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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