Re: raid10 make_request failure during iozone benchmark upon btrfs

[Kerin Millar <kerframil@gmail.com>]

On 03/07/2012 03:47, NeilBrown wrote:

[snip]

> Thanks.  Looks like it is a btrfs bug - so a big "hello" to linux-btrfs :-)
>
> The symptom is that iozone on btrfs on md/raid10 can result in
>
> [  919.893454] md/raid10:md0: make_request bug: can't convert block across chunks or bigger than 256k 6653500160 256
> [  919.893465] btrfs: bdev /dev/mapper/vg0-test errs: wr 1, rd 0, flush 0, corrupt 0, gen 0
>
>
> i.e. RAID10 has a 256K chunk size, but is getting 256K requests which overlap
> two chunks - the last half of one chunk and the first half of the next.
> That isn't allowed and raid10_mergeable_bvec, called by bio_add_page, should
> prevent it.
>
> However btrfs_map_bio() sets ->bi_sector to a new value without verifying
> that the resulting bio is still acceptable - which it isn't.
>
> The core problem is that you cannot build a bio for one location, then use it
> freely at another location.
> md/raid1 handles this by checking each addition to a bio against all the
> possible location that it might read/write it.  Maybe btrfs could do the
> same.
> Alternately we could work with Kent Overstreet (of bcache fame) to remove the
> restriction that the fs must make the bio compatible with the device -
> instead requiring the device to split bios when needed, and making it easy to
> do that (currently it is not easy).
> And there are probably other alternative.
>

Thanks very much for identifying the bug. I'm glad to find that the raid 
subsystem is not at fault. I'll give btrfs a spin at some point in the 
future and see whether anything has changed by then.

Cheers,

--Kerin