Re: [Nbd] Question about the expected behaviour of nbd-server for async ops

[Goswin von Brederlow <goswin-v-b@...186...>]

Alex Bligh <alex@...872...> writes:

> --On 29 May 2011 19:54:35 +0200 Wouter Verhelst <w@...112...> wrote:
>
>>> Which just means that a CoW based filing system or sparse files don't
>>> support FUA. The idea of a FUA is that it is cheaper than a FLUSH. But
>>> if nbd-server does fsync() in both cases then it is pointless to
>>> announce FUA support.
>>
>> No, that's not entirely true. With a FLUSH, you need to ensure that
>> whatever the FLUSH would cover is flushed to disk; with a FUA, you need
>> to ensure the same thing for just one call, which is easier to do.
>
> Indeed. And even if we were limited to fsync() by something other than
> my laziness, then we'd still only need to fsync() one file, rather than
> every file (in a device spanning multiple files).

Ahh, I forgot about multiple files. Ok, so FUA will be cheaper in that
case even if it still does more than it needs too.

MfG
        Goswin