On Sun, 2019-06-30 at 18:08 +0300, Maxim Levitsky wrote:
> 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(-)
>
Ping
Best regards,
Maxim Levitsky
