[dm-devel] dm-cache failure semantics in write-back mode

[Thanos Makatos]

Thanks, Joe. So just to make sure I've understood correctly, if the SSD
cache is configured as a write-back cache but the device cache is
disabled/set to write-though on the HDD and the SSD, then there is no risk
of data loss in the event of a failure. Is my understanding correct?

On Tue, Mar 3, 2015 at 4:19 PM, Joe Thornber <thornber at redhat.com> wrote:

> On Tue, Mar 03, 2015 at 03:53:39PM +0000, Thanos Makatos wrote:
> > On Tue, Feb 17, 2015 at 12:15 PM, Thanos Makatos <
> thanos.makatos at onapp.com>
> > wrote:
> >
> > > Hi,
> > >
> > > I'm trying to understand the failure semantics of dm-cache in
> write-back
> > > mode. In Documentation/device-mapper/cache.txt it is stated:
> > >
> > > "On-disk metadata is committed every time a FLUSH or FUA bio is
> written.
> > > If no such requests are made then commits will occur every second.
> This
> > > means the cache behaves like a physical disk that has a volatile write
> > > cache.  If power is lost you may lose some recent writes.  The metadata
> > > should always be consistent in spite of any crash."
> > >
> > > Which I admit confuses me. Assumie that no FLUSH/FUA requerst is issued
> > > (e.g. the user of the cached device is a Windows VM) and a failure
> occurs
> > > (e.g. there is a power failure but both the HDD and the SSD are fine)
> > > immediatelly after a write I/O request, but before on-disk metadata get
> > > commited (e.g. the failure occurs less than a second after the write
> I/O
> > > request was completed). After the hosts reboots, is this completed
> write
> > > I/O request going to be lost?
> > >
> >
> > I haven't gotten any reply to this so I'll try to rephrase: If the user
> of
> > the cache doesn't issue FLUSH/FUA, is there a chance of (irreversible)
> data
> > loss in the event of a kernel crash or power failure?
>
> If the mappings change, ie. something is promoted to the cache, or
> demoted from it.  Then the metadata update is committed and updated
> before the triggering io is issued.
>
> So power failure will not result in loss of a mapping, it may result
> in loss of data if your physical device has a write cache.  But this
> is also the case with the cache.
>
> - Joe
>
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