Re: increasing the default WAL segment size

From: Beena Emerson <memissemerson(at)gmail(dot)com>
To: Kuntal Ghosh <kuntalghosh(dot)2007(at)gmail(dot)com>
Cc: "pgsql-hackers(at)postgresql(dot)org" <pgsql-hackers(at)postgresql(dot)org>, Stephen Frost <sfrost(at)snowman(dot)net>, David Steele <david(at)pgmasters(dot)net>, tushar <tushar(dot)ahuja(at)enterprisedb(dot)com>, Prabhat Sahu <prabhat(dot)sahu(at)enterprisedb(dot)com>, Ashutosh Sharma <ashu(dot)coek88(at)gmail(dot)com>, Jim Nasby <Jim(dot)Nasby(at)bluetreble(dot)com>, Andres Freund <andres(at)anarazel(dot)de>, Robert Haas <robertmhaas(at)gmail(dot)com>
Subject: Re: increasing the default WAL segment size
Date: 2017-03-24 17:33:15
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On Wed, Mar 22, 2017 at 9:41 PM, Kuntal Ghosh <kuntalghosh(dot)2007(at)gmail(dot)com>

> On Wed, Mar 22, 2017 at 3:14 PM, Beena Emerson <memissemerson(at)gmail(dot)com>
> wrote:
> > PFA an updated patch which fixes a minor bug I found. It only increases
> the
> > string size in pretty_wal_size function.
> > The 01-add-XLogSegmentOffset-macro.patch has also been rebased.
> Thanks for the updated versions. Here is a partial review of the patch:
> In pg_standby.c and pg_waldump.c,
> + XLogPageHeader hdr = (XLogPageHeader) buf;
> + XLogLongPageHeader NewLongPage = (XLogLongPageHeader) hdr;
> +
> + XLogSegSize = NewLongPage->xlp_seg_size;
> It waits until the file is at least XLOG_BLCKSZ, then gets the
> expected final size from XLogPageHeader. This looks really clean
> compared to the previous approach.

thank you for testing. PFA the rebased patch incorporating your comments.

> + * Verify that the min and max wal_size meet the minimum requirements.
> Better to write min_wal_size and max_wal_size.

Updated wherever applicable.

> + errmsg("Insufficient value for \"min_wal_size\"")));
> "min_wal_size %d is too low" may be? Use lower case for error
> messages. Same for max_wal_size.


> + /* Set the XLogSegSize */
> + XLogSegSize = ControlFile->xlog_seg_size;
> +
> A call to IsValidXLogSegSize() will be good after this, no?

Is it necessary? We already have the CRC check for ControlFiles. Is that
not enough?

> + /* Update variables using XLogSegSize */
> + check_wal_size();
> The method name looks somewhat misleading compared to the comment for
> it, doesn't it?

The method name is correct since it only checks if the value provided is
sufficient (at least 2 segment size). I have updated the comment to say
Check and update variables dependent on XLogSegSize

> This patch introduces a new guc_unit having values in MB for
> max_wal_size and min_wal_size. I'm not sure about the upper limit
> which is set to INT_MAX for 32-bit systems as well. Is it needed to
> define something like MAX_MEGABYTES similar to MAX_KILOBYTES?
> It is worth mentioning that GUC_UNIT_KB can't be used in this case
> since MAX_KILOBYTES is INT_MAX / 1024(<2GB) on 32-bit systems. That's
> not a sufficient value for min_wal_size/max_wal_size.

You are right. I can use the same value as upper limit for GUC_UNIT_MB, we
could probably change the name of MAX_KILOBYTES to something more general
for both GUC_UNIT_MB and GUC_UNIT_KB. The max size in 32-bit systems would
be 2TB.

> While testing with pg_waldump, I got the following error.
> bin/pg_waldump -p master/pg_wal/ -s 0/01000000
> Floating point exception (core dumped)
> Stack:
> #0 0x00000000004039d6 in ReadPageInternal ()
> #1 0x0000000000404c84 in XLogFindNextRecord ()
> #2 0x0000000000401e08 in main ()
> I think that the problem is in following code:
> /* parse files as start/end boundaries, extract path if not specified */
> if (optind < argc)
> {
> ....
> + if (!RetrieveXLogSegSize(full_path))
> ...
> }
> In this case, RetrieveXLogSegSize is conditionally called. So, if the
> condition is false, XLogSegSize will not be initialized.
pg_waldump code has been updated.


Beena Emerson

The Enterprise PostgreSQL Company

Attachment Content-Type Size
02-initdb-walsegsize-v8.patch application/octet-stream 41.8 KB

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