 2 2.4GHz Xeon Processors, 2GB RAM, ONE Disk on
a Battery Backed Write Cache SCSI Controller and PostgreSQL 8.1.4
running with the data on a DRBD Device for High Availability. The used
database is also replicated to two similar machines with slony1.

Since the load average is between 1 (most of the time) and 10 (peeks) i
am worried about the load and executed vmstat and iostat which show that
1000-6000 Blocks are writen per second. Please check the attached output
for further details.
top shows that the CPUs are at least 80% idle most of the time so i
think there is an I/O bottleneck. I'm aware that this hardware setup is
probably not sufficient but is would like to investigate how critical
the situation is.

thanks,
Peter

procs -----------memory---------- ---swap-- -----io---- --system-- ----cpu----
 r  b   swpd   free   buff  cache   si   so    bi    bo   in    cs us sy id wa
 0  0 398256  78328 140612 1063556    0    0     0  1472 2029  5081  4  3 92  0
 0  2 398256  78268 140612 1063576    0    0     0  2304 1928  4216  0  2 98  0
 1  2 398256  78100 140612 1063576    0    0     0  1464 1716  3994  1  3 96  0
 0  0 398256  78704 140612 1063592    0    0     0   916 1435  3570  5  3 91  0
 0  0 398256  77876 140612 1063616    0    0     0     0  305  1169  3  1 96  0
 0  1 398256  79200 140612 1063628    0    0     0  2404 2787  6555  7  6 86  0
 0  2 398256  79256 140612 1063660    0    0     0  1564 1640  3577  4  2 94  0
 0  0 398256  79324 140612 1063692    0    0     0  1660 2322  5615  5  2 93  0
 0  0 398256  78668 140612 1063724    0    0     0  1048 1390  4197 16 13 72  0
 1  0 398256  79344 140612 1063748    0    0     0  1896 2416  5675  5  5 89  0
 0  0 398256  79336 140612 1063784    0    0     0  1176 1720  4436  5  3 92  0
 1  2 398256  79280 140612 1063812    0    0     0  1800 1697  3931  3  3 94  0
 0  1 398256  79048 140612 1063832    0    0     0  1384 1733  4137  4  2 93  0
 0  1 398256  78792 140612 1063868    0    0     0  1672 2163  5241  3  2 94  0
 0  0 398256  77608 140612 1063880    0    0     0  1088 1638  3484  4  2 93  0
 0  0 398256  79108 140612 1063884    0    0     0  1568 2103  5382  7  5 88  0
 0  0 398256  79100 140612 1063892    0    0     0  1556 1394  3135  2  1 97  0
 0  2 398256  79084 140612 1063900    0    0     0  1644 2072  4953  2  0 97  0
 0  0 398256  79060 140612 1063932    0    0     0  1240 1714  3888  5  2 92  0
 0  2 398256  79032 140612 1063940    0    0     0  1328 1694  4135  4  2 94  0
 0  1 398256  78452 140612 1063944    0    0     0   620  925  2824  6  7 87  0
 0  0 398256  79036 140612 1063956    0    0     0  1196 1293  2954  6  7 87  0
 0  2 398256  79136 140612 1063964    0    0     0  1736 1959  4494  4  2 94  0
 0  0 398256  79132 140612 1063964    0    0     0     4  260  1039  1  1 98  0
 0  0 398256  79052 140612 1063980    0    0     0  2444 3084  6955  6  5 89  0
 0  2 398256  79060 140612 1063988    0    0     0   948 1146  3616  3  1 96  0
 0  1 398256  78268 140612 1064056    0    0     0  1908 1809  4086  6  5 88  0
 0  1 398256  76728 140612 1064056    0    0     0  6256 6637 15472  5  5 90  0
 0  2 398256  77000 140612 1064064    0    0     0  4916 5840 12107  1  4 95  0
 0  2 398256  76956 140612 1064068    0    0     0  6468 7432 15211  1  3 96  0
 0  6 398256  77388 140612 1064072    0    0     0  8116 7826 18265  1  8 91  0
 0  2 398256  74312 140612 1064076    0    0     0  7032 6886 16136  2  7 91  0
 0  2 398256  74264 140612 1064076    0    0     0  5680 7143 13411  0  5 95  0
 0  2 398256  72980 140612 1064140    0    0     0  5396 6377 13251  6  6 88  0
 0  3 398256  76972 140612 1064148    0    0     0  5652 6793 14079  4  9 87  0
 0  2 398256  77836 140612 1064148    0    0     0  3968 5321 14187 10  8 82  0
 1  0 398256  77280 140612 1064148    0    0     0  1608 3188  8974 21 12 67  0
 1  0 398256  77832 140612 1064152    0    0     0   236  834  2625  7  5 87  0
 0  0 398256  77464 140612 1064152    0    0     0   244  505  1378  2  4 94  0
 1  0 398256  77828 140612 1064164    0    0     0   316  580  1954  7  2 91  0
 0  0 398256  77804 140612 1064180    0    0     0   740  673  2248  2  2 96  0
 0  0 398256  77000 140612 1064180    0    0     0   304  589  1739  1  3 96  0
 0  0 398256  77000 140612 1064184    0    0     0     0  216   886  0  1 99  0
 0  0 398256  75452 140612 1064184    0    0     0   432  755  2032  6  1 93  0
 0  0 398256  76964 140616 1064200    0    0     0  1980 2722  6452  7  2 91  0
 0  0 398256  77040 140616 1064212    0    0     0  1524 1536  3635  5  1 93  0
 0  2 398256  77028 140616 1064216    0    0     0  1136 1489  3342  1  2 97  0
 0  0 398256  76976 140616 1064236    0    0     0  1392 1831  4468  3  2 94  0
 0  0 398256  76944 140616 1064256    0    0     0   932 1374  3318  7  8 85  0
 0  0 398256  76856 140616 1064276    0    0     0  1392 1773  3985  5  4 91  0
 1  4 398256  76536 140616 1064448    0    0     0  1432 1537  3950  4  2 94  0
 0  0 398256  76640 140616 1064308    0    0     0   560  865  2577  6  3 91  0
 0  0 398256  76368 140616 1064328    0    0     0  1752 1973  4424  3  3 94  0
 0  0 398256  74732 140616 1064340    0    0     0  1244 1807  3758  4  6 90  0
 1  2 398256  76244 140616 1064352    0    0     0   876 1189  3579  4  2 94  0
 0  0 398256  76192 140616 1064360    0    0     0  1856 1623  3727  5  4 91  0
 0  0 398256  76236 140616 1064372    0    0     0  1292 1748  3664  5  4 91  0
 0  0 398256  76548 140616 1064392    0    0     0     0  338  1810 11  5 84  0
 0  0 398256  76540 140616 1064392    0    0     0     0  278  1055  2  4 93  0


