Re: Libvirt, quemu, ceph write cache settings

Josh Durgin <josh.durgin@xxxxxxxxxxx> · Wed, 17 Jul 2013 15:04:13 -0700

On 07/17/2013 05:59 AM, Maciej Gałkiewicz wrote:
Hello

Is there any way to verify that cache is enabled? My machine is running
with following parameters:

qemu-system-x86_64 -machine accel=kvm:tcg -name instance-00000302 -S
-machine pc-i440fx-1.5,accel=kvm,usb=off -cpu
Westmere,+rdtscp,+avx,+osxsave,+xsave,+tsc-deadline,+pcid,+pdcm,+xtpr,+tm2,+est,+vmx,+ds_cpl,+monitor,+dtes64,+pclmuldq,+pbe,+tm,+ht,+ss,+acpi,+ds,+vme
-m 1024 -realtime mlock=off -smp 1,sockets=1,cores=1,threads=1 -uuid
a5a1406b-6899-4f2e-9d86-d52814b7e6ff -smbios
type=1,manufacturer=OpenStack Foundation,product=OpenStack
Nova,version=2013.1.2,serial=1b059fc0-5bcb-11d9-9bb6-c860005ff2fe,uuid=a5a1406b-6899-4f2e-9d86-d52814b7e6ff
-no-user-config -nodefaults -chardev
socket,id=charmonitor,path=/var/lib/libvirt/qemu/instance-00000302.monitor,server,nowait
-mon chardev=charmonitor,id=monitor,mode=control -rtc
base=utc,driftfix=slew -no-kvm-pit-reinjection -no-shutdown -device
piix3-usb-uhci,id=usb,bus=pci.0,addr=0x1.0x2 -drive
file=rbd:cinder_volumes/volume-2dbad334-1dd5-4e26-8c6f-0bd79dd43d98:id=cinder_volumes:key=XXX:auth_supported=cephx\;none,if=none,id=drive-virtio-disk0,format=raw,serial=2dbad334-1dd5-4e26-8c6f-0bd79dd43d98,cache=writeback
-device
virtio-blk-pci,scsi=off,bus=pci.0,addr=0x4,drive=drive-virtio-disk0,id=virtio-disk0,bootindex=1
-drive
file=rbd:cinder_volumes/volume-5c310c6a-942f-4039-9594-074ee633656a:id=cinder_volumes:key=XXX:auth_supported=cephx\;none,if=none,id=drive-virtio-disk1,format=raw,serial=5c310c6a-942f-4039-9594-074ee633656a,cache=writeback
-device
virtio-blk-pci,scsi=off,bus=pci.0,addr=0x5,drive=drive-virtio-disk1,id=virtio-disk1
-netdev tap,fd=25,id=hostnet0,vhost=on,vhostfd=26 -device
virtio-net-pci,netdev=hostnet0,id=net0,mac=fa:16:3e:e8:34:4c,bus=pci.0,addr=0x3
-chardev
file,id=charserial0,path=/var/lib/nova/instances/a5a1406b-6899-4f2e-9d86-d52814b7e6ff/console.log
-device isa-serial,chardev=charserial0,id=serial0 -chardev
pty,id=charserial1 -device isa-serial,chardev=charserial1,id=serial1
-device usb-tablet,id=input0 -vnc 127.0.0.1:1 <http://127.0.0.1:1> -k
en-us -device cirrus-vga,id=video0,bus=pci.0,addr=0x2 -device
virtio-balloon-pci,id=balloon0,bus=pci.0,addr=0x6

You can verify that rbd_cache = true is set with the admin socket
command 'config get rbd_cache'. You'd need to set

   admin socket = /path/to/socket

in ceph.conf, then reattach the rbd device. librados will create the
admin socket as long as the user running qemu isn't prevented by unix
permissions, selinux, apparmor, etc.

ceph.conf has entry:
[client]
   rbd_cache = true

The ceph.conf entry isn't needed since qemu 1.2 when you've got
cache=setting in the qemu command line.

Josh
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