Gentoo Archives: gentoo-user

From: Mark Knecht <markknecht@×××××.com>
To: gentoo-user@l.g.o
Subject: Re: [gentoo-user] dmraid, mdraid, lvm, btrfs, what?
Date: Tue, 29 Nov 2011 14:12:22
Message-Id: CAK2H+efUUmZSap7T+p0opBb036tW-KYAdc8r+qx4V8afYRF=9Q@mail.gmail.com
In Reply to: [gentoo-user] dmraid, mdraid, lvm, btrfs, what? by Michael Mol
1 On Mon, Nov 28, 2011 at 8:10 PM, Michael Mol <mikemol@×××××.com> wrote:
2 > I've got four 750GB drives in addition to the installed system drive.
3 >
4 > I'd like to aggregate them and split them into a few volumes. My first
5 > inclination would be to raid them and drop lvm on top.  I know lvm well
6 > enough, but I don't remember md that well.
7 >
8 > Since I don't recall md well, and this isn't urgent, I figure I can look at
9 > the options.
10 >
11 > The obvious ones appear tobe mdraid, dmraid and btrfs. I'm not sure I'm
12 > interested in btrfs until it's got a fsck that will repair errors, but I'm
13 > looking forward to it once it's ready.
14 >
15 > Any options I missed? What are the advantages and disadvantages?
16 >
17 > ZZ
18
19 Hi Michael,
20 Welcome to the world of what ever sort of multi-disk environment
21 you choose. It's a HUGE topic and a conversation I look forward to
22 having as you dig through it.
23
24 My main compute system here at home has six 500GB WD RE3 drives.
25 Five are in use with one as a cold spare. I'm using md. It's pretty
26 mature and you have good access to the main developer through the
27 email list. I don't know much about dm. If this is your first time
28 putting RAID on a box (it was for me) then I think md is a good
29 choice. On the other hand you're more system software savy than I am
30 so go with what you think is best for you.
31
32 1) First lesson - not all hard drives make good RAID hard drives. I
33 started with six 1TB WD Green drives and found they made _terrible_
34 RAID units so I took them out and bought _real_ RAID drives. They were
35 only half as large for the same price but they have worked perfectly
36 for nearly 2 years.
37
38 2) Second lesson - prepare to build a few RAID configurations and
39 TEST, TEST, TEST __BEFORE__ (BEFORE!!!) you make _ANY_ decision about
40 what sort of RAID you really want. There are a LOT of parameter
41 choices that effect performance, reliability, capacity and I think to
42 some extent your ability to change RAID types later on. To name a few:
43 The obvious RAID type (0,1,2,3,4,5,6,10, etc.) but also chunk size,
44 metadata type, physical layout for certain RAID types, etc. I strongly
45 suggest building 5-10 different configurations and testing them with
46 bonnie++ to gauge speed. I didn't do enough of this before I built
47 this system and I've been dealing with the effects ever since.
48
49 3) Third lesson - think deeply about what happens when 1 drive goes
50 bad and you are in the process of fixing the system. Do you have a
51 spare drive ready? Is it in the box? Hot or cold? What happens if a
52 second drive in the system fails while you're rebuilding the RAID?
53 It's from the same manufacturing lot so it probably suffers from the
54 same weaknesses. My decision for the most part was (for data or system
55 drives) 3-drive RAID1 or 5-drive RAID6. For backup I went with 5-drive
56 RAID5. It all makes me feel good, but it's too complicated.
57
58 4) Lastly - as they say all the time on the mdadm list: RAID is not a backup.
59
60 Personally I like your idea of one big RAID with lvm on top but I
61 haven't done it myself. I think it's what I would look at today if I
62 was starting from scratch, but I'm not sure. It would take some study.
63
64 Hope this helps even a little,
65 Mark

Replies

Subject Author
Re: [gentoo-user] dmraid, mdraid, lvm, btrfs, what? Michael Mol <mikemol@×××××.com>