Re: Adding caching to existing volumes?

[Steven Haigh <netwiz-tY1ak9Q0PTWHXe+LvDLADg@public.gmane.org>]

[-- Attachment #1: Type: text/plain, Size: 2483 bytes --]

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-tY1ak9Q0PTWHXe+LvDLADg@public.gmane.org
Web: http://www.crc.id.au
Phone: (03) 9001 6090 - 0412 935 897
Fax: (03) 8338 0299


[-- Attachment #2: S/MIME Cryptographic Signature --]
[-- Type: application/pkcs7-signature, Size: 4965 bytes --]