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Re: [Qemu-block] [PATCH v8 4/9] block: introduce BDRV_REQ_ALLOCATE flag

From: Anton Nefedov
Subject: Re: [Qemu-block] [PATCH v8 4/9] block: introduce BDRV_REQ_ALLOCATE flag
Date: Fri, 16 Mar 2018 17:52:13 +0300
User-agent: Mozilla/5.0 (Windows NT 6.3; WOW64; rv:52.0) Gecko/20100101 Thunderbird/52.6.0

On 16/3/2018 2:01 PM, Alberto Garcia wrote:
On Mon 12 Mar 2018 11:16:53 AM CET, Anton Nefedov wrote:
The flag is supposed to indicate that the region of the disk image has
to be sufficiently allocated so it reads as zeroes.

The call with the flag set must return -ENOTSUP if allocation cannot
be done efficiently.
This has to be made sure of by both
   - the drivers that support the flag
   - and the common block layer (so it will not fall back to any slowpath
     (like writing zero buffers) in case the driver does not support
     the flag).

Signed-off-by: Anton Nefedov <address@hidden>
Reviewed-by: Eric Blake <address@hidden>
Reviewed-by: Alberto Garcia <address@hidden>

Hmm... this is not the patch that I reviewed, please remove the R-b
lines when you submit a modified patch.

Oops. sorry

+    /* The BDRV_REQ_ALLOCATE flag is used to indicate that the driver has to
+     * efficiently allocate the space so it reads as zeroes, or return an 
+     * Must be used together with BDRV_REQ_ZERO_WRITE.

I guess it's understandable from the context but it's not immediately
obvious that you can set BDRV_REQ_ZERO_WRITE but not this one.

Perhaps it could be rephrased as something like "If this is set then
BDRV_REQ_ZERO_WRITE must also be set".

Sound much better, fixed

+     * Contradictory to BDRV_REQ_MAY_UNMAP so the two must not be used 

Or simply "This flag cannot be set together with BDRV_REQ_MAY_UNMAP".


+     */
+    BDRV_REQ_ALLOCATE           = 0x40,


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