Device:            tps   Blk_read/s   Blk_wrtn/s   Blk_read   Blk_wrtn
dev8-0          241.00         0.00      6344.00          0       6344
dev8-1            0.00         0.00         0.00          0          0

Device:            tps   Blk_read/s   Blk_wrtn/s   Blk_read   Blk_wrtn
dev8-0           39.00         0.00      1240.00          0       1240
dev8-1            0.00         0.00         0.00          0          0

Device:            tps   Blk_read/s   Blk_wrtn/s   Blk_read   Blk_wrtn
dev8-0          170.00         0.00      3944.00          0       3944
dev8-1            0.00         0.00         0.00          0          0

Device:            tps   Blk_read/s   Blk_wrtn/s   Blk_read   Blk_wrtn
dev8-0          173.00         0.00      3880.00          0       3880
dev8-1            0.00         0.00         0.00          0          0

Device:            tps   Blk_read/s   Blk_wrtn/s   Blk_read   Blk_wrtn
dev8-0           73.00         0.00      2656.00          0       2656
dev8-1            0.00         0.00         0.00          0          0

Device:            tps   Blk_read/s   Blk_wrtn/s   Blk_read   Blk_wrtn
dev8-0          120.00         0.00      2656.00          0       2656
dev8-1            0.00         0.00         0.00          0          0

Device:            tps   Blk_read/s   Blk_wrtn/s   Blk_read   Blk_wrtn
dev8-0           81.00         0.00      2184.00          0       2184
dev8-1            0.00         0.00         0.00          0          0

Device:            tps   Blk_read/s   Blk_wrtn/s   Blk_read   Blk_wrtn
dev8-0          133.00         0.00      3144.00          0       3144
dev8-1            0.00         0.00         0.00          0          0

Device:            tps   Blk_read/s   Blk_wrtn/s   Blk_read   Blk_wrtn
dev8-0          115.00         0.00      2936.00          0       2936
dev8-1            0.00         0.00         0.00          0          0

Device:            tps   Blk_read/s   Blk_wrtn/s   Blk_read   Blk_wrtn
dev8-0           64.00         0.00      2080.00          0       2080
dev8-1            0.00         0.00         0.00          0          0

Device:            tps   Blk_read/s   Blk_wrtn/s   Blk_read   Blk_wrtn
dev8-0          176.00         0.00      4384.00          0       4384
dev8-1            0.00         0.00         0.00          0          0

Device:            tps   Blk_read/s   Blk_wrtn/s   Blk_read   Blk_wrtn
dev8-0          172.00         0.00      3296.00          0       3296
dev8-1            0.00         0.00         0.00          0          0

Device:            tps   Blk_read/s   Blk_wrtn/s   Blk_read   Blk_wrtn
dev8-0          191.00         0.00      3880.00          0       3880
dev8-1            0.00         0.00         0.00          0          0

Device:            tps   Blk_read/s   Blk_wrtn/s   Blk_read   Blk_wrtn
dev8-0          144.00         0.00      2464.00          0       2464
dev8-1            0.00         0.00         0.00          0          0

#---------------------------------------------------------------------------
# FILE LOCATIONS
#---------------------------------------------------------------------------

# The default values of these variables are driven from the -D command line
# switch or PGDATA environment variable, represented here as ConfigDir.

data_directory = '/clstsql/data'    # btaf: Data Directory on the drbd device, APUS

#hba_file = 'ConfigDir/pg_hba.conf'    # host-based authentication file
#ident_file = 'ConfigDir/pg_ident.conf'    # IDENT configuration file

# If external_pid_file is not explicitly set, no extra pid file is written.
#external_pid_file = '(none)'        # write an extra pid file


