Re: bcache vs enhanceio?

[Kent Overstreet <koverstreet-hpIqsD4AKlfQT0dZR+AlfA@public.gmane.org>]

On Tue, Feb 19, 2013 at 11:42:14AM -0600, Jason Warr wrote:
> 
> 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.

Yeah, that idea's been kicked around before. A couple people have said
they were going to work on it, but nothing's happened.

> >
> > 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.

You actually don't have to switch out of writeback mode to safely detach
- if you detach in writeback mode, it detaches as soon as it's finished
flushing all the dirty data.