Re: Weird Issue with raid 5+0

[Bill Davidsen <davidsen@tmr.com>]

Neil Brown wrote:
> On Sun, 21 Feb 2010 19:16:40 +1100
> Neil Brown <neilb@suse.de> wrote:
>
>   
>> On Sun, 21 Feb 2010 02:26:42 -0500
>> chris <tknchris@gmail.com> wrote:
>>
>>     
>>> That is exactly what I didn't want to hear :( I am running
>>> 2.6.26-2-xen-amd64. Are you sure its a kernel problem and nothing to
>>> do with my chunk/block sizes? If this is a bug what versions are
>>> affected, I'll build a new domU kernel and see if I can get it working
>>> there.
>>>
>>> - chris
>>>       
>> I'm absolutely sure it is a kernel bug.
>>     
>
> And I think I now know what the bug is.
>
> A patch was recently posted to dm-devel which I think addresses exactly this
> problem.
>
> I reproduce it below.
>
> NeilBrown
>
> -------------------
> If the lower device exposes a merge_bvec_fn,
> dm_set_device_limits() restricts max_sectors
> to PAGE_SIZE "just to be safe".
>
> This is not sufficient, however.
>
> If someone uses bio_add_page() to add 8 disjunct 512 byte partial
> pages to a bio, it would succeed, but could still cross a border
> of whatever restrictions are below us (e.g. raid10 stripe boundary).
> An attempted bio_split() would not succeed, because bi_vcnt is 8.
>
> One example that triggered this frequently is the xen io layer.
>   

And I bet this hasn't been fix in RHEL yet... I believe we saw this a 
while ago.

-- 
Bill Davidsen <davidsen@tmr.com>
  "We can't solve today's problems by using the same thinking we
   used in creating them." - Einstein