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
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