Re: Fix(es) for ext2 fsync bug

[sfaibish <sfaibish@emc.com>]

Val,

Maybe it is not only our (FS people) problem. We probably need to
bring the kernel people judge as ext2 and ext3 are the base Linux FS.
I add the kernel list for opinion.

/Sorin

On Wed, 14 Feb 2007 14:54:54 -0500, Valerie Henson  
<val_henson@linux.intel.com> wrote:

> Just some quick notes on possible ways to fix the ext2 fsync bug that
> eXplode found.  Whether or not anyone will bother to implement it is
> another matter.
>
> Background: The eXplode file system checker found a bug in ext2 fsync
> behavior.  Do the following: truncate file A, create file B which
> reallocates one of A's old indirect blocks, fsync file B.  If you then
> crash before file A's metadata is all written out, fsck will complete
> the truncate for file A... thereby deleting file B's data.  So fsync
> file B doesn't guarantee data is on disk after a crash.  Details:
>
> http://www.stanford.edu/~engler/explode-osdi06.pdf
>
> Two possible solutions I can think of:
>
> * Rearrange order of duplicate block checking and fixing file size in
>   fsck.  Not sure how hard this is. (Ted?)
>
> * Keep a set of "still allocated on disk" block bitmaps that gets
>   flushed whenever a sync happens.  Don't allocate these blocks.
>   Journaling file systems already have to do this.
>
> -VAL
> -
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