Re: [gentoo-user] dmraid, mdraid, lvm, btrfs, what?

[Florian Philipp]

Am 29.11.2011 14:44, schrieb Michael Mol:
> On Tue, Nov 29, 2011 at 2:07 AM, Florian Philipp <lists@×××××××××××.net> wrote:
>> Am 29.11.2011 05:10, schrieb Michael Mol:
>>> I've got four 750GB drives in addition to the installed system drive.
>>>
>>> I'd like to aggregate them and split them into a few volumes. My first
>>> inclination would be to raid them and drop lvm on top.  I know lvm well
>>> enough, but I don't remember md that well.
>>>
>>> Since I don't recall md well, and this isn't urgent, I figure I can look
>>> at the options.
>>>
>>> The obvious ones appear tobe mdraid, dmraid and btrfs. I'm not sure I'm
>>> interested in btrfs until it's got a fsck that will repair errors, but
>>> I'm looking forward to it once it's ready.
>>>
>>> Any options I missed? What are the advantages and disadvantages?
>>>
>>> ZZ
>>>
>>
>> Sounds good so far. Of course, you only need mdraid OR dmraid (md
>> recommended).
>
> dmraid looks rather new on the block. Or, at least, I've been more
> aware of md than dm over the years. What's its purpose, as compared to
> dmraid? Why is mdraid recommended over it?
>

dmraid being new? Not really. Anyway: Under the hood, md and dm use the
exactly same code in the kernel. They just provide different interfaces.
mdraid is a linux-specific software RAID implemented on top of ordinary
single-disk disk controllers. It works like a charm and any Linux system
with any disk controller can work with it (if you ever change your
hardware).

dmraid provides a "fake-RAID": A software RAID with support of (or
rather, under control of) a cheap on-board RAID controller.
Performance-wise, it usually doesn't provide any kind of advantage
because the kernel driver still has to do all the heavy lifting
(therefore it uses the same code base as mdraid). Its most important
disadvantage is that it binds you to the vendor of the chipset who
determines the on-disk layout. Apparently, this gets better in the last
few years because of some pretty major consolidations on the chipset
market. It might be helpful if you consider dual-booting Windows on the
same RAID (both systems ought to use the same disk layout by means of
their respective drivers).


>> What kind of RAID level do you want to use, 10 or 5? You
>> can also split it: Use a smaller RAID 10 for performance-critical
>> partitions like /usr and the more space-efficient RAID 5 for bulk like
>> videos. You can handle this with one LVM volume group consisting of two
>> physical volumes. Then you can decide on a per-logical-volume basis
>> where it should allocate space and also migrate LVs between the two PVs.
>
> Since I've got four disks for the pool, I was thinking raid10 with lvm
> on top, and a single lvm pv above that.
>

Yeah, that would also be my recommendation. But if storage efficiency is
more relevant, RAID-5 with 4 disks brings you 750GB more usable storage.