[Users] vzctl chkpnt speed

[Roman Haefeli]

Hi all

Only recently I discovered that online migration seems to work for us
now. CT on NFS or NFS mounted inside a CT is non-issue now.

We are running all our CTs on an NFS filesystem shared between
hostnodes. While checkpointing and restoring works flawlessly with that
setup, I noticed that "vzctl chkpnt CTID" writes slower to the NFS mount
compared to a dd-write, for instance.

'dd if=/dev/zero of=/mnt/nfs/deleteme bs=1M' writes with approx. 70MB/s.
'iotop' shows that 'vzctl chkpnt CTID' writes with only 18MB/s to the
same dir.

However, the speed of writing is similar to the one of dd when I use the
'bs=8k' option for dd. This makes me assume that quite some write
performance could be gained, if checkpointing would write bigger blocks
at time. I haven't read the respective source code to confirm my
assumption that small block sizes are used, as my skills are far too
limited, but if that is really the case, wouldn't it make sense to use
bigger writes in order to improve checkpointing performance?

What do you think?

Roman