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Re: FSFS Issue...

From: John Szakmeister <john_at_szakmeister.net>
Date: 2005-12-04 02:33:53 CET

On Saturday 03 December 2005 15:19, Malcolm Rowe wrote:
> On Sat, Dec 03, 2005 at 06:16:41AM -0500, John Szakmeister wrote:
> > > Yep. I actually noted this on the dev@ list a while back. This
> > > particular file was extremely weird. You can also find a large
> > > repeated block of data in the svndiff too. Here's the gory details:
> > > http://svn.haxx.se/dev/archive-2005-09/0409.shtml
> > >
> > > Note the size of the repeated block. Does your head hurt yet? :-)
>
> I tried to look at that, but my head hurt too much, so I probably didn't
> understand it. I did notice the repeated data block, but I couldn't
> quite tell: is it an exact repeat of the data at the start of the file?

No. The repeated block (starting at offset 1250990) is an exact copy of
offset 1691 to offset 1250989.

> > FWIW, I just tried copying the first 1690 bytes in front of the repeated
> > block. Without touching the actual noderev, the new stream appears to
> > consume the correct amount of data.
>
> Interesting, so it looks like the second delta rep has actually been
> overwritten by the first. Normally I'd be thinking about simultaneous
> writes from two different threads here, but it looks like the transaction
> isolation code is fairly bulletproof (it just creates directories),
> so I can't see two threads getting the same transaction id, much less
> trying to create the same transaction.

Neither can I.

> Ok, here's a scenario I've just thought of:
>
> We create a new transaction, and start writing the delta rep for the
> updated file. During the write, we hit a problem - a disk read or write
> error, perhaps, it doesn't really matter - and we throw an error back
> to the caller.
>
> For whatever reason (handwave, handwave), we don't abort the transaction.
> We start writing the delta rep again, appending to the end of the proto
> rev file. This time, it completes without error (that it's on the
> back-end of the partially-written earlier delta won't actually matter,
> since the partial delta won't ever get referenced by anything).
>
> Now, before the proto revfile is moved into place, we run pool cleanup
> on the pool that contains the allocation for the original file open
> (the one that errored). That closes the file, and as part of the close,
> *flushes the partially-filled buffer to disk*, overwriting the start of
> the 'real' delta rep.
>
> What do you think? In our case, the second delta-rep (where the text
> rep points) starts 19 bytes past a 4k boundary (where 19 bytes is the
> size of the DELTA line, and 4k is the APR buffer size). And yes!
> The get_file_offset() after the write of the DELTA line will call
> apr_file_seek(), which, because we're writing, will flush the buffer to
> disk, so we'd expect the last 4k buffered write from the first delta-rep
> to finish exactly where we see it: at 4k+19 bytes.
>
> That sounds feasible, but why aren't we killing the transaction if we
> hit an error? And when are we running pool cleanup?
>
> I think I might try to see if I can do some fault injection via an
> LD_PRELOAD hack or something: it might help to reproduce the problem.

Very interesting. I think I'm going to have to digest that for a little while
with some code in front of me though. I think I'll first try to recreate the
corruption now that I realize I may have the data to do it.

-John

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Received on Sun Dec 4 02:34:49 2005

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