Block requests priorities?

[Samuel Thibault <admin@xxxxxxxxxxxxxxxxxxx>]

Hello,

Every month, we are getting a RAID check at the same time as VMs backing
up their data. Yes, probably not a good thing to do, but that should
still be working, and what we see is the whole system gets overly
sluggish since we mark all of these idle-prioritized.

The RAID check is happening inside dom0, with the --idle option to make
I/Os deprioritized.

The backups are happening inside domUs, with ionice -c 3 to make I/O
deprioritized as well.

But AIUI, the fact that the backup I/Os should be reprioritized is not
transmitted through the blkif protocol, and thus dom0 considers them
just like other VM I/O requests. Shouldn't we add priorities to the
blkif requests?

Otherwise what I guess is happening is that some large VMs are backing
up a lot of data, and even if *they* prioritize I/O requests inside
the domUs, the flurry of I/O requests are still piling up inside
dom0, at non-idle priority since the idle priority information is not
transmitted. And then I/O requests from other VMs get delayed behind
that flurry of requests. I'm not even sure that bfq happens to have the
proper information in order to correctly establish fair queueing between
domUs?

Samuel