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[Qemu-block] [PATCH v2 0/1] Don't obey the kernel block device max trans
[Qemu-block] [PATCH v2 0/1] Don't obey the kernel block device max transfer len / max segments for raw block devices
Thu, 4 Jul 2019 15:43:41 +0300
Linux block devices, even in O_DIRECT mode don't have any user visible
limit on transfer size / number of segments, which underlying kernel block
device can have.
The kernel block layer takes care of enforcing these limits by splitting the
By limiting the transfer sizes, we force qemu to do the splitting itself which
introduces various overheads.
It is especially visible in nbd server, where the low max transfer size of the
underlying device forces us to advertise this over NBD, thus increasing the
traffic overhead in case of image conversion which benefits from large blocks.
More information can be found here:
Tested this with qemu-img convert over nbd and natively and to my surprise,
even native IO performance improved a bit.
(The device on which it was tested is Intel Optane DC P4800X,
which has 128k max transfer size reported by the kernel)
Images were created using:
Sparse image: qemu-img create -f qcow2 /dev/nvme0n1p3 1G / 10G / 100G
Allocated image: qemu-img create -f qcow2 /dev/nvme0n1p3 -o
preallocation=metadata 1G / 10G / 100G
The test was:
echo "convert native:"
rm -rf /dev/shm/disk.img
time qemu-img convert -p -f qcow2 -O raw -T none $FILE /dev/shm/disk.img >
echo "convert via nbd:"
qemu-nbd -k /tmp/nbd.sock -v -f qcow2 $FILE -x export --cache=none
rm -rf /dev/shm/disk.img
time qemu-img convert -p -f raw -O raw
nbd:unix:/tmp/nbd.sock:exportname=export /dev/shm/disk.img > /dev/zero
1G sparse image:
100G sparse image:
1G preallocated image:
The block limits of max transfer size/max segment size are retained
for the SCSI passthrough because in this case the kernel passes the userspace
directly to the kernel scsi driver, bypassing the block layer, and thus there
is no code to split
Fam, since you was the original author of the code that added
these limits, could you share your opinion on that?
What was the reason besides SCSI passthrough?
* Manually tested to not break the scsi passthrough with a nested VM
* As Eric suggested, refactored the area around the fstat.
* Spelling/grammar fixes
Maxim Levitsky (1):
raw-posix.c - use max transfer length / max segement count only for
block/file-posix.c | 54 ++++++++++++++++++++++++----------------------
1 file changed, 28 insertions(+), 26 deletions(-)
- [Qemu-block] [PATCH v2 0/1] Don't obey the kernel block device max transfer len / max segments for raw block devices,
Maxim Levitsky <=