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. -- To unsubscribe from this list: send the line "unsubscribe linux-bcache" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
