Re: [PATCH/RFC] Allow writing loose objects that are corrupted in a pack file

On Tue, 2009-01-06 at 20:25 -0500, Nicolas Pitre wrote:

On Tue, 6 Jan 2009, R. Tyler Ballance wrote:

quoted

On Tue, 2008-12-09 at 09:36 +0100, Jan Krüger wrote:

quoted

For fixing a corrupted repository by using backup copies of individual
files, allow write_sha1_file() to write loose files even if the object
already exists in a pack file, but only if the existing entry is marked
as corrupted.

I figured I'd reply to this again, since the issue cropped up again.

We started experiencing *large* numbers of corruptions like the ones
that started the thread (one developer was receiving them once or twice
a day) with v1.6.0.4

We went ahead and upgraded to a custom build of v1.6.1 with Jan's patch
(below) and the issues /seem/ to have resolved themselves. I'm not
certain whether Jan's patch was really responsible, or if there was
another issue that caused this to correct itself in v1.6.1. 

I'll back the patch out and redeploy, it's worth mentioning that a
coworker of mine just got the issue as well (on 1.6.1). He was able to
`git pull` and the error went away, but I doubt that it "magically fixed
itself"

Please back it out.  As it stands, that patch is a no op because of the 
way git is used, and even if the patch was to work as intended, its 
purpose is not to magically fix corruptions without special action from 
your part.  If you have corruption problems coming back only because of 
the removal of this patch then something is really really fishy and I 
would really like to know about it.

There were indeed many changes between v1.6.0.4 and v1.6.1: the exact 
number is 1029.  A couple of them are especially addressing increased 
robustness against some kind of pack corruptions.  But in any case you 
still should see error messages appearing about them.

And don't underestimate the power of disk corruptions.  I started to 
work on git corruption resilience simply because I ended up with a 
corrupted pack at some point.  Then a while later I got another 
corrupted pack.  Then another while later I lost my filesystem entirely 
and had to reinstall my system (after buying a new disk).  Turns out 
that my old disk is silently corrupting data without signaling any 
errors to the host.

I highly doubt this, I've got the issue appearing on at least 7
different development boxes (not workstations, 2U quad-core ECC RAM, etc
machines), while that doesn't mean that they all don't have issues, the
probability of them *all* having disk issues, and it somehow only
manifesting itself with Git usage, is low ;)

I've tarred one of the repositories that had it in a reproducible state
so I can create a build and extract the tar and run against that to
verify any patches anybody might have, but unfortunately at 7GB of
company code and assets, I can't exactly share ;)


Cheers


-- 
-R. Tyler Ballance
Slide, Inc.