On Thu, Oct 27, 2011 at 1:45 PM, Nicholson, Brad (Toronto, ON, CA) < bnichol...@hp.com> wrote:
> >From: pgsql-performance-ow...@postgresql.org [mailto: > pgsql-performance-ow...@postgresql.org] On Behalf Of Samuel Gendler > >Sent: Thursday, October 27, 2011 12:47 PM > >To: pgsql-performance@postgresql.org > >Subject: [PERFORM] backups blocking everything > > > >I've got a large mixed-used database, with the data warehouse side of > things consisting of several tables at hundreds of millions of rows, plus a > number of tables with tens of >millions. There is partitioning, but as the > volume of data has risen, individual partitions have gotten quite large. > Hardware is 2x4 core 2.0Ghz Xeon processors, 176GB of RAM, 4 drives in > >raid 10 for WAL logs and 16 or 20 spindles for data, also in RAID 10. > Total database size is currently 399GB - via pg_database_size(). It's also > worth noting that we switched from 8.4 to >9.0.4 only about a month ago, and > we were not seeing this problem on 8.4.x. The database is growing, but not > at some kind of exponential rate. full backup, compressed, on the old > hardware >was 6.3GB and took about 1:45:00 to be written. Recent backups > are 8.3GB and taking 3 or 4 hours. We were not seeing al queries stall out > during the backups on 8.4, so far as I am aware. > > > >The time it takes for pg_dump to run has grown from 1 hour to 3 and even 4 > hours over the last 6 months, with more than half of that increase occurring > since we upgrade to 9.0.x. In the >last several weeks (possibly since the > upgrade to 9.0.4), we are seeing all connections getting used up (our main > apps use connection pools, but monitoring and some utilities are making > >direct connections for each query, and some of them don't check for the > prior query to complete before sending another, which slowly eats up > available connections). Even the connection >pool apps cease functioning > during the backup, however, as all of the connections wind up in parse > waiting state. I also see lots of sockets in close wait state for what > seems to be an >indefinite period while the backup is running and all > connections are used up. I assume all of this is the result of pg_dump > starting a transaction or otherwise blocking other access. I >can get > everything using a pool, that's not a huge problem to solve, but that won't > fix the fundamental problem of no queries being able to finish while the > backup is happening. > > What is the I/O utilization like during the dump? I've seen this situation > in the past and it was caused be excessively bloated tables causing I/O > starvation while they are getting dumped. > There are definitely no bloated tables. The large tables are all insert-only, and old data is aggregated up and then removed by dropping whole partitions. There should be no bloat whatsoever. The OLTP side of things is pretty minimal, and I can pg_dump those schemas in seconds, so they aren't the problem, either. I don't know what the I/O utilization is during the dump, offhand. I'll be doing a more thorough investigation tonight, though I suppose I could go look at the monitoring graphs if I weren't in the middle of 6 other things at the moment. the joys of startup life.
