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Date: Tue, 29 Nov 2011 13:39:45 -0500
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Subject: Re: [gentoo-user] dmraid, mdraid, lvm, btrfs, what?
From: Michael Mol <mikemol@gmail.com>
To: gentoo-user@lists.gentoo.org
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On Tue, Nov 29, 2011 at 1:20 PM, Florian Philipp <lists@binarywings.net> wr=
ote:
> Am 29.11.2011 14:44, schrieb Michael Mol:
>> On Tue, Nov 29, 2011 at 2:07 AM, Florian Philipp <lists@binarywings.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. =C2=A0I know lv=
m 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 lo=
ok
>>>> 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.
>
>
It looks like I'll want to try two different configurations. RAID5 and
RAID10. Not for different storage requirements, but I want to see
exactly what the performance drop is.
I wish lvm striping supported data redundancy. But, then, I wish btrfs
was ready...
--=20
:wq