Re: [CHECKER] crash after fsync causing serious FS corruptions (ext2, 2.6.11)

[Pavel Machek]

On Po 07-03-05 11:45:13, Jens Axboe wrote:
> On Mon, Mar 07 2005, Junfeng Yang wrote:
> > 
> > Hi,
> > 
> > FiSC (our FS checker) issues a warning on ext2, complaining that crash
> > after fsync causes file system to corrupt.  FS corrupts in two different
> > ways: 1. file contains illegal blocks (such as block # -2) 2. one block
> > owned by two different files.
> > 
> > I diagnosed the warning a little bit and it appears that this warning can
> > be triggered by the following steps:
> > 
> > 1. a file is truncated, so several blocks are freed
> > 2. a new file is created, and the blocks freed in step 1 are reused
> > 3. fsync on the new file
> > 4. crash and run fsck to recover.
> > 
> > fsync should guarantee that a specific file is persistent on disk.
> > Presumably, operations on other files should not mess up with the file we
> > just fsync (true ?)  However, I also understand that ext2 by default
> > relies on e2fsck to provide file system consistency.  Do you guys consider
> > the above warning as a bug or not?  Any clarification on this will be very
> > helpful.
> 
> fsync on ext2 only really guarantees that the data has reached
> the disk, what the disk does it outside the realm of the fs.
> If the ide drive has write back caching enabled, the data just
> might only be in cache. If the power is removed right after fsync
> returns, the drive might not get a chance to actually commit the
> write to platter.

I *think* they are using emulation for their checker, so drivers
lying about writes should not be problem.
								Pavel
-- 
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