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

[Thanos Makatos]

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?
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