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>
99bfc1c754