Re: [Qemu-devel] [PATCH] nbd/server: introduce NBD_CMD_CACHE

[Eric Blake]

On 04/19/2018 03:36 AM, Vladimir Sementsov-Ogievskiy wrote:
> 18.04.2018 19:37, Eric Blake wrote:
>> On 04/13/2018 09:31 AM, Vladimir Sementsov-Ogievskiy wrote:
>>> Handle nbd CACHE command. Just do read, without sending read data back.
>>> Cache mechanism should be done by exported node driver chain.
>>>
>>> Signed-off-by: Vladimir Sementsov-Ogievskiy <address@hidden>
>>> ---
>>>   include/block/nbd.h |  3 ++-
>>>   nbd/server.c        | 10 ++++++----
>>>   2 files changed, 8 insertions(+), 5 deletions(-)
>>>
>>> diff --git a/include/block/nbd.h b/include/block/nbd.h
>>> index fcdcd54502..b4793d0a29 100644
>>> --- a/include/block/nbd.h
>>> +++ b/include/block/nbd.h
>>> @@ -135,6 +135,7 @@ typedef struct NBDExtent {
>>>   #define NBD_FLAG_SEND_TRIM         (1 << 5) /* Send TRIM (discard) */
>>>   #define NBD_FLAG_SEND_WRITE_ZEROES (1 << 6) /* Send WRITE_ZEROES */
>>>   #define NBD_FLAG_SEND_DF           (1 << 7) /* Send DF (Do not
>>> Fragment) */
>>> +#define NBD_FLAG_SEND_CACHE        (1 << 8) /* Send CACHE (prefetch) */
>> Hmm, this flag is not documented in the upstream NBD protocol yet; are
>> we sure it matches the xNBD implementation?  We'll want at least a
>> documentation patch proposed for the NBD list before taking this.
>>
> 
> The discussion already on list:
> https://lists.debian.org/nbd/2018/03/msg00040.html

Indeed, and that made me realize that we may want to add NBD_CMD_FLAG_*
values corresponding to other useful posix_fadvise() modes before
finalizing the NBD protocol addition of this feature, in which case we'd
want to make sure the qemu implementation is in line with that.

-- 
Eric Blake, Principal Software Engineer
Red Hat, Inc.           +1-919-301-3266
Virtualization:  qemu.org | libvirt.org

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