min_io_size is currently limited to size uint16_t

[Alex]

I am using LVM VGs on MD-raid1 for hosting my KVM volumes. On the host, a VG looks like this:

Disk /dev/vm/vol202a: 60 GiB, 64424509440 bytes, 125829120 sectors
Units: sectors of 1 * 512 = 512 bytes
Sector size (logical/physical): 512 bytes / 4096 bytes
I/O size (minimum/optimal): 131072 bytes / 131072 bytes

In order to replicate the block device characteristics in the guest, I am using the following extra parameters:

-set device.scsi0-0-0-0.logical_block_size=512
-set device.scsi0-0-0-0.physical_block_size=4096
-set device.scsi0-0-0-0.min_io_size=131072

This doesn't work as qemu complains that min_io_size needs to be of type uint16_t.

[Rubén Rodríguez (ruben-gnu)]

The value is set to uint16_t by mistake. The value is passed to Qemu in bytes, but then it is divided by the sector size and passed to the vm in sectors through a 16 bit register field. The vm kernel then multiplies it again by sector size and shows (through /sys/block/x/queue) the value in bytes. Because of this, the input value from the qemu side should be uint32_t.

A simple patch is attached, correcting the problem. I think some extra logic should be added after dividing by the page size, to prevent overflowing the 16 bit register.

With that patch, I can pass a 4MB min and optimal io sizes successfully, to match the stripe size used by RBD/Ceph. Sadly though, I see no improvement coming out of it, but I'll document that in a separate bug report.

root@cephvm:~# lsblk -t
NAME ALIGNMENT MIN-IO OPT-IO PHY-SEC LOG-SEC ROTA SCHED RQ-SIZE RA WSAME
sda 0 4194304 4194304 512 512 1 noop 128 4096 2G

[Thomas Huth (th-huth)]

The QEMU project is currently considering to move its bug tracking to
another system. For this we need to know which bugs are still valid
and which could be closed already. Thus we are setting older bugs to
"Incomplete" now.

If you still think this bug report here is valid, then please switch
the state back to "New" within the next 60 days, otherwise this report
will be marked as "Expired". Or please mark it as "Fix Released" if
the problem has been solved with a newer version of QEMU already.

Thank you and sorry for the inconvenience.

[Launchpad Janitor (janitor)]

[Expired for QEMU because there has been no activity for 60 days.]