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Tsirkin" , Kevin Wolf , Hanna Reitz , Max Makarov Subject: [PATCH] vhost-user-blk-server: fix opt_io_size=1 causing severe Windows I/O degradation Date: Mon, 30 Mar 2026 22:27:01 +0300 Message-ID: <20260330192701.75780-1-maxpain@linux.com> X-Mailer: git-send-email 2.50.1 MIME-Version: 1.0 Content-Transfer-Encoding: quoted-printable Received-SPF: pass (zohomail.com: domain of gnu.org designates 209.51.188.17 as permitted sender) client-ip=209.51.188.17; envelope-from=qemu-devel-bounces+importer=patchew.org@nongnu.org; helo=lists.gnu.org; Received-SPF: pass client-ip=2a00:1450:4864:20::134; envelope-from=maxpain177@gmail.com; helo=mail-lf1-x134.google.com X-Spam_score_int: -17 X-Spam_score: -1.8 X-Spam_bar: - X-Spam_report: (-1.8 / 5.0 requ) BAYES_00=-1.9, DKIM_SIGNED=0.1, DKIM_VALID=-0.1, DKIM_VALID_AU=-0.1, DKIM_VALID_EF=-0.1, FREEMAIL_ENVFROM_END_DIGIT=0.25, FREEMAIL_FROM=0.001, RCVD_IN_DNSWL_NONE=-0.0001, SPF_HELO_NONE=0.001, SPF_PASS=-0.001 autolearn=unavailable autolearn_force=no X-Spam_action: no action X-BeenThere: qemu-devel@nongnu.org X-Mailman-Version: 2.1.29 Precedence: list List-Id: qemu development List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Errors-To: qemu-devel-bounces+importer=patchew.org@nongnu.org Sender: qemu-devel-bounces+importer=patchew.org@nongnu.org X-ZohoMail-DKIM: pass (identity @gmail.com) X-ZM-MESSAGEID: 1774898863724158500 Content-Type: text/plain; charset="utf-8" The QSD vhost-user-blk export sets opt_io_size=3D1 and min_io_size=3D1 in the virtio config. These values are reported to the guest through the VPD Block Limits page as OptimalTransferLength=3D1 block (512 bytes) and OptimalTransferLengthGranularity=3D1 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. Buglink: https://github.com/spdk/spdk/issues/3199 Signed-off-by: Max Makarov --- 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 827502d..e421f72 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 =3D cpu_to_le32(blk_size); config->size_max =3D cpu_to_le32(4096); config->seg_max =3D cpu_to_le32(128 - 2); - config->min_io_size =3D cpu_to_le16(1); - config->opt_io_size =3D cpu_to_le32(1); + config->min_io_size =3D cpu_to_le16(0); + config->opt_io_size =3D cpu_to_le32(0); config->num_queues =3D cpu_to_le16(num_queues); config->max_discard_sectors =3D cpu_to_le32(VIRTIO_BLK_MAX_DISCARD_SECTORS); --=20 2.53.0