Describe the bug
Block device sequential write performance fluctuates widely

To Reproduce
Steps to reproduce the behaviour:
Repeat fio seq-write-test in guest OS，the results may be different each time, but the performance of a single time is relatively stable.

Observe the disk statistics through iostat on the host. It seems that the request size is a random value and has nothing to do with fio -bs. The higher the value, the better the test result.

The same test is performed on a qemu VM, the request size is consistent with the fio -bs, and the results of multiple tests are also close.

fio -direct=1 -iodepth=32 -rw=write -ioengine=libaio -bs=1024k -numjobs=1 -runtime=300 -group_reporting -filename=/dev/vdb -name=Write_BandWidth_Test --time_based

TEST1

TEST2

Version
v28.0.0

VM configuration

./cloud-hypervisor-scsi
--cpus boot=4
--memory size=4096M
--kernel vmlinux-5.10.25-88
--cmdline "console=ttyS0 root=/dev/vda1 rw iommu=off"
--disk path=focal-server-cloudimg-amd64-2.raw path=/dev/rbd0,direct=on,num_queues=4,queue_size=1024
--console off
--serial tty

Guest OS version details:
NAME="Ubuntu"
VERSION="20.04.5 LTS (Focal Fossa)"
ID=ubuntu
ID_LIKE=debian
PRETTY_NAME="Ubuntu 20.04.5 LTS"
VERSION_ID="20.04"
HOME_URL="https://www.ubuntu.com/"
SUPPORT_URL="https://help.ubuntu.com/"
BUG_REPORT_URL="https://bugs.launchpad.net/ubuntu/"
PRIVACY_POLICY_URL="https://www.ubuntu.com/legal/terms-and-policies/privacy-policy"
VERSION_CODENAME=focal
UBUNTU_CODENAME=focal

Host OS version details:
NAME="CentOS Linux"
VERSION="7 (Core)"
ID="centos"
ID_LIKE="rhel fedora"
VERSION_ID="7"
PRETTY_NAME="CentOS Linux 7 (Core)"
ANSI_COLOR="0;31"
CPE_NAME="cpe:/o:centos:centos:7"
HOME_URL="https://www.centos.org/"
BUG_REPORT_URL="https://bugs.centos.org/"

CENTOS_MANTISBT_PROJECT="CentOS-7"
CENTOS_MANTISBT_PROJECT_VERSION="7"
REDHAT_SUPPORT_PRODUCT="centos"
REDHAT_SUPPORT_PRODUCT_VERSION="7"