Re: dm-cache: dirty state of blocks in writethrough mode

[Heinz Mauelshagen <heinzm@redhat.com>]

On 07/24/2013 03:02 PM, Kumar amit mehta wrote:
> On Wed, Jul 24, 2013 at 07:05:59PM +0200, Heinz Mauelshagen wrote:
>> dm-cache maintains it's own metadata keeping track of any cached
>> blocks properties
>> such as a block being dirty in case of writeback.
>>
>> If any write in writeback mode hits a cache block, the cache metadata will
>> reflect that dirty state before the write's being reported to the
>> application.
>>
>> After a crashed system rebooted, that information is available to flush a
>> dirty block out on eviction.
> Ahh, so It is the metadata device. Maybe overkill, but is it
> possible to keep redundant copies of this metadata, like user space
> utilities such as LVM2 does?

You can set up a mirrored metadata device to be resilient against SSD 
failures.

Plus you can take metadata snapshots at arbitrary points in time;
see the targets kernel documentation in thin-provisioning.txt for this
and the recent thin_dump support metadata snapshots  in the rawhide
device-mapper-persistent-data package.

Either is subject to future LVM2 support and isn't supported as yet.

Heinz

>
> !!amit