Hi,
I am currently working on a data migration for a client.
The general plan is :
- Read data from a postgresql database
- Convert them to the new application
- Insert in another database (same postgresql instance).
The source database is rather big (~40GB, wo indexes), and the
conversion process takes some time. It is done by multiple workers
on a separate Linux environnement, piece by piece.
When we start the migration, at first it looks good.
Performances are good, and it ran smoothly. After a few hours,
we noticed that things started to slow down. Some queries seemed
to be stuck, so we waited for them to end, and restarted the server.
After that it went well for some time (~10 minutes), then it slowed
down again. We tried again (a few times), and the pattern repeats.
My postgresql specific problem is that it looks like the server gets
stuck. CPU usage is <10%, RAM usage is under 50% max, there is
no noticeable disk usage. But, there are some (<10) active queries,
some of which may take several hours to complete. Those queries
work properly (i.e < 1min) right after the server restarts.
So my question is : What could slow the queries from ~1min to 2hours
which does not involve CPU, Memory, or disk usage, and which would
"reset" when restarting the server ?
For information, the number of processes does not seem to be the
problem, there are ~20 connections with max_connection set to 100.
We noticed at some point that the hard drive holding the target
database was heavily fragmented (100%...), but defrag did not
seem to change anything.
Also, the queries that appear to get stuck are "heavy" queries,
though after a fresh restart they execute in a reasonable time.
Finally, whatever causes the database to wait also causes the
Windows instance to slow down. But restarting Postgresql fixes
this as well.
Configuration :
The Postgresql server runs on a Windows Virtual Machine under
VMWare. The VM has dedicated resources, and the only other
VM on the host is the applicative server (which runs idle while
waiting for the database). There is nothing else running on the
server except postgresql (well, there were other things, but we
stopped everything to no avail).
PostgreSQL 9.3.5, compiled by Visual C++ build 1600, 64-bit
Windows 2008R2 (64 bits)
10 Go RAM
4 vCPU
Host : VMWare ESXi 5.5.0 build-2068190
CPU Intel XEON X5690 3.97GHz
HDD 3x Nearline SAS 15K RAID0
Please let me know if any other information may be useful.