#---------------------------------------------------------------------------
# CONNECTIONS AND AUTHENTICATION
#---------------------------------------------------------------------------

# - Connection Settings -

listen_addresses = '*'            # btaf: '*' = Listen to all addresses
port = 5432                         # btaf: Use default port for PostgreSQL, APUS

max_connections = 400            # btaf: APUS
# note: increasing max_connections costs ~400 bytes of shared memory per
# connection slot, plus lock space (see max_locks_per_transaction).  You
# might also need to raise shared_buffers to support more connections.
#superuser_reserved_connections = 2
unix_socket_directory = '/var/run/postgresql'
#unix_socket_group = ''
#unix_socket_permissions = 0777        # octal
#bonjour_name = ''            # defaults to the computer name

# - Security & Authentication -

#authentication_timeout = 60        # 1-600, in seconds

ssl = false                             # btaf: Do not use ssl for connections, APUS

#password_encryption = on
#db_user_namespace = off

# Kerberos
#krb_server_keyfile = ''
#krb_srvname = 'postgres'
#krb_server_hostname = ''        # empty string matches any keytab entry
#krb_caseins_users = off

# - TCP Keepalives -
# see 'man 7 tcp' for details

#tcp_keepalives_idle = 0        # TCP_KEEPIDLE, in seconds;
                    # 0 selects the system default
#tcp_keepalives_interval = 0        # TCP_KEEPINTVL, in seconds;
                    # 0 selects the system default
#tcp_keepalives_count = 0        # TCP_KEEPCNT;
                    # 0 selects the system default


#---------------------------------------------------------------------------
# RESOURCE USAGE (except WAL)
#---------------------------------------------------------------------------

# - Memory -

shared_buffers = 40000            # min 16 or max_connections*2, 8KB each, APUS
temp_buffers = 8000            # min 100, 8KB each, APUS
#max_prepared_transactions = 5        # can be 0 or more
# note: increasing max_prepared_transactions costs ~600 bytes of shared memory
# per transaction slot, plus lock space (see max_locks_per_transaction).
work_mem = 20480            # min 64, size in KB, APUS
maintenance_work_mem = 51200        # min 1024, size in KB, APUS
#max_stack_depth = 2048            # min 100, size in KB

# - Free Space Map -

max_fsm_pages = 40000            # min max_fsm_relations*16, 6 bytes each, APUS
#max_fsm_relations = 1000        # min 100, ~70 bytes each

# - Kernel Resource Usage -

#max_files_per_process = 1000        # min 25
#preload_libraries = ''

# - Cost-Based Vacuum Delay -

#vacuum_cost_delay = 0            # 0-1000 milliseconds
#vacuum_cost_page_hit = 1        # 0-10000 credits
#vacuum_cost_page_miss = 10        # 0-10000 credits
#vacuum_cost_page_dirty = 20        # 0-10000 credits
#vacuum_cost_limit = 200        # 0-10000 credits

# - Background writer -

bgwriter_delay = 100            # 10-10000 milliseconds between rounds, APUS
bgwriter_lru_percent = 2.0        # 0-100% of LRU buffers scanned/round, APUS
bgwriter_lru_maxpages = 10        # 0-1000 buffers max written/round, APUS
bgwriter_all_percent = 1        # 0-100% of all buffers scanned/round, APUS
bgwriter_all_maxpages = 10        # 0-1000 buffers max written/round, APUS


#---------------------------------------------------------------------------
# WRITE AHEAD LOG
#---------------------------------------------------------------------------

# - Settings -

#fsync = on                # turns forced synchronization on or off
#wal_sync_method = fsync        # the default is the first option
                    # supported by the operating system:
                    #   open_datasync
                    #   fdatasync
                    #   fsync
                    #   fsync_writethrough
                    #   open_sync
#full_page_writes = on            # recover from partial page writes
#wal_buffers = 8            # min 4, 8KB each
#commit_delay = 0            # range 0-100000, in microseconds
#commit_siblings = 5            # range 1-1000

# - Checkpoints -

checkpoint_segments = 32        # in logfile segments, min 1, 16MB each, APUS
#checkpoint_timeout = 300        # range 30-3600, in seconds
checkpoint_warning = 100        # in seconds, 0 is off, APUS

# - Archiving -

#archive_command = ''            # command to use to archive a logfile
                    # segment


#---------------------------------------------------------------------------
# QUERY TUNING
#---------------------------------------------------------------------------

# - Planner Method Configuration -

#enable_bitmapscan = on
#enable_hashagg = on
#enable_hashjoin = on
#enable_indexscan = on
#enable_mergejoin = on
#enable_nestloop = on
#enable_seqscan = on
#enable_sort = on
#enable_tidscan = on

# - Planner Cost Constants -

effective_cache_size = 89600        # typically 8KB each, APUS
#random_page_cost = 4            # units are one sequential page fetch
                    # cost
#cpu_tuple_cost = 0.01            # (same)
#cpu_index_tuple_cost = 0.001        # (same)
#cpu_operator_cost = 0.0025        # (same)

# - Genetic Query Optimizer -

#geqo = on
#geqo_threshold = 12
#geqo_effort = 5            # range 1-10
#geqo_pool_size = 0            # selects default based on effort
#geqo_generations = 0            # selects default based on effort
#geqo_selection_bias = 2.0        # range 1.5-2.0

# - Other Planner Options -

#default_statistics_target = 10        # range 1-1000
#constraint_exclusion = off
#from_collapse_limit = 8
#join_collapse_limit = 8        # 1 disables collapsing of explicit
                    # JOINs


#---------------------------------------------------------------------------
# ERROR REPORTING AND LOGGING
#---------------------------------------------------------------------------

# - Where to Log -

#log_destination = 'stderr'        # Valid values are combinations of
                    # stderr, syslog and eventlog,
                    # depending on platform.

