block device journal durability

[Phil Carns <carns@mcs.anl.gov>]

I noticed that Ceph issues a warning if it detects that you are using a 
raw block device as the journal and write caching is enabled on that 
device.

When it opens the block device file, however, the FileJournal is using 
O_DIRECT|O_SYNC.  In recent kernels, syncing a block device file 
actually triggers a proper write barrier operation
(http://lxr.linux.no/linux+v2.6.34/fs/block_dev.c#L420).  The barrier 
operation is also supported on MD and LVM now as well if you happen to 
have a journal on a multi-disk volume.

Does this mean that if you have a new enough kernel, and a block device 
that understands barriers, that you can safely leave the write cache 
enabled for the journal device?  It seems that way to me, but I wanted 
to make sure that I am not missing a more subtle issue related to how 
Ceph performs its journaling.

Kudos on the user-friendly warning messages in Ceph in general, by the way.

thanks,
-Phil