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List:       linux-raid
Subject:    Re: Extendible RAID10
From:       David Brown <david.brown () hesbynett ! no>
Date:       2011-03-31 19:10:30
Message-ID: in2jj7$hn6$1 () dough ! gmane ! org
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On 31/03/11 19:42, Keld Jørn 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, 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.
>

I can't see any reason for that, but I'll try some googling for 
references.  And at some point I will do some testing myself.  Thanks 
for the warning.

> 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.
>

As far as I understand it, there is currently no way to extend RAID10 - 
either by using larger partitions (such as after migrating to larger 
disks) or by adding new disks.  It is possible to add new disks to a 
RAID 1 array, and to grow the disks in a RAID 1 array, but there is no 
resizing or reshaping of RAID 0 arrays at the moment.

I also thought that resizing and reshaping RAID10 arrays /was/ on Neil's 
wishlist, precisely because these are becoming a popular choice.


> best regards
> keld


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