From mboxrd@z Thu Jan 1 00:00:00 1970 From: Heinz Mauelshagen Subject: Re: dm-cache: dirty state of blocks in writethrough mode Date: Thu, 25 Jul 2013 12:45:31 +0200 Message-ID: <51F101CB.20900@redhat.com> References: <51EE8470.6070302@redhat.com> <20130724102452.GA6963@gmail.com> <51F00977.7060802@redhat.com> <20130724130220.GC6963@gmail.com> Reply-To: device-mapper development Mime-Version: 1.0 Content-Type: text/plain; charset="us-ascii"; Format="flowed" Content-Transfer-Encoding: 7bit Return-path: In-Reply-To: <20130724130220.GC6963@gmail.com> List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Sender: dm-devel-bounces@redhat.com Errors-To: dm-devel-bounces@redhat.com To: Kumar amit mehta Cc: dm-devel@redhat.com List-Id: dm-devel.ids 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