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Re: [Qemu-devel] [PATCH v4 7/8] nbd: Implement NBD_INFO_BLOCK_SIZE on se
Re: [Qemu-devel] [PATCH v4 7/8] nbd: Implement NBD_INFO_BLOCK_SIZE on server
Thu, 23 Feb 2017 09:38:56 -0600
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On 02/23/2017 07:13 AM, Paolo Bonzini wrote:
> On 22/02/2017 21:34, Eric Blake wrote:
>>> Oh, so it's the smallest "good" transfer size, or the preferred
>>> alignment. That's not the same as the SCSI definition, which is:
>>> If a device server receives one of these commands with a transfer
>>> size greater than this value, then the device server may incur
>>> delays in processing the command. An OPTIMAL TRANSFER LENGTH field
>>> set to 0000_0000h indicates that the device server does not report
>>> an optimal transfer size.
>> Hmm - that's yet another limit. I don't know if our block layer exposes
>> it, or if it should expose it.
> It exposes it in opt_transfer. :) The only driver that sets
> opt_transfer is the iSCSI driver and uses the SCSI meaning.
>> Does that mean our BlockLimits structure documentation needs a tweak,
>> too? It currently reads:
>> /* Optimal transfer length in bytes. A power of 2 is best but not
>> * mandatory. Must be a multiple of bl.request_alignment, or 0 if
>> * no preferred size */
>> uint32_t opt_transfer;
>> Are we trying to track both optimum size in the SCSI sense _and_ block
>> size in the O_DIRECT sense?
> As of now, just in the SCSI sense.
Okay, sounds like I need a pre-requisite patch to beef up the
BlockLimits documentation, and maybe adding in a preferred size for yet
another parameter that we learn from stat() on files (there really are
performance benefits for performing I/O on a file in aligned requests,
even if it can do byte-wise manipulations). And maybe the NBD
documentation needs a tweak too - which values are the most beneficial
to expose over the wire? Should NBD be exposing more than just
Eric Blake eblake redhat com +1-919-301-3266
Libvirt virtualization library http://libvirt.org
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[Qemu-devel] [PATCH v4 5/8] nbd: Implement NBD_OPT_GO on server, Eric Blake, 2017/02/20
[Qemu-devel] [PATCH v4 8/8] nbd: Implement NBD_INFO_BLOCK_SIZE on client, Eric Blake, 2017/02/20
- Re: [Qemu-devel] [PATCH v4 4/8] nbd: Expose and debug more NBD constants, (continued)