Gentoo Archives: gentoo-user

From: Michael Mol <mikemol@×××××.com>
To: gentoo-user@l.g.o
Subject: Re: [gentoo-user] dmraid, mdraid, lvm, btrfs, what?
Date: Tue, 29 Nov 2011 18:41:42
Message-Id: CA+czFiDO6cqs=2LWRbZWg_v2KDaCbu+JaSbB4_gGpY7BFdvJZg@mail.gmail.com
In Reply to: Re: [gentoo-user] dmraid, mdraid, lvm, btrfs, what? by Florian Philipp
1 On Tue, Nov 29, 2011 at 1:20 PM, Florian Philipp <lists@×××××××××××.net> wrote:
2 > Am 29.11.2011 14:44, schrieb Michael Mol:
3 >> On Tue, Nov 29, 2011 at 2:07 AM, Florian Philipp <lists@×××××××××××.net> wrote:
4 >>> Am 29.11.2011 05:10, schrieb Michael Mol:
5 >>>> I've got four 750GB drives in addition to the installed system drive.
6 >>>>
7 >>>> I'd like to aggregate them and split them into a few volumes. My first
8 >>>> inclination would be to raid them and drop lvm on top.  I know lvm well
9 >>>> enough, but I don't remember md that well.
10 >>>>
11 >>>> Since I don't recall md well, and this isn't urgent, I figure I can look
12 >>>> at the options.
13 >>>>
14 >>>> The obvious ones appear tobe mdraid, dmraid and btrfs. I'm not sure I'm
15 >>>> interested in btrfs until it's got a fsck that will repair errors, but
16 >>>> I'm looking forward to it once it's ready.
17 >>>>
18 >>>> Any options I missed? What are the advantages and disadvantages?
19 >>>>
20 >>>> ZZ
21 >>>>
22 >>>
23 >>> Sounds good so far. Of course, you only need mdraid OR dmraid (md
24 >>> recommended).
25 >>
26 >> dmraid looks rather new on the block. Or, at least, I've been more
27 >> aware of md than dm over the years. What's its purpose, as compared to
28 >> dmraid? Why is mdraid recommended over it?
29 >>
30 >
31 > dmraid being new? Not really. Anyway: Under the hood, md and dm use the
32 > exactly same code in the kernel. They just provide different interfaces.
33 > mdraid is a linux-specific software RAID implemented on top of ordinary
34 > single-disk disk controllers. It works like a charm and any Linux system
35 > with any disk controller can work with it (if you ever change your
36 > hardware).
37 >
38 > dmraid provides a "fake-RAID": A software RAID with support of (or
39 > rather, under control of) a cheap on-board RAID controller.
40 > Performance-wise, it usually doesn't provide any kind of advantage
41 > because the kernel driver still has to do all the heavy lifting
42 > (therefore it uses the same code base as mdraid). Its most important
43 > disadvantage is that it binds you to the vendor of the chipset who
44 > determines the on-disk layout. Apparently, this gets better in the last
45 > few years because of some pretty major consolidations on the chipset
46 > market. It might be helpful if you consider dual-booting Windows on the
47 > same RAID (both systems ought to use the same disk layout by means of
48 > their respective drivers).
49 >
50 >
51 >>> What kind of RAID level do you want to use, 10 or 5? You
52 >>> can also split it: Use a smaller RAID 10 for performance-critical
53 >>> partitions like /usr and the more space-efficient RAID 5 for bulk like
54 >>> videos. You can handle this with one LVM volume group consisting of two
55 >>> physical volumes. Then you can decide on a per-logical-volume basis
56 >>> where it should allocate space and also migrate LVs between the two PVs.
57 >>
58 >> Since I've got four disks for the pool, I was thinking raid10 with lvm
59 >> on top, and a single lvm pv above that.
60 >>
61 >
62 > Yeah, that would also be my recommendation. But if storage efficiency is
63 > more relevant, RAID-5 with 4 disks brings you 750GB more usable storage.
64 >
65 >
66
67 It looks like I'll want to try two different configurations. RAID5 and
68 RAID10. Not for different storage requirements, but I want to see
69 exactly what the performance drop is.
70
71 I wish lvm striping supported data redundancy. But, then, I wish btrfs
72 was ready...
73
74 --
75 :wq

Replies

Subject Author
Re: [gentoo-user] dmraid, mdraid, lvm, btrfs, what? Florian Philipp <lists@×××××××××××.net>