Re: [PATCH] Always report a writeback error once

Andres Freund
2018-04-23T21:50:37Z

On 2018-04-23 14:43:48 -0700, Matthew Wilcox wrote:
> On Mon, Apr 23, 2018 at 01:57:30PM -0700, Andres Freund wrote:
> > I've never really looked at this code in any depth before, but won't
> > this potentially lead to the same error being reported on multiple FDs?
> > Imagine two fds (potentially in different processes) getting the 0
> > returned by errseq_sample() because it's not ERRSEQ_SEEN. Afaict
> > file_check_and_advance_wb_err() will return an error that's always
> > unlike 0 in that case, and thus the error will returned on both fds?
> >
> > I'm personally perfectly fine with that, but it's not necessarily what's
> > described as desired in your email?.
>
> That's what I was trying to say with this paragraph:
>
> > > This patch restores that behaviour by reporting errors to file descriptors
> > > which are opened after the error occurred, but before it was reported
> > > to any file descriptor.
>
> Maybe it was a little unclear? I'd appreciate a suggestion on making
> it clearer.

I think I was thinking of the following paragraph from your commit
message:

> Before errseq_t, a writeback error would be reported exactly once (as
> long as the inode remained in memory), so Postgres could open a file,
> call fsync() and find out whether there had been a writeback error on
> that file from another process.

Note the "exactly once", making "that behaviour" in the following
paragraph potentially refer to exactly once:

> This patch restores that behaviour by reporting errors to file descriptors
> which are opened after the error occurred, but before it was reported
> to any file descriptor.

Just adding a sentence here saying something like "This means that the
error might be reported to more fds than before." or such would address
that potential ambiguity?

> I think this behaviour is perfectly justifiable.

I agree.

Greetings,

Andres Freund