# This is used when logging to stderr:
#redirect_stderr = off            # Enable capturing of stderr into log
                    # files

# These are only used if redirect_stderr is on:
#log_directory = 'pg_log'        # Directory where log files are written
                    # Can be absolute or relative to PGDATA
#log_filename = 'postgresql-%Y-%m-%d_%H%M%S.log' # Log file name pattern.
                    # Can include strftime() escapes
#log_truncate_on_rotation = off # If on, any existing log file of the same
                    # name as the new log file will be
                    # truncated rather than appended to. But
                    # such truncation only occurs on
                    # time-driven rotation, not on restarts
                    # or size-driven rotation. Default is
                    # off, meaning append to existing files
                    # in all cases.
#log_rotation_age = 1440        # Automatic rotation of logfiles will
                    # happen after so many minutes.  0 to
                    # disable.
#log_rotation_size = 10240        # Automatic rotation of logfiles will
                    # happen after so many kilobytes of log
                    # output.  0 to disable.

# These are relevant when logging to syslog:
#syslog_facility = 'LOCAL0'
#syslog_ident = 'postgres'


# - When to Log -

#client_min_messages = notice        # Values, in order of decreasing detail:
                    #   debug5
                    #   debug4
                    #   debug3
                    #   debug2
                    #   debug1
                    #   log
                    #   notice
                    #   warning
                    #   error

#log_min_messages = notice        # Values, in order of decreasing detail:
                    #   debug5
                    #   debug4
                    #   debug3
                    #   debug2
                    #   debug1
                    #   info
                    #   notice
                    #   warning
                    #   error
                    #   log
                    #   fatal
                    #   panic

#log_error_verbosity = default        # terse, default, or verbose messages

#log_min_error_statement = panic    # Values in order of increasing severity:
                     #   debug5
                    #   debug4
                    #   debug3
                    #   debug2
                    #   debug1
                     #   info
                    #   notice
                    #   warning
                    #   error
                    #   panic(off)

log_min_duration_statement = 1000    # -1 is disabled, 0 logs all statements
                    # and their durations, in milliseconds.

#silent_mode = off            # DO NOT USE without syslog or
                    # redirect_stderr

# - What to Log -

#debug_print_parse = off
#debug_print_rewritten = off
#debug_print_plan = off
#debug_pretty_print = off
#log_connections = off
#log_disconnections = off
#log_duration = off
log_line_prefix = '[%d %m]'            # Special values:
                    #   %u = user name
                    #   %d = database name
                    #   %r = remote host and port
                    #   %h = remote host
                    #   %p = PID
                    #   %t = timestamp (no milliseconds)
                    #   %m = timestamp with milliseconds
                    #   %i = command tag
                    #   %c = session id
                    #   %l = session line number
                    #   %s = session start timestamp
                    #   %x = transaction id
                    #   %q = stop here in non-session
                    #        processes
                    #   %% = '%'
                    # e.g. '<%u%%%d> '
#log_statement = 'none'            # none, mod, ddl, all
#log_hostname = off


#---------------------------------------------------------------------------
# RUNTIME STATISTICS
#---------------------------------------------------------------------------

# - Statistics Monitoring -

#log_parser_stats = off
#log_planner_stats = off
#log_executor_stats = off
#log_statement_stats = off

# - Query/Index Statistics Collector -

#stats_start_collector = on
stats_command_string = on
#stats_block_level = off
stats_row_level = on
#stats_reset_on_server_start = off


#---------------------------------------------------------------------------
# AUTOVACUUM PARAMETERS
#---------------------------------------------------------------------------

autovacuum = on                # enable autovacuum subprocess?
autovacuum_naptime = 30            # time between autovacuum runs, in secs, APUS
autovacuum_vacuum_threshold = 500    # min # of tuple updates before, APUS
                    # vacuum
autovacuum_analyze_threshold = 250    # min # of tuple updates before, APUS
                    # analyze
autovacuum_vacuum_scale_factor = 0.2    # fraction of rel size before, APUS
                    # vacuum
autovacuum_analyze_scale_factor = 0.1    # fraction of rel size before, APUS
                    # analyze
#autovacuum_vacuum_cost_delay = -1    # default vacuum cost delay for
                    # autovac, -1 means use
                    # vacuum_cost_delay
#autovacuum_vacuum_cost_limit = -1    # default vacuum cost limit for
                    # autovac, -1 means use
                    # vacuum_cost_limit


#---------------------------------------------------------------------------
# CLIENT CONNECTION DEFAULTS
#---------------------------------------------------------------------------

# - Statement Behavior -

#search_path = '$user,public'        # schema names
#default_tablespace = ''        # a tablespace name, '' uses
                    # the default
#check_function_bodies = on
#default_transaction_isolation = 'read committed'
#default_transaction_read_only = off
statement_timeout = 60000        # 0 is disabled, in milliseconds, APUS

