[v2] vhost-user-blk-server: fix opt_io_size=1 causing severe Windows I/O degradation

[PATCH v2] vhost-user-blk-server: fix opt_io_size=1 causing severe Windows I/O degradation

Max Makarov posted 1 patch 2 days, 4 hours ago

Diff against v1
Download mbox

Patches applied successfully (tree, apply log)
git fetch https://github.com/patchew-project/qemu tags/patchew/20260330193451.76037-1-maxpain@linux.com

Maintainers: Coiby Xu <Coiby.Xu@gmail.com>, "Michael S. Tsirkin" <mst@redhat.com>, Stefano Garzarella <sgarzare@redhat.com>, Kevin Wolf <kwolf@redhat.com>, Hanna Reitz <hreitz@redhat.com>

block/export/vhost-user-blk-server.c | 4 ++--
1 file changed, 2 insertions(+), 2 deletions(-)

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[PATCH v2] vhost-user-blk-server: fix opt_io_size=1 causing severe Windows I/O degradation

Posted by Max Makarov 2 days, 4 hours ago

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>
---
v2: Rebased on clean master (v1 depended on a separate size_max patch).
    Removed unrelated Buglink.
 block/export/vhost-user-blk-server.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/block/export/vhost-user-blk-server.c b/block/export/vhost-user-blk-server.c
index e89422b..67912a3 100644
--- a/block/export/vhost-user-blk-server.c
+++ b/block/export/vhost-user-blk-server.c
@@ -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);
-- 
2.53.0