On Wed, Mar 30, 2011 at 01:57:36PM +0200, David Brown wrote: > RAID10 with far layout is a very nice raid level - it gives you read > speed like RAID0, write speed no slower than other RAID1 mirrors, and of > course you have the mirror redundancy. > > But it is not extendible - once you have made your layout, you are stuck > with it. There is no way (at the moment) to migrate over to larger drives. > > As far as I can see, you can grow RAID1 sets to larger disks. But you > can't grow RAID0 sets. As far as I can see, there is some inconsistency > in the mdadm manual pages as to whether or not you can grow the size of > a RAID4 array. If it is possible to grow a RAID4, then it should be > possible to use a degraded RAID4 (with a missing parity disk) as a RAID0. > > > I'm planning a new server in the near future, and I think I'll get a > reasonable balance of price, performance, capacity and redundancy using > a 3-drive RAID10,f2 setup (with a small boot partition on each drive, > all three as a RAID1, so that grub will work properly). On the main md > device I then have an LVM physical volume, with logical partitions for > different virtual machines or other data areas. I've used such an > arrangement before, and been happy with it. > > But as an alternative solution that is expandable, I am considering > using LVM to do the striping. Ignoring the boot partition for > simplicity, I would partition each disk into two equal parts - sda1, > sda2, sdb1, sdb2, sdc1 and sdc2. Then I would form a set of RAID1 > devices - md1 = sda1 + sdb2, md2 = sdb1 + sdc2, md3 = sdc1 + sda2. I > would make an lvm physical volume on each of these md devices, and put > all those physical volumes into a single volume group. Whenever I make > a new logical volume, I specify that it should have three stripes. > > If I then want to replace the disks with larger devices, it is possible > to add a new disk, partition it into two larger partitions, add these > partitions to two of the existing raids, sync, fail then remove the > now-redundant drive. After three rounds, the RAID1 sets can then be > grown to match the new partition sizes. Then the lvm physical volumes > can be grown to match the new raid sizes. > > > Any opinions? Have I missed anything here, perhaps some issues that > will make this arrangement slower or less efficient than a normal > RAID10,f2 with lvm on top? I am not sure RAID10,f2 works well with LVM. I believe I have seen reports to the contrary. It should be possible to extend RAID10 arrays with more disks. I do not think it is so difficult. But I think neil does not have it on his wish list. best regards keld -- To unsubscribe from this list: send the line "unsubscribe linux-raid" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
