On Wed, 16 Jun 2010, Phil Carns wrote: > 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. You're correct. The only concern is that the data is safely on disk when the write returns, and it sounds like recent kernels issue the barriers to make that happen. Depending on how recent that behavior is, we can probably either remove the warning entirely, or try to guess based on kernel version. > Kudos on the user-friendly warning messages in Ceph in general, by the way. Thanks! sage -- To unsubscribe from this list: send the line "unsubscribe ceph-devel" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
