From: | Stephen Frost <sfrost(at)snowman(dot)net> |
---|---|
To: | Robert Haas <robertmhaas(at)gmail(dot)com> |
Cc: | Michael Paquier <michael(dot)paquier(at)gmail(dot)com>, Peter Geoghegan <pg(at)heroku(dot)com>, Jim Nasby <Jim(dot)Nasby(at)bluetreble(dot)com>, "Joshua D(dot) Drake" <jd(at)commandprompt(dot)com>, Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>, Petr Jelinek <petr(dot)jelinek(at)2ndquadrant(dot)com>, Magnus Hagander <magnus(at)hagander(dot)net>, PostgreSQL-development <pgsql-hackers(at)postgresql(dot)org> |
Subject: | Re: Checksums by default? |
Date: | 2017-01-26 00:32:41 |
Message-ID: | 20170126003240.GA9812@tamriel.snowman.net |
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* Robert Haas (robertmhaas(at)gmail(dot)com) wrote:
> On Wed, Jan 25, 2017 at 7:19 PM, Michael Paquier
> <michael(dot)paquier(at)gmail(dot)com> wrote:
> > On Thu, Jan 26, 2017 at 9:14 AM, Peter Geoghegan <pg(at)heroku(dot)com> wrote:
> >> On Wed, Jan 25, 2017 at 3:30 PM, Stephen Frost <sfrost(at)snowman(dot)net> wrote:
> >>> As it is, there are backup solutions which *do* check the checksum when
> >>> backing up PG. This is no longer, thankfully, some hypothetical thing,
> >>> but something which really exists and will hopefully keep users from
> >>> losing data.
> >>
> >> Wouldn't that have issues with torn pages?
> >
> > Why? What do you foresee here? I would think such backup solutions are
> > careful enough to ensure correctly the durability of pages so as they
> > are not partially written.
>
> Well, you'd have to keep a read(fd, buf, 8192) performed by the backup
> tool from overlapping with a write(fd, buf, 8192) performed by the
> backend.
As Michael mentioned, that'd depend on if things are atomic from a
user's perspective at certain sizes (perhaps 4k, which wouldn't be too
surprising, but may also be system-dependent), in which case verifying
that the page is in the WAL would be sufficient.
Thanks!
Stephen
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