# - Locale and Formatting -

#datestyle = 'iso, mdy'
#timezone = unknown            # actually, defaults to TZ
                    # environment setting
#australian_timezones = off
#extra_float_digits = 0            # min -15, max 2
#client_encoding = sql_ascii        # actually, defaults to database
                    # encoding

# These settings are initialized by initdb -- they might be changed
lc_messages = 'C'            # locale for system error message
                    # strings
lc_monetary = 'C'            # locale for monetary formatting
lc_numeric = 'C'            # locale for number formatting
lc_time = 'C'                # locale for time formatting

# - Other Defaults -

#explain_pretty_print = on
#dynamic_library_path = '$libdir'


#---------------------------------------------------------------------------
# LOCK MANAGEMENT
#---------------------------------------------------------------------------

#deadlock_timeout = 1000        # in milliseconds
#max_locks_per_transaction = 64        # min 10
# note: each lock table slot uses ~220 bytes of shared memory, and there are
# max_locks_per_transaction * (max_connections + max_prepared_transactions)
# lock table slots.


#---------------------------------------------------------------------------
# VERSION/PLATFORM COMPATIBILITY
#---------------------------------------------------------------------------

# - Previous Postgres Versions -

add_missing_from = on               # btaf: Add missing FROM clauses to improve compatibility with 7.4.x versions

#backslash_quote = safe_encoding    # on, off, or safe_encoding
#default_with_oids = off
#escape_string_warning = off
#regex_flavor = advanced        # advanced, extended, or basic
#sql_inheritance = on

# - Other Platforms & Clients -

#transform_null_equals = off


#---------------------------------------------------------------------------
# CUSTOMIZED OPTIONS
#---------------------------------------------------------------------------

#custom_variable_classes = ''        # list of custom variable class names

Re: System Load analyze

From
"Scott Marlowe"
Date:
On Nov 24, 2007 10:57 AM, Peter Bauer <peter.bauer@apus.co.at> wrote:
>
> i have a system here with 2 2.4GHz Xeon Processors, 2GB RAM, ONE Disk on
> a Battery Backed Write Cache SCSI Controller and PostgreSQL 8.1.4
> running with the data on a DRBD Device for High Availability. The used
> database is also replicated to two similar machines with slony1.

Why are you running a version of PostgreSQL with known data eating
bugs? If you care for your data, you will keep up to date on releases.
 8.1.10 was released on 2007-09-17.  8.1.4 was released on 2006-05-23.
 That's 16 months of bug fixes you're missing.  Go here:
http://www.postgresql.org/docs/8.1/static/release.html and read up on
the fixes you're missing.  Then update.  Or just update.

OK, on the the issue at hand.

> Since the load average is between 1 (most of the time) and 10 (peeks) i
> am worried about the load and executed vmstat and iostat which show that
> 1000-6000 Blocks are writen per second. Please check the attached output
> for further details.
> top shows that the CPUs are at least 80% idle most of the time so i
> think there is an I/O bottleneck. I'm aware that this hardware setup is
> probably not sufficient but is would like to investigate how critical
> the situation is.

Yes.  Battery backed cache can only do so much, it's not magic pixie
dust.  Once it's full, the drive becomes the bottle neck.  Real db
servers have more than one disk drive.  They usually have at least 4
or so, and often dozens to hundreds. Also, not all battery backed
caching RAID controllers are created equal.

> procs -----------memory---------- ---swap-- -----io---- --system-- ----cpu----
>  r  b   swpd   free   buff  cache   si   so    bi    bo   in    cs us sy id wa
>  0  0 398256  78328 140612 1063556    0    0     0  1472 2029  5081  4  3 92  0
>  0  2 398256  78268 140612 1063576    0    0     0  2304 1928  4216  0  2 98  0
>  1  2 398256  78100 140612 1063576    0    0     0  1464 1716  3994  1  3 96  0
>  0  0 398256  78704 140612 1063592    0    0     0   916 1435  3570  5  3 91  0
>  0  0 398256  77876 140612 1063616    0    0     0     0  305  1169  3  1 96  0

See that dip in the last line above where the blocks in drop to 0,
idle jumps to 96, and blocks out drops, and context switches drop?
That's most likely where postgresql is checkpointing.  Checkpointing
is where it writes out all the dirty buffers to disk.  If the bgwriter
is not tuned aggresively enough, checkpoints happen and make the whole
database slow down for a few seconds.  If it's tuned too aggresively
then the db spends too much CPU time tracking the dirty buffers and
then writing them.  If tuned just right, it will write out the dirty
buffers just fast enough that a checkpoint is never needed.

