Re: increasing the default WAL segment size

From: Beena Emerson <memissemerson(at)gmail(dot)com>
To: Andres Freund <andres(at)anarazel(dot)de>
Cc: "pgsql-hackers(at)postgresql(dot)org" <pgsql-hackers(at)postgresql(dot)org>
Subject: Re: increasing the default WAL segment size
Date: 2017-09-14 06:01:33
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On Wed, Sep 13, 2017 at 2:58 PM, Andres Freund <andres(at)anarazel(dot)de> wrote:
> Hi,
> On 2017-09-06 20:24:16 +0530, Beena Emerson wrote:
>> > - pg_standby's RetrieveWALSegSize() does too much for it's name. It
>> > seems quite weird that a function named that way has the section below
>> > "/* check if clean up is necessary */"
>> we set 2 cleanup related variables once WalSegSize is set, namely
>> need_cleanup and exclusiveCleanupFileName. Does
>> SetWALSegSizeAndCleanupValues look good?
> It's better, but see below.
>> > - the way you redid the ReadControlFile() invocation doesn't quite seem
>> > right. Consider what happens if XLOGbuffers isn't -1 - then we
>> > wouldn't read the control file, but you unconditionally copy it in
>> > XLOGShmemInit(). I think we instead should introduce something like
>> > XLOGPreShmemInit() that reads the control file unless in bootstrap
>> > mode. Then get rid of the second ReadControlFile() already present.
>> I did not think it was necessary to create a new function, I have
>> simply added the check and
>> function call within the XLOGShmemInit().
> Which is wrong. XLogShmemSize() already needs to know the actual size,
> otherwise we allocate the wrong shmem size. You may sometimes succeed
> nevertheless because we leave some slop unused shared memory space, but
> it's not ok to rely on. See the refactoring I did in 0001.
> Changes:
> - refactored the way the control file was handled, moved it to separate
> phase. I wrote this last and it's late, so I'm not yet fully confident
> in it, but it survives plain and EXEC_BACKEND builds. This also gets
> rid of ferrying wal_segment_size through the EXEC_BACKEND variable
> stuff, which didn't really do much, given how many other parts weren't
> carried over.
> - renamed all the non-postgres binary version of wal_segment_size to
> WalSegSz, diverging seems pointless, and the WalSegsz seems
> inconsistent.
> - changed malloc in pg_waldump's search_directory() to a stack
> allocation. Less because of efficiency, more because there wasn't any
> error handling.
> - removed redundant char * -> XLogPageHeader -> XLogLongPageHeader casting.
> - replace new malloc with pg_malloc in initdb (failure handling!)
> - replaced the floating point logic in pretty_wal_size with a, imo much
> simpler, (sz % 1024) == 0
> - it's inconsistent that the new code for pg_standby was added to the
> top of the file, where all the customizable stuff resides.
> - other small changes
> Issues:
> - I think the pg_standby stuff isn't correct. And it's hard to
> understand. Consider the case where the first file restored is *not* a
> timeline history file, but *also* not a complete file. We'll start to
> spew "not enough data in file" errors and such, which we previously
> didn't. My preferred solution would be to remove pg_standby ([1]),
> but that's probably not quick enough. Unless we can quickly agree on
> that, I think we need to refactor this a bit, I've done so in the
> attached, but it's untested. Could you please verify it works and if
> not fix it up?
> What do you think?

The change looks good and is working as expected.
PFA the updated patch after running pgindent.

Thank you,

Beena Emerson

The Enterprise PostgreSQL Company

Attachment Content-Type Size
0002-Make-wal-segment-size-configurable-at-initdb-time_v4.patch application/octet-stream 123.3 KB

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