On Sun, Jan 20, 2013 at 07:02:46PM +1100, Steven Haigh wrote: > On 20/01/2013 5:08 AM, Roy Sigurd Karlsbakk wrote: > >Hi all > > > >As far as I can understand from the bcache docs, a volume cached with > >bcache, must be formatted and setup for bcache in the first place. I come > >from a ZFS environment, where adding SLOG or L2ARC is done dynamically, so I > >have a few questions: > > > >- Would it be somewhat possible to add caching to an existing volume and its > >data? > >- What would happen if the cache device dies - does the whole filesystem > >become inaccessible? > > I've actually been wondering a bit about this - its not exactly > clear in the docs as to what I should do to set up bcache. > > In my case, I have a RAID6 over 4 x 2Tb drives. It lives as > /dev/md2. /dev/md[01] are RAID1 on a pair of 80Gb drives for boot > and LVM. > > As the system is a Xen Dom0, all the DomU (guests) run from their > own LV on the RAID6. So - it would make sense to add bcache to > /dev/md2. > > I'm a bit confused from reading the docs if I can attach to the > existing /dev/md2 or I have to create something from scratch. > Obviously, attaching to an existing RAID device is going to be the > preferred method. > > Covering this in the docs or even on the web site would probably be > beneficial for a lot of people - especially as I feel that this is > getting closer to be merged with the upstream kernel.
You've got to start from scratch, unfortunately. The reason is that there needs to be a bcache specific superblock on the backing device so bcache can keep the cache and backing device in sync - and especially so you can't accidentally mount and use the backing device without the cache. That would be bad. I just added an explanation to the faq - thanks for pointing it out. http://bcache.evilpiepirate.org/FAQ -- To unsubscribe from this list: send the line "unsubscribe linux-bcache" in the body of a message to [email protected] More majordomo info at http://vger.kernel.org/majordomo-info.html
