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Re: [PATCH v4 2/4] block/nbd: define new max_write_zero_fast limit

From: Eric Blake
Subject: Re: [PATCH v4 2/4] block/nbd: define new max_write_zero_fast limit
Date: Thu, 23 Jul 2020 15:32:45 -0500
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:68.0) Gecko/20100101 Thunderbird/68.10.0

On 6/11/20 11:26 AM, Vladimir Sementsov-Ogievskiy wrote:
The NBD spec was recently updated to clarify that max_block doesn't
mirrors Qemu flag BDRV_REQ_NO_FALLBACK).

bs->bl.max_write_zero_fast is zero by default which means using
max_pwrite_zeroes. Update nbd driver to allow larger requests with

Signed-off-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
  block/nbd.c | 1 +
  1 file changed, 1 insertion(+)

diff --git a/block/nbd.c b/block/nbd.c
index 4ac23c8f62..b0584cf68d 100644
--- a/block/nbd.c
+++ b/block/nbd.c
@@ -1956,6 +1956,7 @@ static void nbd_refresh_limits(BlockDriverState *bs, 
Error **errp)
bs->bl.request_alignment = min;
      bs->bl.max_pdiscard = QEMU_ALIGN_DOWN(INT_MAX, min);
+    bs->bl.max_pwrite_zeroes_fast = bs->bl.max_pdiscard;
      bs->bl.max_pwrite_zeroes = max;

Do we even need max_pwrite_zeroes_fast? Doesn't qemu behave correctly if we just blindly assign max_pdiscard and max_pwrite_zeroes to the same value near 2G?

      bs->bl.max_transfer = max;

Eric Blake, Principal Software Engineer
Red Hat, Inc.           +1-919-301-3226
Virtualization:  qemu.org | libvirt.org

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