Re: Strange behavior when attempting to convert a file to extents using chattr

[Eric Sandeen <sandeen@redhat.com>]

On 4/28/11 3:14 PM, Peter Uchno wrote:
> Hello,
> I recently converted an ext3 filesystem to ext4 in the hopes of
> performance increases. However, I ran into a snag:
> 
> In theory, chattr can be used to convert a file to extents using
> "chattr +e foo". However, on my system (Archlinux, kernel 2.6.37.4),
> the behavior of this command is unexpected. Given a file baz created
> when the filesystem was still ext3 (and thus not using extents),
> "chattr +e baz" returns zero but "lsattr baz" reveals the file is not
> using extents. Run "chattr +e baz" a second time, the same thing
> happens. A third time, however, it comes back with:
>     "chattr: Input/output error while setting flags on baz"
> and an error in the kernel log:
>     attempt to access beyond end of device
>     loop1: rw=0, want=204802, limit=204800
> The file is still not using extents and remounting the filesystem
> doesn't seem to change much. Running e2fsck on the filesystem after
> unmounting it comes back with some errors. Letting it fix them and
> then mounting the FS up again shows that the file is now using
> extents.
> I attempted this on an Arch system with both kernels 2.6.37.4 and
> 2.6.38.4 and was able to recreate it by creating a small loopback
> filesystem using dd, making a few files on it, converting the FS to
> ext4 using "tune2fs -O extents,uninit_bg,dir_index /tmp/fs.ext; e2fsck
> -fDC0 /tmp/fs.ext", and then mounting it again as ext4 and attempting
> the conversion to extents with chattr. However, on a debian system
> with kernel 2.6.32.5, it works as intended (chattr +e bar; lsattr bar
> reveals the file converted to extents). I ran strace on the chattr
> command on the Arch system and came up with the following:
> 
> http://pastebin.com/EpwhLF3t
> for the first attempt at running the command;
> http://pastebin.com/qB7DiDgF
> for the second try; and
> http://pastebin.com/jjAdPtBr
> for the final attempt.
> 
> After doing so, running e2fsck on the filesystem revealed the following:
> 
> http://pastebin.com/83q60Wik
> 
> And after mounting the filesystem again, the file is now using extents.
> 
> My kernel config, if it is of interest, is at:
> http://pastebin.com/uk1Hs53P
> 
> I'm using e2fsprogs 1.41.14. The Debian system (where it works
> normally) has e2fsprogs 1.41.12.
> 
> I'm not sure what would cause the filesystem to behave in this
> fashion. Does anyone have any insights?

I can't speak directly to the problem because I haven't investigated, but I can tell you that chattr +e foo is probably almost never tested.

If any other ext4 dev does test it, speak up. :)

And I would strongly suggest that if you want an ext4 filesystem, mkfs.ext4 and migratind data to it is the best route to a consistent, featureful, well-tested setup.

-Eric

> Regards,
> Peter