Re: increasing the default WAL segment size

From: Beena Emerson <memissemerson(at)gmail(dot)com>
To: Michael Paquier <michael(dot)paquier(at)gmail(dot)com>
Cc: Robert Haas <robertmhaas(at)gmail(dot)com>, Jim Nasby <Jim(dot)Nasby(at)bluetreble(dot)com>, "pgsql-hackers(at)postgresql(dot)org" <pgsql-hackers(at)postgresql(dot)org>
Subject: Re: increasing the default WAL segment size
Date: 2017-01-23 06:05:11
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Please find attached an updated WIP patch. I have incorporated almost all
comments. This is to be applied over Robert's patches. I will post
performance results later on.

1. shift (>>) and AND (&) operations: The assign hook of wal_segment_size
sets the WalModMask and WalShiftBit. All the modulo and division operations
using XLogSegSize has been replaced with these. However, there are many
preprocessors which divide with XLogSegSize in xlog_internal.h. I have not
changed these because it would mean I will have to reassign the WalShiftBit
along with XLogSegSize in all the modules which use these macros. That does
not seem to be a good idea. Also, this means shift operator can be used
only in couple of places.

2. pg_standby: it deals with WAL files, so I have used the file size to set
the XLogSegSize (similar to pg_xlogdump). Also, macro
MaxSegmentsPerLogFile using XLOG_SEG_SIZE is now defined
in SetWALFileNameForCleanup where it is used. Since XLOG_SEG_SIZE is not
preset, the code which throws an message if the file size is greater than
XLOG_SEG_SIZE had to be removed.

3. XLOGChooseNumBuffers: This function, called during the creation of
Shared Memory Segment, requires XLogSegSize which is set from the
ControlFile. Hence temporarily read the ControlFile in XLOGShmemSize before
invoking XLOGChooseNumBuffer. The ControlFile is read again and stored on
the Shared Memory later on.

4. IsValidXLogSegSize: This is a macro to verify the XLogSegSize. This is
used in initdb, pg_xlogdump, pg_standby.

5. Macro for power2: There were couple of ideas to make it centralised.
For now, I have just defined it in xlog_internal.

6. Since CRC is used to verify the ControlFile before reading all the
contents from it as is and I do not see the need to re-verify the

7. min/max_wal_size still take values in KB unit and internally store it as
segment count. Though the calculation is now shifted to their respective
assign hook as the GUC_UNIT_XSEGS had to be removed.

8. Declaring XLogSegSize: There are 2 internal variables for the same
parameter. In original code XLOG_SEG_SIZE is defined in the auto-generated
file src/include/pg_config.h. And xlog_internal.h defines:

#define XLogSegSize ((uint32) XLOG_SEG_SIZE)

To avoid renaming all parts of code, I made the following change in

+ extern uint32 XLogSegSize;

+#define XLOG_SEG_SIZE XLogSegSize

would it be better to just use one variable XLogSegSize everywhere. But
few external modules could be using XLOG_SEG_SIZE. Thoughts?

9. Documentation will be added in next version of patch.

Beena Emerson

On Sat, Jan 21, 2017 at 5:30 AM, Michael Paquier <michael(dot)paquier(at)gmail(dot)com>

> On Sat, Jan 21, 2017 at 4:50 AM, Robert Haas <robertmhaas(at)gmail(dot)com>
> wrote:
> > On Fri, Jan 20, 2017 at 2:34 AM, Michael Paquier
> > <michael(dot)paquier(at)gmail(dot)com> wrote:
> >> On Fri, Jan 20, 2017 at 12:49 AM, Robert Haas <robertmhaas(at)gmail(dot)com>
> wrote:
> >>> On Wed, Jan 18, 2017 at 12:42 PM, Robert Haas <robertmhaas(at)gmail(dot)com>
> wrote:
> >>>> Problems 2-4 actually have to do with a DestReceiver of type
> >>>> DestRemote really, really wanting to have an associated Portal and
> >>>> database connection, so one approach is to create a stripped-down
> >>>> DestReceiver that doesn't care about those things and then passing
> >>>> that to GetPGVariable.
> >>>
> >>> I tried that and it worked out pretty well, so I'm inclined to go with
> >>> this approach. Proposed patches attached. 0001 adds the new
> >>> DestReceiver type, and 0002 is a revised patch to implement the SHOW
> >>> command itself.
> >>>
> >>> Thoughts, comments?
> >>
> >> This looks like a sensible approach to me. DestRemoteSimple could be
> >> useful for background workers that are not connected to a database as
> >> well. Isn't there a problem with PGC_REAL parameters?
> >
> > No, because the output of SHOW is always of type text, regardless of
> > the type of the GUC.
> Thinking about that over night, that looks pretty nice. What would be
> nicer though would be to add INT8OID and BYTEAOID in the list, and
> convert as well the other replication commands. At the end, I think
> that we should finish by being able to remove all pq_* routine
> dependencies in walsender.c, saving quite a couple of lines.
> --
> Michael

Thank you,

Beena Emerson

Have a Great Day!

Attachment Content-Type Size
initdb-walsegsize-v2_WIP.patch application/octet-stream 42.7 KB

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