On Thu, Jul 04, 2019 at 03:43:41PM +0300, Maxim Levitsky wrote: > 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 > >
Reviewed-by: Stefan Hajnoczi <stefa...@redhat.com>
signature.asc
Description: PGP signature
