default filesystem options?

[dswartz]

I noticed that proxmox formats the user data partition (/var/lib/vz) using data=ordered. This is the slowest option possible. I tried modifying the fstab entry to use data=writeback instead (and did the tune2fs too), and rebooted. Doing a bulk write to a file on that partition speeded up by 40%. I'm aware of the tradeoffs of speed vs reliability, but it seems to me that with guests writing to raw and/or qcow2 files on this partition, performance would be improved by going with writeback. Just throwing this out there for debate

 

[dietmar]

I noticed that proxmox formats the user data partition (/var/lib/vz) using data=ordered. This is the slowest option possible.

But this is the only reliable mode. Or are there other reliable options?

 

[dswartz]

Actually, I mis-spoke. data=journal is the slowest. That said, my reading indicates that writeback will never result in filesystem corruption, although if you crash and reboot, recently modified files might be missing data for those changes. The problem is that almost any intelligent guest OS is going to be doing caching of modified data before it 'syncs' it to the host, so you already are at risk of that. I guess I'm not sure what the right thing is, although I have never been happy with the write performance of ext3 and its handling of big files (which a vhd is), but that's a larger dicussion (e.g. which FS to use for data partition...)

 

[dietmar]

The problem is that almost any intelligent guest OS is going to be doing caching of modified data before it 'syncs' it to the host, so you already are at risk of that.

No, no risk, because any reasonable application do fsync (databases).

 

[dswartz]

Not all apps are databases. An application doing, say, a several GB file copy is going to have some data 'in the air', and subject to being lost entirely. That all said, if you really virtualizing things you care about, you have a UPS and such, so I think this is not as much of a concern as years ago.

 

[dietmar]

Not all apps are databases.

But most application care about there data, and so they use fsync to make sure that data gets written to disk (that is why writeback will never be the default mode).