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Re: [Qemu-devel] [Nbd] Is NBD_CMD_FLAG_FUA valid during NBD_CMD_FLUSH?

From: Paolo Bonzini
Subject: Re: [Qemu-devel] [Nbd] Is NBD_CMD_FLAG_FUA valid during NBD_CMD_FLUSH?
Date: Fri, 1 Apr 2016 09:43:46 +0200
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:38.0) Gecko/20100101 Thunderbird/38.6.0

On 31/03/2016 22:17, Alex Bligh wrote:
>> > In qemu, read+FUA just triggers blk_co_flush() prior to reading; but
>> > that's the same function it calls for write+FUA.
> That's harmless, but unnecessary in the sense that current documented
> behaviour doesn't require it. Perhaps it should?
> I suppose TRIM etc. should support FUA too?

TRIM is an advisory operation, so it doesn't make sense to force access
to the medium.  The closest you could get would be to add FUA to
WRITE_ZEROES.  But since WRITE_ZEROES is not a particularly common
operation there isn't much to gain compared to FLUSHing after the write
has completed; in fact SCSI doesn't have a FUA bit on its WRITE SAME


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