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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 09:49:50 +0200
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:38.0) Gecko/20100101 Thunderbird/38.6.0

On 31/03/2016 22:34, Eric Blake wrote:
> give me possibly stale data because I accessed
> the underlying storage rather than paying attention to in-flight writes
> that would change what I read".

Overlapping I/O is always unspecified, so you should expect stale data
if a read overlaps an in-flight write.  Not only there's no guarantee of
atomicity; the disk is a "safe register" rather than a "regular

The way QEMU does "flush first" for FUA on reads provides the guarantee
that "no one else can read something older than what I read", but that
assumes that no one else is doing an overlapping write.


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