Re: Online verification of checksums

Hi,

Am Freitag, den 01.03.2019, 18:03 -0500 schrieb Robert Haas:
> On Tue, Sep 18, 2018 at 10:37 AM Michael Banck
> <michael(dot)banck(at)credativ(dot)de> wrote:
> > I have added a retry for this as well now, without a pg_sleep() as well.
> > This catches around 80% of the half-reads, but a few slip through. At
> > that point we bail out with exit(1), and the user can try again, which I
> > think is fine?
>
> Maybe I'm confused here, but catching 80% of torn pages doesn't sound
> robust at all.

The chance that pg_verify_checksums hits a torn page (at least in my
tests, see below) is already pretty low, a couple of times per 1000
runs. Maybe 4 out 5 times, the page is read fine on retry and we march
on. Otherwise, we now just issue a warning and skip the file (or so was
the idea, see below), do you think that is not acceptable?

I re-ran the tests (concurrent createdb/pgbench -i -s 50/dropdb and
pg_verify_checksums in tight loops) with the current patch version, and
I am seeing short reads very, very rarely (maybe every 1000th run) with
a warning like:

|1174
|pg_verify_checksums: warning: could not read block 374 in file "data/base/18032/18045": read 4096 of 8192
|pg_verify_checksums: warning: could not read block 375 in file "data/base/18032/18045": read 4096 of 8192
|Files skipped: 2

The 1174 is the sequence number, the first 1173 runs of
pg_verify_checksums only skipped blocks.

However, the fact it shows two warnings for the same file means there is
something wrong here. It was continueing to the next block while I think
it should just skip to the next file on read failures. So I have changed
that now, new patch attached.

Michael

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Michael Banck
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