From: | Jakub Wartak <jakub(dot)wartak(at)enterprisedb(dot)com> |
---|---|
To: | Andres Freund <andres(at)anarazel(dot)de> |
Cc: | Michael Paquier <michael(at)paquier(dot)xyz>, Majid Garoosi <amoomajid99(at)gmail(dot)com>, pgsql-hackers(at)postgresql(dot)org, Heikki Linnakangas <hlinnaka(at)iki(dot)fi>, Fujii Masao <masao(dot)fujii(at)gmail(dot)com> |
Subject: | Re: GUC-ify walsender MAX_SEND_SIZE constant |
Date: | 2024-04-24 06:57:55 |
Message-ID: | CAKZiRmwu=Tnc0Sz4+g6Zsb7UyMJw+8iu_sCkfCe=T1A=NmE1Dw@mail.gmail.com |
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Lists: | pgsql-hackers |
Hi,
> My understanding of Majid's use-case for tuning MAX_SEND_SIZE is that the
> bottleneck is storage, not network. The reason MAX_SEND_SIZE affects that is
> that it determines the max size passed to WALRead(), which in turn determines
> how much we read from the OS at once. If the storage has high latency but
> also high throughput, and readahead is disabled or just not aggressive enough
> after crossing segment boundaries, larger reads reduce the number of times
> you're likely to be blocked waiting for read IO.
>
> Which is also why I think that making MAX_SEND_SIZE configurable is a really
> poor proxy for improving the situation.
>
> We're imo much better off working on read_stream.[ch] support for reading WAL.
Well then that would be a consistent message at least, because earlier
in [1] it was rejected to have prefetch the WAL segment but on the
standby side, where the patch was only helping in configurations
having readahead *disabled* for some reason.
Now Majid stated that he uses "RBD" - Majid, any chance to specify
what that RBD really is ? What's the tech? What fs? Any ioping or fio
results? What's the blockdev --report /dev/XXX output ? (you stated
"high" latency and "high" bandwidth , but it is relative, for me 15ms+
is high latency and >1000MB/s sequential, but it would help others in
future if you could specify it by the exact numbers please). Maybe
it's just a matter of enabling readahead (line in [1]) there and/or
using a higher WAL segment during initdb.
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