[Qemu-devel] [PATCH v2 0/1] Don't obey the kernel block device max transfer len / max segments for raw block devices

[Maxim Levitsky]

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 
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 reported by the kernel)

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.

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?

V2:

*  Manually tested to not break the scsi passthrough with a nested VM
*  As Eric suggested, refactored the area around the fstat.
*  Spelling/grammar fixes

Best regards,
        Maxim Levitsky

Maxim Levitsky (1):
  raw-posix.c - use max transfer length / max segement count only for
    SCSI passthrough

 block/file-posix.c | 54 ++++++++++++++++++++++++----------------------
 1 file changed, 28 insertions(+), 26 deletions(-)

-- 
2.17.2