[SOLVED] writeback vs writeback (unsafe)

[adamb]

The wiki's seem to be missing anything related to the "writeback (unsafe)" cache options. Could someone explain the difference between just normal writeback and writeback (unsafe)? I am thinking there is a difference.

Am I also correct in thinking that cache=writeback is now safe so long your guest is using a file system which supports write barriers?

I appreciate the input!

 

[spirit]

>>The wiki's seem to be missing anything related to the "writeback (unsafe)" cache options. Could someone explain the difference between just normal writeback and writeback (unsafe)? I am thinking there is a difference.

unsafe : when the qemu vm is doing the fsync, to flush the datas to disk, the host acknowledge before datas is really writen. If you have a power loss, you'll have data corruption.


>>Am I also correct in thinking that cache=writeback is now safe so long your guest is using a file system which supports write barriers?

yes. barrier enabled , or > kernel 2.6.37 with fua support

http://monolight.cc/2011/06/barriers-caches-filesystems/

 

[TauriRed]

>> unsafe : when the qemu vm is doing the fsync, to flush the datas to disk, the host acknowledge before datas is really writen. If you have a power loss, you'll have data corruption.

So if host itself do have UPS - it's ok to use unsafe instead of writeback, right? (setup: regular lvm, no zfs)