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

[Thanos Makatos]

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?
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