Re: pg_combinebackup does not detect missing files

On 5/17/24 14:20, Robert Haas wrote:
> On Fri, May 17, 2024 at 1:18 AM David Steele <david@pgmasters.net> wrote:
>> However, I think allowing the user to optionally validate the input
>> would be a good feature. Running pg_verifybackup as a separate step is
>> going to be a more expensive then verifying/copying at the same time.
>> Even with storage tricks to copy ranges of data, pg_combinebackup is
>> going to aware of files that do not need to be verified for the current
>> operation, e.g. old copies of free space maps.
> 
> In cases where pg_combinebackup reuses a checksums from the input
> manifest rather than recomputing it, this could accomplish something.
> However, for any file that's actually reconstructed, pg_combinebackup
> computes the checksum as it's writing the output file. I don't see how
> it's sensible to then turn around and verify that the checksum that we
> just computed is the same one that we now get. It makes sense to run
> pg_verifybackup on the output of pg_combinebackup at a later time,
> because that can catch bits that have been flipped on disk in the
> meanwhile. But running the equivalent of pg_verifybackup during
> pg_combinebackup would amount to doing the exact same checksum
> calculation twice and checking that it gets the same answer both
> times.
> 
>> One more thing occurs to me -- if data checksums are enabled then a
>> rough and ready output verification would be to test the checksums
>> during combine. Data checksums aren't very good but something should be
>> triggered if a bunch of pages go wrong, especially since the block
>> offset is part of the checksum. This would be helpful for catching
>> combine bugs.
> 
> I don't know, I'm not very enthused about this. I bet pg_combinebackup
> has some bugs, and it's possible that one of them could involve
> putting blocks in the wrong places, but it doesn't seem especially
> likely. Even if it happens, it's more likely to be that
> pg_combinebackup thinks it's putting them in the right places but is
> actually writing them to the wrong offset in the file, in which case a
> block-checksum calculation inside pg_combinebackup is going to think
> everything's fine, but a standalone tool that isn't confused will be
> able to spot the damage.
> 

Perhaps more importantly, can you even verify data checksums before the
recovery is completed? I don't think you can (pg_checksums certainly
does not allow doing that). Because who knows in what shape you copied
the block?


regards

-- 
Tomas Vondra
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