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vhost-user-blk-server: fix opt_io_size=1 causing severe Windows I/O degradation

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The QSD vhost-user-blk export sets opt_io_size=1 and min_io_size=1 in
the virtio config. These values are reported to the guest through the
VPD Block Limits page as OptimalTransferLength=1 block (512 bytes)
and OptimalTransferLengthGranularity=1 block.

Windows respects these hints and splits all I/O into ~512-byte
requests, causing ~100x sequential throughput degradation (150 MB/s
instead of 15+ GB/s). Linux is unaffected as its block layer ignores
these values.

Set both to 0 which means "not reported" per the SCSI Block Limits
VPD spec, allowing Windows to use its own optimal I/O size defaults.

Signed-off-by: Max Makarov <maxpain@linux.com>
Message-ID: <20260330193451.76037-1-maxpain@linux.com>
Reviewed-by: Kevin Wolf <kwolf@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>
This commit is contained in:
Max Makarov
2026-03-30 22:34:51 +03:00
committed by Kevin Wolf
parent fc1a2ec7da
commit 99bfc1c754
1 changed files with 2 additions and 2 deletions
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4
block/export/vhost-user-blk-server.c
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@@ -242,8 +242,8 @@ vu_blk_initialize_config(BlockDriverState *bs,
config->blk_size = cpu_to_le32(blk_size);
config->size_max = cpu_to_le32(0);
config->seg_max = cpu_to_le32(128 - 2);
config->min_io_size = cpu_to_le16(1);
config->opt_io_size = cpu_to_le32(1);
config->min_io_size = cpu_to_le16(0);
config->opt_io_size = cpu_to_le32(0);
config->num_queues = cpu_to_le16(num_queues);
config->max_discard_sectors =
cpu_to_le32(VIRTIO_BLK_MAX_DISCARD_SECTORS);