From mboxrd@z Thu Jan 1 00:00:00 1970 From: David Brown Subject: Re: Extendible RAID10 Date: Thu, 31 Mar 2011 21:10:30 +0200 Message-ID: References: <20110331174230.GA10981@www2.open-std.org> Mime-Version: 1.0 Content-Type: text/plain; charset=ISO-8859-1; format=flowed Content-Transfer-Encoding: QUOTED-PRINTABLE Return-path: In-Reply-To: <20110331174230.GA10981@www2.open-std.org> Sender: linux-raid-owner@vger.kernel.org To: linux-raid@vger.kernel.org List-Id: linux-raid.ids On 31/03/11 19:42, Keld J=F8rn Simonsen wrote: > 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, an= d of >> course you have the mirror redundancy. >> >> But it is not extendible - once you have made your layout, you are s= tuck >> 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 y= ou >> can't grow RAID0 sets. As far as I can see, there is some inconsist= ency >> 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 R= AID0. >> >> >> 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 us= ing >> 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 f= or >> 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 =3D sda1 + sdb2, md2 =3D sdb1 + sdc2, md3 =3D sdc1 + s= da2. I >> would make an lvm physical volume on each of these md devices, and p= ut >> all those physical volumes into a single volume group. Whenever I m= ake >> 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 possi= ble >> to add a new disk, partition it into two larger partitions, add thes= e >> 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 volum= es >> 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. > I can't see any reason for that, but I'll try some googling for=20 references. And at some point I will do some testing myself. Thanks=20 for the warning. > It should be possible to extend RAID10 arrays with more disks. I do n= ot > think it is so difficult. But I think neil does not have it on his wi= sh > list. > As far as I understand it, there is currently no way to extend RAID10 -= =20 either by using larger partitions (such as after migrating to larger=20 disks) or by adding new disks. It is possible to add new disks to a=20 RAID 1 array, and to grow the disks in a RAID 1 array, but there is no=20 resizing or reshaping of RAID 0 arrays at the moment. I also thought that resizing and reshaping RAID10 arrays /was/ on Neil'= s=20 wishlist, precisely because these are becoming a popular choice. > best regards > keld -- To unsubscribe from this list: send the line "unsubscribe linux-raid" i= n the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html