You tune the bgwriter to your machine and I/O subsystem.  If you're
planning on getting more hard drives, do that first.  Then tune the
bgwriter.

btw, if this is "vmstat 1" running, it's showing a checkpoint every 20
or so seconds I think

>  0  2 398256  79136 140612 1063964    0    0     0  1736 1959  4494  4  2 94  0
checkpoint here:
>  0  0 398256  79132 140612 1063964    0    0     0     4  260  1039  1  1 98  0
>  0  0 398256  79052 140612 1063980    0    0     0  2444 3084  6955  6  5 89  0
>  0  2 398256  79060 140612 1063988    0    0     0   948 1146  3616  3  1 96  0
>  0  1 398256  78268 140612 1064056    0    0     0  1908 1809  4086  6  5 88  0
>  0  1 398256  76728 140612 1064056    0    0     0  6256 6637 15472  5  5 90  0
>  0  2 398256  77000 140612 1064064    0    0     0  4916 5840 12107  1  4 95  0
>  0  2 398256  76956 140612 1064068    0    0     0  6468 7432 15211  1  3 96  0
>  0  6 398256  77388 140612 1064072    0    0     0  8116 7826 18265  1  8 91  0
>  0  2 398256  74312 140612 1064076    0    0     0  7032 6886 16136  2  7 91  0
>  0  2 398256  74264 140612 1064076    0    0     0  5680 7143 13411  0  5 95  0
>  0  2 398256  72980 140612 1064140    0    0     0  5396 6377 13251  6  6 88  0
>  0  3 398256  76972 140612 1064148    0    0     0  5652 6793 14079  4  9 87  0
>  0  2 398256  77836 140612 1064148    0    0     0  3968 5321 14187 10  8 82  0
>  1  0 398256  77280 140612 1064148    0    0     0  1608 3188  8974 21 12 67  0
>  1  0 398256  77832 140612 1064152    0    0     0   236  834  2625  7  5 87  0
>  0  0 398256  77464 140612 1064152    0    0     0   244  505  1378  2  4 94  0
>  1  0 398256  77828 140612 1064164    0    0     0   316  580  1954  7  2 91  0
>  0  0 398256  77804 140612 1064180    0    0     0   740  673  2248  2  2 96  0
>  0  0 398256  77000 140612 1064180    0    0     0   304  589  1739  1  3 96  0
20 rows later, checkpoint here:
>  0  0 398256  77000 140612 1064184    0    0     0     0  216   886  0  1 99  0
>  0  0 398256  75452 140612 1064184    0    0     0   432  755  2032  6  1 93  0

> max_fsm_pages = 40000                   # min max_fsm_relations*16, 6 bytes each, APUS

This seems a little low for a busy server.

> # - Background writer -
>
> bgwriter_delay = 100                    # 10-10000 milliseconds between rounds, APUS
> bgwriter_lru_percent = 2.0              # 0-100% of LRU buffers scanned/round, APUS
> bgwriter_lru_maxpages = 10              # 0-1000 buffers max written/round, APUS
> bgwriter_all_percent = 1                # 0-100% of all buffers scanned/round, APUS
> bgwriter_all_maxpages = 10              # 0-1000 buffers max written/round, APUS

So, bgwriter wakes up 10 times a second, and each time it processes 2%
of the Least Recently Used pages for writing, and writes up to 10 of
those pages.  And it only checks 1% of the total pages and writes 10
of those at the most.  This is not aggresive enough, and given how
much spare CPU you have left over, you can be a fair bit more
aggresive.  The main thing to increase is the maxes.  Try changing
them to the 100 to 300 range, and maybe increase your percentages to
5% or so.  What we're shooting for is to see those checkpoints go
away.

Then, when running your benchmark, after a few minutes, run a
checkpoint by hand and see if you get one of those slow downs like we
saw in vmstat above.  If your bgwriter is tuned properly, you should
get an almost instant response from the checkpoint and no noticeable
slow down in the vmstat numbers for context switches per second.

Once you reach the point where the bgwriter is just keeping ahead of
check points, there's little to be gained in more aggressive tuning of
the bgwriter and you'll just be chewing up memory and cpu bandwidth if
you do get too aggressive with it.

