Re: [Qemu-devel] [PATCH] proto: add 'shift' extension.

[Eric Blake]

On 09/26/2016 07:46 AM, Vladimir Sementsov-Ogievskiy wrote:
> This extension allows big requests for TRIM and WRITE_ZEROES through
> special 'shift' parameter, which means that offset and length should be
> shifted left by several bits.
> 
> This is needed for efficient clearing large regions of the disk (up to
> the whole disk).
> 
> Signed-off-by: Vladimir Sementsov-Ogievskiy <address@hidden>
> ---
> 
> Here mentioned WRITE_ZEROES command which is only an experemental
> extension for now.
> 
> To dicuss:

> +- `NBD_OPT_SHIFT` (10)
> +
> +    Defines shift used to calculate request offset and length if
> +    `NBD_CMD_FLAG_SHIFT` is set.
> +
> +    Data:
> +
> +    - 32 bits, shift (unsigned); Must not be larger than 32.

Uggh. You're making the protocol stateful (the server has to remember
what the client has previously requested via NBD_CMD_FLAG_SHIFT, rather
than having ALL information it needs immediately available in the
current NBD_CMD_WRITE_ZEROES).

I'd much rather support a single flag that says to zero the entire disk
than to introduce stateful variable-amount shifting.

-- 
Eric Blake   eblake redhat com    +1-919-301-3266
Libvirt virtualization library http://libvirt.org

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