Re: bcache vs enhanceio?

[Jason Warr <jason-/cow75dQlsI@public.gmane.org>]

On 02/19/2013 11:17 AM, Joseph Glanville wrote:
> I am not Kent.. but I can answer your questions.
>
> 8<--- snip ---->8
>> A question for Kent, once you have bcache and it's tools built,
>> installed and running, is there anything to stop a user from always
>> tagging devices of whatever type you choose from having the superblock
>> info to accept a cache dynamically?  Example, if I create an MD RAID
>> device and before I pvcreate or anything else with it I prep it for
>> bcache but don't actually attach a cache device, is there any negative
>> effects that can come from that?  Can I then at anytime attach a cache
>> device to it?  I realize that once attached in writeback it becomes
>> non-detachable.  Same question for raw sd devices and LVM volumes.
> In short yes, there are no detrimental effects for having backing
> devices with superblocks that don't have associated cache sets.

That's what I thought.  This could be an argument for integration with
DM or MD.  Up-rev the superblock or metadata version and have the bcache
bits in it by default.

>
> To touch on the second point about writeback - it's not so much that
> it's non-detachable it's that you don't want the backing device to be
> used while the cache is not attached and is dirty (contains unflushed
> data).
>
> You can detach the cache safely from a writeback device by first
> switching the cache to writethrough (or none from memory) and waiting
> for the data to flush to the backing device.
> Once that is done you can either continue to use it in writethrough
> mode or you can detach it completely.
>
:) Typing faster than I am thinking.  I should have said non-detachable
while in writeback mode, or rather while it contains "dirty" blocks.