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[Qemu-discuss] Slow read IO with a 9P virtio fs

From: Alastair Hughes
Subject: [Qemu-discuss] Slow read IO with a 9P virtio fs
Date: Fri, 24 Jun 2016 18:44:04 +1200

I've encountered slow IO (~25MB/s reading from a 100MB file) with both a MIPS 
and an i386 qemu vm, on an x86_64 host.
The system generally sits at around 30% CPU during this operation (dd 
if=/bigfile of=/dev/null bs=1M) so the operation does not appear to be 
The root filesystem is a 9P share; it is in tmpfs in the host, so reading 
should be extremely quick (I get ~1GB/s on the host).
Writing is fairly fast; I get around 100MB/s for writing the large test file.
The kernel in the vm is not configured to do any fs caching, as I didn't expect 
to need it... I'd rather keep the VM with a small amount of memory.

Is this as expected? Am I missing some "magic sauce" to make this faster?

I am running QEMU using:
qemu-system-mips \
-kernel "/tmp/builder-mips//fs/boot/vmlinuz" \
-append "console=ttyS0 init=/usr/bin/sinit panic=1 rootfstype=9p rw 
rootflags=trans=virtio,version=9p2000.L" \
-fsdev local,id=root,security_model=none,path=/tmp/builder-mips//fs \
-device virtio-9p-pci,fsdev=root,mount_tag=/dev/root \
-m "128" \
-no-reboot -nographic

Alastair Hughes

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