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

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Тема Re: [PERFORM] [GENERAL] Questionaire: Common WAL write rates on busy servers.
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Msg-id CAGrpgQ-kdUawMakxoe-yd=3YKTGBgNhkd7DLYTUG4oMZevWD7g@mail.gmail.com
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Ответ на [PERFORM] Questionaire: Common WAL write rates on busy servers.  (Andres Freund <andres@anarazel.de>)
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On Mon, Apr 24, 2017 at 9:17 PM, Andres Freund <andres@anarazel.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.

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