Is vzdump using O_DIRECT IO?


Well-Known Member
Dec 22, 2008
Budapest, Hungary
I am currently in the process of benchmarking Facebook's flashcache with the latest Proxmox kernel to decide on it's long-term usefulness. While studying the flashcache documentation I stumbled upon a whitelist/blacklist feature:

2) You can make the global cache setting "cache everything" and add
tgids (or pids) of other applications that may issue IOs on this
volume to the blacklist, which will make those un-interesting IOs not

Note that this only works for O_DIRECT IOs. For buffered IOs, pdflush,
kswapd would also do the writes, with flashcache caching those.

For a Proxmox server doing daily backups, it would be essential to use the blacklist feature on vzdump so the cache would not be overwritten every night during the backup run.

So my question is: does vzdump only use direct IO, or does it use buffered?

I guess I'm only interested in reads, as the writes would go to NFS which would not be cached.

Also, do I need to add any other PIDs to the blacklist (like tar), as they seem to be doing disk IO as well during backups?
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