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Re: [Qemu-block] [Qemu-devel] [PATCH v2 0/1] Don't obey the kernel block
Re: [Qemu-block] [Qemu-devel] [PATCH v2 0/1] Don't obey the kernel block device max transfer len / max segments for raw block devices
Fri, 12 Jul 2019 04:32:00 -0400 (EDT)
> 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
> introduces various overheads.
> It is especially visible in nbd server, where the low max transfer size of
> underlying device forces us to advertise this over NBD, thus increasing the
> traffic overhead in case of image conversion which benefits from large
> More information can be found here:
> Tested this with qemu-img convert over nbd and natively and to my
> surprise,Reviewed-by: Stefano Garzarella <address@hidden>
> 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 >
> 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:
> before: 0.027s
> after: 0.027s
> before: 0.287s
> after: 0.035s
> 100G sparse image:
> before: 0.028s
> after: 0.028s
> before: 23.796s
> after: 0.109s
> 1G preallocated image:
> before: 0.454s
> after: 0.427s
> 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
> 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?
> * 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(-)
I am not familiar with SCSI passthrough special case. But overall looks good to
Feel free to add:
Reviewed-by: Pankaj Gupta <address@hidden>