On Wed, 2011-06-29 at 10:29 +0100, Ric Wheeler wrote: > On 06/27/2011 07:46 AM, NeilBrown wrote: > > On Thu, 23 Jun 2011 12:53:37 +0200 Nico Schottelius > > <nico-lkml-20110...@schottelius.org> wrote: > > > >> Good morning devs, > >> > >> I'm wondering whether the raid- and volume-management-builtin of btrfs is > >> actually a sane idea or not. > >> Currently we do have md/device-mapper support for raid > >> already, btrfs lacks raid5 support and re-implements stuff that > >> has already been done. > >> > >> I'm aware of the fact that it is very useful to know on which devices > >> we are in a filesystem. But I'm wondering, whether it wouldn't be > >> smarter to generalise the information exposure through the VFS layer > >> instead of replicating functionality: > >> > >> Physical: USB-HD SSD USB-Flash | Exposes information to > >> Raid: Raid1, Raid5, Raid10, etc. | higher levels > >> Crypto: Luks | > >> LVM: Groups/Volumes | > >> FS: xfs/jfs/reiser/ext3 v > >> > >> Thus a filesystem like ext3 could be aware that it is running > >> on a USB HD, enable -o sync be default or have the filesystem > >> to rewrite blocks when running on crypto or optimise for an SSD, ... > > I would certainly agree that exposing information to higher levels is a good > > idea. To some extent we do. But it isn't always as easy as it might sound. > > Choosing exactly what information to expose is the challenge. If you lack > > sufficient foresight you might expose something which turns out to be > > very specific to just one device, so all those upper levels which make use > > of > > the information find they are really special-casing one specific device, > > which isn't a good idea. > > > > > > However it doesn't follow that RAID5 should not be implemented in BTRFS. > > The levels that you have drawn are just one perspective. While that has > > value, it may not be universal. > > I could easily argue that the LVM layer is a mistake and that filesystems > > should provide that functionality directly. > > I could almost argue the same for crypto. > > RAID1 can make a lot of sense to be tightly integrated with the FS. > > RAID5 ... I'm less convinced, but then I have a vested interest there so > > that > > isn't an objective assessment. > > > > Part of "the way Linux works" is that s/he who writes the code gets to make > > the design decisions. The BTRFS developers might create something truly > > awesome, or might end up having to support a RAID feature that they > > subsequently think is a bad idea. But it really is their decision to make. > > > > NeilBrown > > > > One more thing to add here is that I think that we still have a chance to > increase the sharing between btrfs and the MD stack if we can get those > changes > made. No one likes to duplicate code, but we will need a richer interface > between the block and file system layer to help close that gap. > > Ric > Is there a possibility that one could have a 3 disk RAID5 array, and then add a 4th disk and then do a balance, growing the RAID5 onto 4 disks and gaining the space still with RAID5? It seems that to be consistent, BTRFS would have to do this.
If this is the case, then I think that the BTRFS implementation of RAID5 would have to be quite different to the MD implementation. James. > > -- > To unsubscribe from this list: send the line "unsubscribe linux-btrfs" in > the body of a message to majord...@vger.kernel.org > More majordomo info at http://vger.kernel.org/majordomo-info.html -- To unsubscribe from this list: send the line "unsubscribe linux-btrfs" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html
