On 7/3/19 4:52 AM, Stefan Hajnoczi wrote: > On Sun, Jun 30, 2019 at 06:08:54PM +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.
s/The block layer/The kernel block layer/ >> >> By limiting the transfer sizes, we force qemu to do the splitting itself >> which double space >> 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 Long line for a commit message. >> 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: >> I'm sorry I didn't see this before softfreeze, but as a performance improvement, I think it still classes as a bug fix and is safe for inclusion in rc0. >> 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? Seems like a reasonable explanation. >> >> 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(-) > > Adding Eric Blake, who implemented the generic request splitting in the > block layer and may know if there were any other reasons aside from SCSI > passthrough why file-posix.c enforces the host block device's maximum > transfer size. No, I don't have any strong reasons for why file I/O must be capped to a specific limit other than size_t (since the kernel does just fine at splitting things up). > > Reviewed-by: Stefan Hajnoczi <stefa...@redhat.com> > -- Eric Blake, Principal Software Engineer Red Hat, Inc. +1-919-301-3226 Virtualization: qemu.org | libvirt.org
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