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Increase msize for 9p mount to 512KB (~30% read performance increase)

Merged Lee Garrett requested to merge lgarrett/autopkgtest:master into master

I have verified that this changen actually works on my sid qemu VM and that there is a measurable performance increase (124s -> 86s)

Prior to this change, the mount command would emit this warning on autopkgtest runs:

qemu-system-x86_64: warning: 9p: degraded performance: a reasonable high msize should be chosen on client/guest side (chosen msize is <= 8192). See https://wiki.qemu.org/Documentation/9psetup#msize for details.

512KB is currently the kernel limit, though the intention is to remove that. See https://lists.gnu.org/archive/html/qemu-devel/2021-03/msg00989.html for discussion about the values, and https://lists.gnu.org/archive/html/qemu-devel/2021-02/msg07258.html for the discussion about removing the max value.

I have verified that it actually improves performance within the VM with a 10GB file "foo":

root@sid:~# mount -t 9p
autopkgtest on /run/autopkgtest/shared type 9p (rw,relatime,access=any,trans=virtio)
root@sid:~# time cat /run/autopkgtest/shared/foo > /dev/null

real	2m4.218s
user	0m0.142s
sys	0m16.212s
root@sid:~# umount /run/autopkgtest/shared/
root@sid:~# mount -t 9p -o trans=virtio,access=any,msize=512000 autopkgtest /run/autopkgtest/shared
root@sid:~# mount -t 9p
autopkgtest on /run/autopkgtest/shared type 9p (rw,relatime,access=any,msize=512000,trans=virtio)
root@sid:~# time cat /run/autopkgtest/shared/foo > /dev/null

real	1m26.010s
user	0m0.048s
sys	0m11.038s
Edited by Lee Garrett

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