Re: System Load analyze

From
Peter Bauer
Date:
Am Dienstag 27 November 2007 schrieb Scott Marlowe:
> On Nov 24, 2007 10:57 AM, Peter Bauer <peter.bauer@apus.co.at> wrote:
> > i have a system here with 2 2.4GHz Xeon Processors, 2GB RAM, ONE Disk on
> > a Battery Backed Write Cache SCSI Controller and PostgreSQL 8.1.4
> > running with the data on a DRBD Device for High Availability. The used
> > database is also replicated to two similar machines with slony1.
>
> Why are you running a version of PostgreSQL with known data eating
> bugs? If you care for your data, you will keep up to date on releases.
>  8.1.10 was released on 2007-09-17.  8.1.4 was released on 2006-05-23.
>  That's 16 months of bug fixes you're missing.  Go here:
> http://www.postgresql.org/docs/8.1/static/release.html and read up on
> the fixes you're missing.  Then update.  Or just update.
>
> OK, on the the issue at hand.
>
> > Since the load average is between 1 (most of the time) and 10 (peeks) i
> > am worried about the load and executed vmstat and iostat which show that
> > 1000-6000 Blocks are writen per second. Please check the attached output
> > for further details.
> > top shows that the CPUs are at least 80% idle most of the time so i
> > think there is an I/O bottleneck. I'm aware that this hardware setup is
> > probably not sufficient but is would like to investigate how critical
> > the situation is.
>
> Yes.  Battery backed cache can only do so much, it's not magic pixie
> dust.  Once it's full, the drive becomes the bottle neck.  Real db
> servers have more than one disk drive.  They usually have at least 4
> or so, and often dozens to hundreds. Also, not all battery backed
> caching RAID controllers are created equal.
>
> > procs -----------memory---------- ---swap-- -----io---- --system--
> > ----cpu---- r  b   swpd   free   buff  cache   si   so    bi    bo   in
> >  cs us sy id wa 0  0 398256  78328 140612 1063556    0    0     0  1472
> > 2029  5081  4  3 92  0 0  2 398256  78268 140612 1063576    0    0     0
> > 2304 1928  4216  0  2 98  0 1  2 398256  78100 140612 1063576    0    0
> >   0  1464 1716  3994  1  3 96  0 0  0 398256  78704 140612 1063592    0
> >  0     0   916 1435  3570  5  3 91  0 0  0 398256  77876 140612 1063616
> >  0    0     0     0  305  1169  3  1 96  0
>
> See that dip in the last line above where the blocks in drop to 0,
> idle jumps to 96, and blocks out drops, and context switches drop?
> That's most likely where postgresql is checkpointing.  Checkpointing
> is where it writes out all the dirty buffers to disk.  If the bgwriter
> is not tuned aggresively enough, checkpoints happen and make the whole
> database slow down for a few seconds.  If it's tuned too aggresively
> then the db spends too much CPU time tracking the dirty buffers and
> then writing them.  If tuned just right, it will write out the dirty
> buffers just fast enough that a checkpoint is never needed.
>
> You tune the bgwriter to your machine and I/O subsystem.  If you're
> planning on getting more hard drives, do that first.  Then tune the
> bgwriter.
>
> btw, if this is "vmstat 1" running, it's showing a checkpoint every 20
> or so seconds I think
>
> >  0  2 398256  79136 140612 1063964    0    0     0  1736 1959  4494  4  2
> > 94  0
>
> checkpoint here:
> >  0  0 398256  79132 140612 1063964    0    0     0     4  260  1039  1  1
> > 98  0 0  0 398256  79052 140612 1063980    0    0     0  2444 3084  6955
> > 6  5 89  0 0  2 398256  79060 140612 1063988    0    0     0   948 1146
> > 3616  3  1 96  0 0  1 398256  78268 140612 1064056    0    0     0  1908
> > 1809  4086  6  5 88  0 0  1 398256  76728 140612 1064056    0    0     0
> > 6256 6637 15472  5  5 90  0 0  2 398256  77000 140612 1064064    0    0
> >   0  4916 5840 12107  1  4 95  0 0  2 398256  76956 140612 1064068    0
> >  0     0  6468 7432 15211  1  3 96  0 0  6 398256  77388 140612 1064072
> >  0    0     0  8116 7826 18265  1  8 91  0 0  2 398256  74312 140612
> > 1064076    0    0     0  7032 6886 16136  2  7 91  0 0  2 398256  74264
> > 140612 1064076    0    0     0  5680 7143 13411  0  5 95  0 0  2 398256
> > 72980 140612 1064140    0    0     0  5396 6377 13251  6  6 88  0 0  3
> > 398256  76972 140612 1064148    0    0     0  5652 6793 14079  4  9 87  0
> > 0  2 398256  77836 140612 1064148    0    0     0  3968 5321 14187 10  8
> > 82  0 1  0 398256  77280 140612 1064148    0    0     0  1608 3188  8974
> > 21 12 67  0 1  0 398256  77832 140612 1064152    0    0     0   236  834
> > 2625  7  5 87  0 0  0 398256  77464 140612 1064152    0    0     0   244
> > 505  1378  2  4 94  0 1  0 398256  77828 140612 1064164    0    0     0
> > 316  580  1954  7  2 91  0 0  0 398256  77804 140612 1064180    0    0
> >  0   740  673  2248  2  2 96  0 0  0 398256  77000 140612 1064180    0
> > 0     0   304  589  1739  1  3 96  0
>
> 20 rows later, checkpoint here:
> >  0  0 398256  77000 140612 1064184    0    0     0     0  216   886  0  1
> > 99  0 0  0 398256  75452 140612 1064184    0    0     0   432  755  2032
> > 6  1 93  0
> >
> > max_fsm_pages = 40000                   # min max_fsm_relations*16, 6
> > bytes each, APUS
>
> This seems a little low for a busy server.
>
> > # - Background writer -
> >
> > bgwriter_delay = 100                    # 10-10000 milliseconds between
> > rounds, APUS bgwriter_lru_percent = 2.0              # 0-100% of LRU
> > buffers scanned/round, APUS bgwriter_lru_maxpages = 10              #
> > 0-1000 buffers max written/round, APUS bgwriter_all_percent = 1
> >      # 0-100% of all buffers scanned/round, APUS bgwriter_all_maxpages =
> > 10              # 0-1000 buffers max written/round, APUS
>
> So, bgwriter wakes up 10 times a second, and each time it processes 2%
> of the Least Recently Used pages for writing, and writes up to 10 of
> those pages.  And it only checks 1% of the total pages and writes 10
> of those at the most.  This is not aggresive enough, and given how
> much spare CPU you have left over, you can be a fair bit more
> aggresive.  The main thing to increase is the maxes.  Try changing
> them to the 100 to 300 range, and maybe increase your percentages to
> 5% or so.  What we're shooting for is to see those checkpoints go
> away.
>
> Then, when running your benchmark, after a few minutes, run a
> checkpoint by hand and see if you get one of those slow downs like we
> saw in vmstat above.  If your bgwriter is tuned properly, you should
> get an almost instant response from the checkpoint and no noticeable
> slow down in the vmstat numbers for context switches per second.
>
> Once you reach the point where the bgwriter is just keeping ahead of
> check points, there's little to be gained in more aggressive tuning of
> the bgwriter and you'll just be chewing up memory and cpu bandwidth if
> you do get too aggressive with it.

