Re: layering question.

Jens-U. Mozdzen
2015-08-07T16:24:28Z

Hi James,

Zitat von "A. James Lewis" <james@fsck.co.uk>:
> OK, but in that case bcache is not between your MD RAID and it's
> disks, so if your disks are dropping out of the MD array, that has
> to be either an independent problem, or a very complex bug.

My guess is that it's a rather simple timeout / locking problem, which
leads to an expiring timer in the MD code. And bcache has a well-known
history for locking problems, according to the mailing list.

Regards,
Jens

> James
>
>
> On 07/08/15 16:36, Jens-U. Mozdzen wrote:
>> Hi James,
>>
>> Zitat von "A. James Lewis" <james@fsck.co.uk>:
>>> That's interesting, are you putting your MD on top of multiple
>>> bcache devices... rather than bcache on top of an MD device... I
>>> wonder what the rationale behind this is?
>>
>> Hi James, no such thing here...
>>
>> bcache is running on top of two MD-RAIDs - RAID6 with 7 spinning
>> drives and RAID1 with two SSDs.
>>
>> The stack is, from bottom to top:
>>
>> - MD-RAID6 data, MD-RAID1 cache
>> - bcache (/dev/bcache0, used as an LVM PV)
>> - LVM
>> - many LVs
>> - DRBD on top of most of the LVs
>> - Ext4 on each of the DRBD devices
>> - SCST / NFS / SMB sharing these file systems
>>
>> In the referenced incidents, SCST reports that (many) writes failed
>> due to time-out, and MD reports a single disk faulty. No other
>> traces in syslog, especially no stalled processes, locking problems
>> or kernel bugs.
>>
>> The i/o pattern is highly parallel reads and writes, mostly via SCST.
>>
>> Regards,
>> Jens