From: Bossart, Nathan <bossartn@amazon.com>
To: Andrey Borodin <x4mmm@yandex-team.ru>
To: Kyotaro Horiguchi <horikyota.ntt@gmail.com>
Cc: Anastasia Lubennikova <a.lubennikova@postgrespro.ru>
Cc: hlinnaka@iki.fi <hlinnaka@iki.fi>
Cc: matsumura.ryo@fujitsu.com <matsumura.ryo@fujitsu.com>
Cc: masao.fujii@gmail.com <masao.fujii@gmail.com>
Cc: pgsql-hackers@postgresql.org <pgsql-hackers@postgresql.org>
Subject: Re: archive status ".ready" files may be created too early
Date: Tue, 26 Jan 2021 19:13:57 +0000
Message-ID: <68120830-3A34-4C4F-942F-6739DAA664CF@amazon.com> (raw)
In-Reply-To: <60C21920-AB0D-43AF-BDB9-98E70153B654@yandex-team.ru>
References: <EFF40306-8E8A-4259-B181-C84F3F06636C@amazon.com>
<20201215.193257.1534297881230796788.horikyota.ntt@gmail.com>
<20201216.110120.887433782054853494.horikyota.ntt@gmail.com>
<20201218.144205.14089619073535731.horikyota.ntt@gmail.com>
<60C21920-AB0D-43AF-BDB9-98E70153B654@yandex-team.ru>
On 12/17/20, 9:15 PM, "Kyotaro Horiguchi" <horikyota.ntt@gmail.com> wrote:
> At Thu, 17 Dec 2020 22:20:35 +0000, "Bossart, Nathan" <bossartn@amazon.com> wrote in
>> On 12/15/20, 2:33 AM, "Kyotaro Horiguchi" <horikyota.ntt@gmail.com> wrote:
>> > You're right in that regard. There's a window where partial record is
>> > written when write location passes F0 after insertion location passes
>> > F1. However, remembering all spanning records seems overkilling to me.
>>
>> I'm curious why you feel that recording all cross-segment records is
>> overkill. IMO it seems far simpler to just do that rather than try to
>
> Sorry, my words are not enough. Remembering all spanning records in
> *shared memory* seems to be overkilling. Much more if it is stored in
> shared hash table. Even though it rarely the case, it can fail hard
> way when reaching the limit. If we could do well by remembering just
> two locations, we wouldn't need to worry about such a limitation.
I don't think it will fail if we reach max_size for the hash table.
The comment above ShmemInitHash() has this note:
* max_size is the estimated maximum number of hashtable entries. This is
* not a hard limit, but the access efficiency will degrade if it is
* exceeded substantially (since it's used to compute directory size and
* the hash table buckets will get overfull).
> Another concern about the concrete patch:
>
> NotifySegmentsReadyForArchive() searches the shared hashacquiaing a
> LWLock every time XLogWrite is called while segment archive is being
> held off. I don't think it is acceptable and I think it could be a
> problem when many backends are competing on WAL.
This is a fair point. I did some benchmarking with a few hundred
connections all doing writes, and I was not able to discern any
noticeable performance impact. My guess is that contention on this
new lock is unlikely because callers of XLogWrite() must already hold
WALWriteLock. Otherwise, I believe we only acquire ArchNotifyLock no
more than once per segment to record new record boundaries.
Nathan
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To: pgsql-hackers@postgresql.org
Cc: bossartn@amazon.com, x4mmm@yandex-team.ru, horikyota.ntt@gmail.com, a.lubennikova@postgrespro.ru, hlinnaka@iki.fi, matsumura.ryo@fujitsu.com, masao.fujii@gmail.com
Subject: Re: archive status ".ready" files may be created too early
In-Reply-To: <68120830-3A34-4C4F-942F-6739DAA664CF@amazon.com>
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