Hi Scott,

thank you for the great suggestions, i will keep the list informed.

br,
Peter

--
Peter Bauer
APUS Software G.m.b.H.
A-8074 Raaba, Bahnhofstrasse 1/1
Email: peter.bauer@apus.co.at
Tel: +43 316 401629 24
Fax: +43 316 401629 9

Re: System Load analyze

From
Greg Smith
Date:
On Sat, 24 Nov 2007, Peter Bauer wrote:

> top shows that the CPUs are at least 80% idle most of the time so i
> think there is an I/O bottleneck.

top also shows that you're never waiting for I/O which is usually evidence
there isn't an I/O bottleneck.  You passed along most of the right data,
but some useful additional things to know are:

-Actual brand/model of SCSI controller
-Operating system
-What time interval the vmstat and iostat information you gave were
produced at.

I agree with Scott that checkpoints should be considered as a possibility
here.  I'd suggest you set checkpoint_warning to a high value so you get a
note in the logs every time one happens, then see if those happen at the
same time as your high load average.  More on that topic and how to adjust
the background writer if that proves to be the cause of your slowdown is
at http://www.westnet.com/~gsmith/content/postgresql/chkp-bgw-83.htm

--
* Greg Smith gsmith@gregsmith.com http://www.gregsmith.com Baltimore, MD

Re: System Load analyze

From
Peter Bauer
Date:
Hi Greg,

Am Mittwoch 28 November 2007 schrieb Greg Smith:
> On Sat, 24 Nov 2007, Peter Bauer wrote:
> > top shows that the CPUs are at least 80% idle most of the time so i
> > think there is an I/O bottleneck.
>
> top also shows that you're never waiting for I/O which is usually evidence
> there isn't an I/O bottleneck.  You passed along most of the right data,
> but some useful additional things to know are:
>
> -Actual brand/model of SCSI controller
> -Operating system
> -What time interval the vmstat and iostat information you gave were
> produced at.

here are the hardware specs:
2x POWEREDGE 2850 - XEON 3.0GHZ/2MB, 800FSB
2048MB SINGLE RANK DDR2
73 GB SCSI-Disk , 15.000 rpm, UL
PERC4E/DI DC ULTRA320 SCSI RAID, 256MB

Its Debian sarge with kernel 2.4.26.050719-686 #1 SMP.

vmstat and iostat were running with 1 second intervals.

> I agree with Scott that checkpoints should be considered as a possibility
> here.  I'd suggest you set checkpoint_warning to a high value so you get a
> note in the logs every time one happens, then see if those happen at the
> same time as your high load average.  More on that topic and how to adjust
> the background writer if that proves to be the cause of your slowdown is
> at http://www.westnet.com/~gsmith/content/postgresql/chkp-bgw-83.htm
>
> --
> * Greg Smith gsmith@gregsmith.com http://www.gregsmith.com Baltimore, MD
>
> ---------------------------(end of broadcast)---------------------------
> TIP 4: Have you searched our list archives?
>
>                http://archives.postgresql.org/

thank you,
Peter




--
Peter Bauer
APUS Software G.m.b.H.
A-8074 Raaba, Bahnhofstrasse 1/1
Email: peter.bauer@apus.co.at
Tel: +43 316 401629 24
Fax: +43 316 401629 9

Re: System Load analyze

From
Greg Smith
Date:
On Wed, 28 Nov 2007, Peter Bauer wrote:

> PERC4E/DI DC ULTRA320 SCSI RAID, 256MB
> Its Debian sarge with kernel 2.4.26.050719-686 #1 SMP.

OK, so I'd expect you're using the megaraid2 driver.  That and kernel
2.4.26 are a few years behind current at this point, and there have been
plenty of performance improvements in Linux and that driver since then.
Since you're asking about this before it's a serious problem and don't
need an immediate fix, you may want to consider whether upgrading to a
more modern 2.6 kernel is in your future; that change alone may resolve
some of the possibly too high load you're having.  I doubt any 2.4 kernel
is really getting the best from your fairly modern server hardsare.

Also, the PERC4E and similar Megaraid cards are known to be generally
sluggish on write throughput compared with some of the competing products
out there.  I don't know that I'd replace it though, as spending the same
amount of money adding disks would probably be more useful.

There's actually a way to combine these two ideas and get an upgrade
transition plan.  Buy a second disk, find a downtime window, install a
newer Linux onto it and test.  If that works well switch to it.  If it
doesn't you still have the original unmodified system around and you can
at least split the disk load between the two drives.

> vmstat and iostat were running with 1 second intervals.

Good, that means the responses you've already gotten made the right
assumptions.  There's some additional fun statistics you can collect with
the Linux iostat, but sadly that needs a newer kernel as well.

--
* Greg Smith gsmith@gregsmith.com http://www.gregsmith.com Baltimore, MD