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Re: [Qemu-block] [PATCH v7 3/9] block: introduce BDRV_REQ_ALLOCATE flag

From: Anton Nefedov
Subject: Re: [Qemu-block] [PATCH v7 3/9] block: introduce BDRV_REQ_ALLOCATE flag
Date: Thu, 1 Feb 2018 16:34:39 +0300
User-agent: Mozilla/5.0 (Windows NT 6.3; WOW64; rv:52.0) Gecko/20100101 Thunderbird/52.5.2

On 31/1/2018 8:31 PM, Max Reitz wrote:
On 2018-01-30 13:34, Anton Nefedov wrote:
Offtop: does REQ_ZERO_WRITE override REQ_WRITE_COMPRESSED in this
function? at least with !REQ_MAY_UNMAP it looks wrong

Looks like zero detection will indeed override compression.  I think
that was intended, but I don't even have an opinion either way.

Of course, it wouldn't be so nice if you tried to compress something and
then, because the zero write failed, you actually write uncompressed
zeroes...  But since zero detection is an optional feature, it might be
your own fault if you enable it when you want compression anyway, and if
you write to some format/protocol combination that doesn't allow zero


Is it detection only? I guess in case the caller sets
(REQ_ZERO_WRITE | REQ_COMPRESSED) it also fires here.

Looks like noone does that though, but for example backup might;
it supports compression, and it also does a zero detection itself and
calls write_zeroes() when it can.
It sets REQ_MAY_UNMAP which should indeed override a compression, but
unmap might be not supported.


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