[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index]
[Qemu-devel] [PATCH 0/1] RFC: don't obey the block device max transfer l
From: |
Maxim Levitsky |
Subject: |
[Qemu-devel] [PATCH 0/1] RFC: don't obey the block device max transfer len / max segments for block devices |
Date: |
Sun, 30 Jun 2019 18:08:54 +0300 |
It looks like Linux block devices, even in O_DIRECT mode don't have any user
visible
limit on transfer size / number of segments, which underlying block device can
have.
The block layer takes care of enforcing these limits by splitting the bios.
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:
https://bugzilla.redhat.com/show_bug.cgi?id=1647104
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)
The benchmark:
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 >
/dev/zero
echo "convert via nbd:"
qemu-nbd -k /tmp/nbd.sock -v -f qcow2 $FILE -x export --cache=none
--aio=native --fork
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
The results:
=========================================
1G sparse image:
native:
before: 0.027s
after: 0.027s
nbd:
before: 0.287s
after: 0.035s
=========================================
100G sparse image:
native:
before: 0.028s
after: 0.028s
nbd:
before: 23.796s
after: 0.109s
=========================================
1G preallocated image:
native:
before: 0.454s
after: 0.427s
nbd:
before: 0.649s
after: 0.546s
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
request
directly to the kernel scsi driver, bypassing the block layer, and thus there
is no code to split
such requests.
What do you think?
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?
Best regards,
Maxim Levitsky
Maxim Levitsky (1):
raw-posix.c - use max transfer length / max segemnt count only for
SCSI passthrough
block/file-posix.c | 16 +++++++---------
1 file changed, 7 insertions(+), 9 deletions(-)
--
2.17.2
- [Qemu-devel] [PATCH 0/1] RFC: don't obey the block device max transfer len / max segments for block devices,
Maxim Levitsky <=