[dm-devel] dm mirror with failed device

[malahal at us.ibm.com]

Stephen Beynon [stephen at octopus-technologies.com] wrote:
> This is probably a stupid question, in which case I apologise in advance.
> 
> I am looking to set up a number of machines with internal disks mirrored onto 
> an iscsi server.  I aim to provide resiliance against disk failures, and the 
> iscsi server will be a handy single point to backup.
> 
> I set up the following test mirror.
> 
> # dmsetup status mirror
> 0 16777216 mirror 2 254:8 8:16 32768/32768 0 1 core
> # dmsetup table mirror
> 0 16777216 mirror core 1 512 2 254:8 0 8:16 0
> 
> As a test I shutdown the iscsi server and continued writes to the mirror 
> device.  As expected syslog recorded i/o errors on the iscsi lun, but writes 
> to the mirror continued.  After waiting a few minutes I expected to see that 
> parts of the mirror were out of sync, however dmsetup status gave exactly the 
> same results as above.
> 
> When I restarted the iscsi server blocks started to be written to the iscsi 
> server again, but the changes that happened during the iscsi server down time 
> were never replicated.  Clearly this left the mirror in an inconsistent state 
> with the only indication of problems being in syslog.
> 
> Is there any way I can configure dm-mirror to either mark the blocks as out of 
> sync when a write fails, and re-try the write later ?
> 
> I am currently running Ubuntu gutsy on the client machines, so it does not 
> have dmeventd, but from reading the docs it it not clear that dmeventd would 
> help.

What kernel version?