Rich Freeman <ri...@gentoo.org> writes: > On Mon, Dec 29, 2014 at 8:55 AM, lee <l...@yagibdah.de> wrote: >> >> Just why can't you? ZFS apparently can do such things --- yet what's >> the difference in performance of ZFS compared to hardware raid? >> Software raid with MD makes for quite a slowdown. >> > > Well, there is certainly no reason that you couldn't serialize a > logical volume as far as design goes. It just isn't implemented (as > far as I'm aware), though you certainly can just dd the contents of a > logical volume.
You can use dd to make a copy. Then what do you do with this copy? I suppose you can't just use dd to write the copy into another volume group and have it show up as desired. You might destroy the volume group instead ... > ZFS performs far better in such situations because you're usually just > snapshotting and not copying data at all (though ZFS DOES support > serialization which of course requires copying data, though it can be > done very efficiently if you're snapshotting since the filesystem can > detect changes without having to read everything). How's the performance of software raid vs. hardware raid vs. ZFS raid (which is also software raid)? > Incidentally, other than lacking maturity btrfs has the same > capabilities. IIRC, there are things that btrfs can't do and ZFS can, like sending a FS over the network. > The reason ZFS (and btrfs) are able to perform better is that they > dictate the filesystem, volume management, and RAID layers. md has to > support arbitrary data being stored on top of it - it is just a big > block device which is just a gigantic array. ZFS actually knows what > is in all those blocks, and it doesn't need to copy data that it knows > hasn't changed, protect blocks when it knows they don't contain data, > and so on. You could probably improve on mdadm by implementing > additional TRIM-like capabilities for it so that filesystems could > inform it better about the state of blocks, which of course would have > to be supported by the filesystem. However, I doubt it will ever work > as well as something like ZFS where all this stuff is baked into every > level of the design. Well, I'm planning to make some tests with ZFS. Particularly, I want to see how it performs when NFS clients write to an exported ZFS file system. How about ZFS as root file system? I'd rather create a pool over all the disks and create file systems within the pool than use something like ext4 to get the system to boot. And how do I convert a system installed on an ext4 FS (on a hardware raid-1) to ZFS? I can plug in another two disks, create a ZFS pool from them, make file systems (like for /tmp, /var, /usr ...) and copy everything over. But how do I make it bootable? -- Again we must be afraid of speaking of daemons for fear that daemons might swallow us. Finally, this fear has become reasonable.
