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From: | Eric Blake |
Subject: | Re: [Qemu-block] [PATCH 3/4] nbd/client: Support requests of additional block sizing info |
Date: | Wed, 23 May 2018 10:40:55 -0500 |
User-agent: | Mozilla/5.0 (X11; Linux x86_64; rv:52.0) Gecko/20100101 Thunderbird/52.7.0 |
On 05/22/2018 10:33 AM, Vladimir Sementsov-Ogievskiy wrote:
02.05.2018 00:13, Eric Blake wrote:The NBD spec is clarifying [1] that a server may want to advertise different limits for READ/WRITE (in our case, 32M) than for TRIM/ZERO (in our case, nearly 4G). Implement the client side support for these alternate limits, by always requesting the new information (a compliant server must ignore the request if it is unknown), and by validating anything reported by the server before populating the block layer limits.
I still need to do a v2 of this series based on feedback from the NBD list, but answering your question in the meantime:
bs->bl.request_alignment = min ? min : BDRV_SECTOR_SIZE; - bs->bl.max_pdiscard = max; - bs->bl.max_pwrite_zeroes = max; + if (s->info.max_trim) {+ bs->bl.max_pdiscard = MIN(s->info.max_trim, BDRV_REQUEST_MAX_BYTES);+ } else { + bs->bl.max_pdiscard = max; + } + bs->bl.pdiscard_alignment = s->info.min_trim; + if (s->info.max_zero) { + bs->bl.max_pwrite_zeroes = MIN(s->info.max_zero, + BDRV_REQUEST_MAX_BYTES); + } else { + bs->bl.max_pwrite_zeroes = max; + } + bs->bl.pwrite_zeroes_alignment = s->info.min_zero; bs->bl.max_transfer = max;Should not max_transfer be updated too? Looks like it limits all requests, is it right?
max_transfer affects read and write requests, but not write_zero requests (where max_pwrite_zeroes is used instead) or trim requests (where max_pdiscard is used instead). Which is precisely the semantics we want - the NBD extension is mirroring the fact that qemu already has a way to track independent limits for trim/zero that can be larger than the normal limits for read/write, because trim/zero do not involve a transfer of a large data buffer.
-- Eric Blake, Principal Software Engineer Red Hat, Inc. +1-919-301-3266 Virtualization: qemu.org | libvirt.org
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