Re: [GENERAL] Questionaire: Common WAL write rates on busy servers.

On Mon, Apr 24, 2017 at 9:17 PM, Andres Freund <andres(at)anarazel(dot)de> wrote:

>
> Questions (answer as many you can comfortably answer):
> - How many MB/s, segments/s do you see on busier servers?
> - What generates the bulk of WAL on your servers (9.5+ can use
> pg_xlogdump --stats to compute that)?
> - Are you seeing WAL writes being a bottleneck?OA
> - What kind of backup methods are you using and is the WAL volume a
> problem?
> - What kind of replication are you using and is the WAL volume a
> problem?
> - What are your settings for wal_compression, max_wal_size (9.5+) /
> checkpoint_segments (< 9.5), checkpoint_timeout and wal_buffers?
> - Could you quickly describe your workload?
>

* Postgresql 9.3
* 1500+ db servers
* Daily peak for busy databases: 75 WALs switched per second (less than 10%
of the servers experience this)
* Avg per db: 2 WALs/s
* Mainly generated by large batch sync processes that occur throughout the
day, and by a legacy archiving process to purge older data (potentially
many millions of cascading deletes).
*Half the servers have (encrypted) pg_dump backups, WAL volume hasn't
proved to be a problem there, though dump size is a problem for a few of
the larger databases (less than 1TB).
* Inter-data-centre replication is all streaming, across DC's (over the
WAN) WAL shipping is over compressed SSH tunnels.
Occasionally the streaming replication falls behind, but more commonly it
is the cross-DC log shipping that becomes a problem. Some of the servers
will generate 50+ GBs of WAL in a matter of minutes and that backs up
immediately on the masters. Occasionally this has a knock-on effect for
other servers and slows down their log shipping due to network saturation.
* checkpoint_segments: 64, checkpoint_timeout: 5 mins, wal_buffers: 16MB

Workload:
70% of servers are generally quiet, with occasional bursty reads and writes.
20% are medium use, avg a few hundred transactions/second
10% average around 5k txns/s, with bursts up to 25k txns/s for several
minutes.
All servers have about 80% reads / 20% writes, though those numbers flip
during big sync jobs and when the purging maintenance kicks off.