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




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


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