Hi, On 08/27/2015 07:21 PM, Jean Cavallo wrote:
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.
If you're moving a lot of data (especially if the destination database is empty), one possible problem is statistics. This generally is not a problem in regular operation, because the data growth is gradual and autovacuum analyzes the tables regularly, but in batch processes this is often a big issue.
The usual scenario is that there's an empty (or very small) table, where indexes are inefficient so PostgreSQL plans the queries with sequential scans. The table suddenly grows, which would make indexes efficient, but the planner has no idea about that until autovacuum kicks in. But before that happens, the batch process executes queries on that table.
Try adding ANALYZE after steps that add a lot of data.
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.
That's a bit strange. Essentially what you're saying is that the workload is neither CPU nor I/O bound. To make it CPU bound, at least one CPU would have to be 100% utilized, and with 4 CPUs that's 25%, but you're saying there's only 10% used. But you're saying I/O is not the bottleneck either.
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 ?
A lot of things, unfortunately, and the fact that this is a migration moving data between two databases makes it even more complicated. The virtualization does not make it less complex either.
For example, are you sure it's not stuck on the other database? I assume you're running some long queries, so maybe it's stuck there and the destination database is just waiting for data? That's be consistent with the low CPU and I/O usage you observe.
Locking is another possibility, although it probably is not the only cause - it'd be utilizing at least one CPU otherwise.
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.
If it was a problem, you'd see high I/O usage. And that's not the case.
Also, the queries that appear to get stuck are "heavy" queries, though after a fresh restart they execute in a reasonable time.
Does the plan change? If not, check waiting locks in pg_locks.
Finally, whatever causes the database to wait also causes the Windows instance to slow down. But restarting Postgresql fixes this as well.
That's a bit strange, I guess. If you're not observing light CPU and I/O usage, then the instance should not be slow, unless there's something else going on - possibly at the virtualization level (e.g. another busy instance on the same hardware, some sort of accounting that limits the resources after a time, etc.)
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
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