Re: layering question.

Jens-U. Mozdzen
2015-08-07T15:36:18Z

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