From: | Stephen Frost <sfrost(at)snowman(dot)net> |
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To: | Robert Haas <robertmhaas(at)gmail(dot)com> |
Cc: | Andres Freund <andres(at)anarazel(dot)de>, 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 03:38:08 |
Message-ID: | 20170126033808.GL9812@tamriel.snowman.net |
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* Robert Haas (robertmhaas(at)gmail(dot)com) wrote:
> On Wed, Jan 25, 2017 at 7:37 PM, Andres Freund <andres(at)anarazel(dot)de> wrote:
> > On 2017-01-25 19:30:08 -0500, Stephen Frost wrote:
> >> * 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?
> >>
> >> No, why would it? The page has either been written out by PG to the OS,
> >> in which case the backup s/w will see the new page, or it hasn't been.
> >
> > Uh. Writes aren't atomic on that granularity. That means you very well
> > *can* see a torn page (in linux you can e.g. on 4KB os page boundaries
> > of a 8KB postgres page). Just read a page while it's being written out.
>
> Yeah. This is also why backups force full page writes on even if
> they're turned off in general.
I've got a question into David about this, I know we chatted about the
risk at one point, I just don't recall what we ended up doing (I can
imagine a few different possible things- re-read the page, which isn't a
guarantee but reduces the chances a fair bit, or check the LSN, or
perhaps the plan was to just check if it's in the WAL, as I mentioned)
or if we ended up concluding it wasn't a risk for some, perhaps
incorrect, reason and need to revisit it.
Thanks!
Stephen
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