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

From: Alberto Garcia
Subject: Re: [Qemu-block] [PATCH v8 4/9] block: introduce BDRV_REQ_ALLOCATE flag
Date: Fri, 16 Mar 2018 12:01:02 +0100
User-agent: Notmuch/0.18.2 (http://notmuchmail.org) Emacs/24.4.1 (i586-pc-linux-gnu)

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.

> +    /* 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 
> error.
> +     * 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".

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

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

> +     */
> +    BDRV_REQ_ALLOCATE           = 0x40,


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