On 23/01/2013 10:15 AM, Kent Overstreet wrote:
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
Great! Thanks Kent. Although this now makes me wonder how to shift the 2Tb+ of data to try this... That being said, I don't have a spare SSD at this stage, so the main thing was looking at getting a patch file to then add to the kernel during compile time of the RPM packages.
The hard part would be redoing the entire LVM structure again for the guest OS's.
-- Steven Haigh Email: netwiz@xxxxxxxxx Web: http://www.crc.id.au Phone: (03) 9001 6090 - 0412 935 897 Fax: (03) 8338 0299
Attachment:
smime.p7s
Description: S/MIME Cryptographic Signature
