Re: Offline enabling/disabling of data checksums

From: Michael Banck <michael(dot)banck(at)credativ(dot)de>
To: Fabien COELHO <coelho(at)cri(dot)ensmp(dot)fr>
Cc: Postgres hackers <pgsql-hackers(at)postgresql(dot)org>, Michael Paquier <michael(at)paquier(dot)xyz>
Subject: Re: Offline enabling/disabling of data checksums
Date: 2019-02-17 18:31:38
Message-ID: 1550428298.12689.12.camel@credativ.de
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Hi,

sorry for letting this slack.

First off, thanks for the review!

Am Mittwoch, den 09.01.2019, 07:07 +0100 schrieb Fabien COELHO:
> > I changed that to the switches -c/--verify (-c for check as -v is taken,
> > should it be --check as well? I personally like verify better), 
> > -d/--disable and -e/--enable.
>
> I agree that checking the checksum sounds repetitive, but I think that for
> consistency --check should be provided.

Ok then. The enum is currently called PG_ACTION_VERIFY, I changed that
to PG_ACTION_CHECK as well.

> About the patch: applies, compiles, global & local "make check" are ok.
>
> There is still no documentation.

I've added that now, though I did that blindly and have not checked the
output yet.

> I think that there is a consensus about renaming the command.

I think so as well, but doing that right now will make the patch
difficult to review, so I'd prefer to leave it to the committer to do
that. 

I can submit a patch with the directory/file rename if that is
preferred.

> The --help string documents --action which does not exists anymore.

Fixed that.

> The code in "updateControlFile" seems to allow to create the file
> (O_CREAT). I do not think that it should, it should only apply to an
> existing file.

Removed that.

> ISTM that some generalized version of this function should be in
> "src/common/controldata_utils.c" instead of duplicating it from command to
> command (as suggested by Michaël as well).

Haven't done that yet.

> In "scan_file" verbose output, ISTM that the checksum is more computed
> than enabled on the file. It is really enabled at the cluster level in the
> end.

It's certainly not just computed but also written. It's true that it
will be only meaningful if the control file is updated accordingly at
the end, but I don't think that message is very incorrect, so left it
as-is for now.

> Maybe there could be only one open call with a ?: for RO vs RW.

Done that.

> Non root check: as files are only manipulated RW, ISTM that there is no
> reason why the ownership would be changed, so I do not think that this
> constraint is useful.

Now that we no longer unlink() pg_control, I believe you are right and I
have removed it.
`
> There is kind of a copy paste for enabling/disabling, I'd consider
> skipping the scan when not necessary and merge both branches.

Done so.

> > > Also, the full page is rewritten... would it make sense to only overwrite
> > > the checksum part itself?
> >
> > So just writing the page header? I find that a bit scary and don't
> > expect much speedup as the OS would write the whole block anyway I
> > guess? I haven't touched that yet.
>
> Possibly the OS would write its block size, which is not necessary the
> same as postgres page size?

I haven't changed that yet, I think Andres was also of the opinion that
this is not necessary?

> > > It seems that the control file is unlinked and then rewritten. If the
> > > rewritting fails, or the command is interrupted, the user has a problem.
> > >
> > > Could the control file be simply opened RW?

I've done that now.

New patch attached.

Michael

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Attachment Content-Type Size
offline-activation-of-checksums_V3.patch text/x-patch 19.6 KB

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