I have a Xen Dom0 (3.2) with a bunch of domUs (paravirt). DomU VBDs are on local LVM volumes. Now I want to get backups of the LVM volumes using snapshots. I use rsync ioniced to idle (ionice -c3).
I have verified that the rsync processes are really in idle mode and the
blkback kernel threads are on best efford (-c2 -n4).
As I understand the IO scheduling system, the so configured rsync process should never be able to steal IO bandwidth from the
blkback processes and therefor the impact on the domUs should be zero.
But that's not what I see. In the domUs
vmstat shows up to 20% IO wait during rsync filelist operation and about 10% on rsync transferring data:
r b swpd free buff cache si so bi bo in cs us sy id wa 1 3 456 48468 103004 1550936 0 0 75 484 2112 1741 3 0 81 16
Without rsync running the IO wait is around 1%:
r b swpd free buff cache si so bi bo in cs us sy id wa 1 0 456 43876 105564 1553224 0 0 19 365 2063 1718 4 0 95 1
dom0 shows steady 11% IO wait on rsync filelist operation and 5% on data transfer:
r b swpd free buff cache si so bi bo in cs us sy id wa 0 0 23536 220220 73256 558856 0 0 1471 239 5271 310 0 0 89 11
So, has anyone an idea about what's happening here? Is ionice simply broken or am I expecting the wrong effects?
Thanks for any hint.