On Tue, Feb 17, 2015 at 12:15 PM, Thanos Makatos <thanos.makatos@xxxxxxxxx> 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. Thismeans the cache behaves like a physical disk that has a volatile writecache. If power is lost you may lose some recent writes. The metadatashould 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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