Re: patch to allow disable of WAL recycling

From: Alvaro Herrera <alvherre(at)2ndquadrant(dot)com>
To: Jerry Jelinek <jerry(dot)jelinek(at)joyent(dot)com>
Cc: Michael Paquier <michael(at)paquier(dot)xyz>, Peter Eisentraut <peter(dot)eisentraut(at)2ndquadrant(dot)com>, Tomas Vondra <tomas(dot)vondra(at)2ndquadrant(dot)com>, Thomas Munro <thomas(dot)munro(at)enterprisedb(dot)com>, Andres Freund <andres(at)anarazel(dot)de>, Robert Haas <robertmhaas(at)gmail(dot)com>, Pg Hackers <pgsql-hackers(at)postgresql(dot)org>
Subject: Re: patch to allow disable of WAL recycling
Date: 2019-02-27 23:10:10
Message-ID: 20190227231010.GA21574@alvherre.pgsql
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On 2019-Feb-05, Jerry Jelinek wrote:

> First, since last fall, we have found another performance problem related
> to initializing WAL files. I've described this issue in more detail below,
> but in order to handle this new problem, I decided to generalize the patch
> so the tunable refers to running on a Copy-On-Write filesystem instead of
> just being specific to WAL recycling. Specifically, I renamed the GUC
> tunable from 'wal_recycle' to 'wal_cow_fs'. Hopefully this will make it
> more obvious what is being tuned and will also be more flexible if there
> are other problems in the future which are related to running on a COW
> filesystem. I'm happy to choose a different name for the tunable if people
> don't like 'wal_cow_fs'.

I think the idea of it being a generic tunable for assorted behavior
changes, rather than specific to WAL recycling, is a good one. I'm
unsure about your proposed name -- maybe "wal_cow_filesystem" is better?

I'm rewording your doc addition a little bit. Here's my proposal:

This parameter should only be set to <literal>on</literal> when the WAL
resides on a <firstterm>Copy-On-Write</firstterm> (<acronym>COW</acronym>)
Enabling this option adjusts behavior to take advantage of the
filesystem characteristics (for example, recycling WAL files and
zero-filling new WAL files are disabled).

This part sounds good enough to me -- further suggestions welcome.

I'm less sure about this phrase:

This setting is only appropriate for filesystems which
allocate new disk blocks on every write.

Is "... which allocate new disk blocks on every write" a technique
distinct from CoW itself? I'm confused as to what it means, or how can
the user tell whether they are on such a filesystem.

Obviously you're thinking that ZFS is such a filesystem and everybody
who has pg_wal on ZFS should enable this option. What about, say, Btrfs
-- should they turn this option on? Browsing the wikipedia, I find that
Windows has this ReFS thing that apparently is also CoW, but NTFS isn't.
I don't think either Btrfs or ReFS are realistic options to put pg_wal
on, so let's just list the common filesystems for which users are
supposed to enable this option ... which I think nowadays is just ZFS.
All in all, I would replace this phrase with something like: "This
setting should be enabled when pg_wal resides on a ZFS filesystem or
similar." That should be weasely enough that it's clear that we expect
users to do the homework when on unusual systems, while actively pointing
out the most common use case.

> Finally, the patch now includes bypassing the zero-fill for new WAL files
> when wal_cow_fs is true.

That makes sense. I think all these benchmarks Tomas Vondra run are not
valid anymore ...

The attached v2 has assorted cosmetic cleanups. If you can validate it,
I would appreciate it.

Álvaro Herrera
PostgreSQL Development, 24x7 Support, Remote DBA, Training & Services

Attachment Content-Type Size
v2-0001-pg_wal-on-COW-fs.patch text/x-diff 8.8 KB

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