Is anybody actually using XLR_BKP_REMOVABLE?

From: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>
To: pgsql-hackers(at)postgreSQL(dot)org
Subject: Is anybody actually using XLR_BKP_REMOVABLE?
Date: 2011-12-12 15:17:17
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Back in 2007 (commit a8d539f12498de52453c8113892cbf48cc62478d), we
reduced the maximum number of backup blocks per WAL record from 4 to 3,
in order to permit addition of an XLR_BKP_REMOVABLE flag bit that
purports to show whether it's safe to suppress full-page-image backup
blocks in an external WAL-filtering program. I'm having a problem with
this now because I don't see any way to get SP-GiST's leaf page
splitting operation to touch less than four pages. So I'd like to
propose reverting that decision and again allowing a WAL record to touch
as many as four pages.

As the above commit message points out, compression of this sort could
only be done externally if that external program knew the complete
details of every single type of WAL record, so that it could figure out
whether any data needed to be extracted from the full-page images and
reinserted in the WAL record. I'm not convinced that anybody has
written such a thing or will be able to maintain it into the future,
as we feel free to whack around the contents of WAL on a regular basis.
The commit message claims that such a program would be posted and
maintained on pgfoundry, but I couldn't find any trace of it (not that
pgfoundry's search tools are very good, but neither "compress", "xlog"
or "wal" produces any hits on such a thing). In any case I think the
modern theory about this is you should get a filesystem that prevents
torn-page writes, and then you can just turn off full_page_writes.

Furthermore, what the XLR_BKP_REMOVABLE bit actually reports is just
whether a backup operation is in progress, and I think we have now (or
easily could) add reporting records to the WAL stream that tell when a
backup starts or stops. So external compression would still be possible
if it kept a bit more state around.

So: is there actually any such compression program out there?
Would anybody really cry if this flag went away?

regards, tom lane


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