Re: Performance considerations for very heavy INSERT - Mailing list pgsql-performance

>From: Brandon Black <blblack@gmail.com>
>Sent: Sep 12, 2005 5:04 PM
>To: pgsql-performance@postgresql.org
>Subject: [PERFORM] Performance considerations for very heavy INSERT traffic

>I'm in the process of developing an application which uses PostgreSQL for
>data storage. Our database traffic is very atypical, and as a result it has
>been rather challenging to figure out how to best tune PostgreSQL on what
>development hardware we have, as well as to figure out exactly what we
>should be evaluating and eventually buying for production hardware.

>The vast, overwhelming majority of our database traffic is pretty much a
>non-stop stream of INSERTs filling up tables. It is akin to data
>acquisition. Several thousand clients are sending once-per-minute updates
>full of timestamped numerical data at our central server, which in turn
>performs INSERTs into several distinct tables as part of the transaction for
>that client. We're talking on the order of ~100 transactions per second,
>each containing INSERTs to multiple tables (which contain only integer and
>floating point columns and a timestamp column - the primary key (and only
>index) is on a unique integer ID for the client and the timestamp). The
>transaction load is spread evenly over time by having the clients send their
>per-minute updates at random times rather than on the exact minute mark.

I have built two such systems.  TBF, neither used PostgreSQL.  OTOH, the principles are the same.

One perhaps non-obvious point: You definitely are going to want a way to adjust exactly when a specific client is
sendingits approximately once-per-minute update via functionality similar to adjtime().  Such functionality will be
neededto smooth the traffic across the clients as much as possible over the 1 minute polling period given the
real-worldvagracies of WAN connections. 

Put your current active data acquisition table on its own RAID 10 array, and keep it _very_ small and simple in
structure(see below for a specific suggestion regarding this).  If you must have indexes for this table, put them on a
_different_array.  Basically, you are going to treat the active table like you would an log file: you want as little HD
headmovement as possible so appending to the table is effectively sequential IO. 

As in a log file, you will get better performance if you batch your updates to HD into reasonably sized chunks rather
thanwriting to HD every INSERT. 

The actual log file will have to be treated in the same way to avoid it becoming a performance bottleneck.


>There will of course be users using a web-based GUI to extract data from
>these tables and display them in graphs and whatnot, but the SELECT query
>traffic will always be considerably less frequent and intensive than the
>incessant INSERTs, and it's not that big a deal if the large queries take a
>little while to run.

More details here would be helpful.


>This data also expires - rows with timestamps older than X days will be
>DELETEd periodically (once an hour or faster), such that the tables will
>reach a relatively stable size (pg_autovacuum is handling vacuuming for now,
>but considering our case, we're thinking of killing pg_autovacuum in favor
>of having the periodic DELETE process also do a vacuum of affected tables
>right after the DELETE, and then have it vacuum the other low traffic tables
>once a day while it's at it).

Design Idea: split the data into tables where _at most_ the tables are of the size that all the data in the table
expiresat the same time and DROP the entire table rather than scanning a big table for deletes at the same time you
wantto do inserts to said.  Another appropriate size for these tables may be related to the chunk you want to write
INSERTSto HD in.   

This will also have the happy side effect of breaking the data into smaller chunks that are more likely to be cached in
theirentirety when used.  


>There is an aggregation layer in place which proxies the inbound data from
>the clients into a small(er) number of persistent postgresql backend
>processes. Right now we're doing one aggregator per 128 clients (so instead
>of 128 seperate database connections over the course of a minute for a small
>transaction each, there is a single database backend that is constantly
>committing transactions at a rate of ~ 2/second). At a test load of ~1,000
>clients, we would have 8 aggregators running and 8 postgresql backends.
>Testing has seemed to indicate we should aggregate even harder - the planned
>production load is ~5,000 clients initially, but will grow to almost double
>that in the not-too-distant future, and that would mean ~40 backends at 128
>clients each initially. Even on 8 cpus, I'm betting 40 concurrent backends
>doing 2 tps is much worse off than 10 backends doing 8 tps.

Experience has taught me that the above is not likely to be the proper architecture for this kind of application.

The best exact approach is dependent on The Details, but in general you want to optimize the amount of data sent from
NICto CPU per transfer (multiple small copies and lots of interrupts _kill_ system performance) and use a combined
Threadingand Event Queue model with Processor Affinity being used to optimize the NIC <-> CPU path for a given NIC.
Haveeach physical NIC+CPU Affinity set be hidden underneath an overall Virtual NIC+CPU abstraction for load balancing
purposes. The traffic thinks it's talking to one NIC attached to one CPU. 

This architecture allowed for a 4P Xeon system (Compaq Proliant 8500s) to handle 250K+ simultaneous active web
connections. In 2000 using Windows 2K.  As HW + SW has gotten better, it has scaled very well.  It's a very high
performancemodel. 

Such an architecture using AMD64 CPUs running a 64b Linux 2.6 distro now in late 2005 should _easily_ handle your
projecteddemand with plenty of room for growth. 


>Test hardware right now is a dual Opteron with 4G of ram, which we've barely
>gotten 1,000 clients running against. Current disk hardware in testing is
>whatever we could scrape together (4x 3-ware PCI hardware RAID controllers,
>with 8 SATA drives in a RAID10 array off of each - aggregated up in a 4-way
>stripe with linux md driver and then formatted as ext3 with an appropriate
>stride parameter and data=writeback). Production will hopefully be a 4-8-way
>Opteron, 16 or more G of RAM, and a fiberchannel hardware raid array or two
>(~ 1TB available RAID10 storage) with 15krpm disks and battery-backed write
>cache.

For deployment, get a 2P or 4P 16 DIMM slot mainboard and stuff it with at least 16GB of RAM.  A 2P 16 DIMM slot
mainboardlike the IWill DK88 (Tyan and a few others also have such boards in the works) is IMHO your best bet. 

IME, you will not need CPU for this application as much as you will need RAM first and HD IO bandwidth second.
Get a 1C2P AMD64 system and don't worry about CPU until you are CPU bound.  I highly doubt you will need more CPU than
a2C2P (4 cores total) based on say 4400+'s, 4600+'s, or 4800+'s can provide. 


>I know I haven't provided a whole lot of application-level detail here, but
>does anyone have any general advice on tweaking postgresql to deal with a
>very heavy load of concurrent and almost exclusively write-only
>transactions? Increasing shared_buffers seems to always help, even out to
>half of the dev box's ram (2G). A 100ms commit_delay seemed to help, but
>tuning it (and _siblings) has been difficult. We're using 8.0 with the
>default 8k blocksize, but are strongly considering both developing against
>8.1 (seems it might handle the heavy concurrency better), and re-compiling
>with 32k blocksize since our storage arrays will inevitably be using fairly
>wide stripes. Any advice on any of this (other than drop the project while
>you're still a little bit sane)?

Since you are doing more writing than reading, and those writes are going to be relatively small, you may not get as
muchout of block sizes larger than the average DB write. 

My initial instinct on this point is that you should keep the size of the "chunk" the same from NIC to CPU to HD, and
makesaid chunk as large as possible. 


Hope this helps,
Ron Peacetree

PS.  Yes, I'm available under the proper circumstances for a consulting gig.