Gentoo Archives: gentoo-user

From: Rich Freeman <rich0@g.o>
To: gentoo-user@l.g.o
Subject: Re: [gentoo-user] Hard drive storage questions
Date: Tue, 28 Apr 2015 18:31:37
Message-Id: CAGfcS_=Yrkmc=+oDAY+vJaUUExL94QO7JfoDreKN3C1oJfFL7A@mail.gmail.com
In Reply to: Re: [gentoo-user] Hard drive storage questions by Neil Bothwick
1 On Tue, Apr 28, 2015 at 2:11 PM, Neil Bothwick <neil@××××××××××.uk> wrote:
2 > On Tue, 28 Apr 2015 13:38:55 -0400, Rich Freeman wrote:
3 >
4 >> > The same is also possible with BTRFS, including built in RAID. RAID5
5 >> > in btrfs is expermiental, but its RAID1 is like RAID5 in some ways,
6 >> > such as giving the capacity of n-1 disks and tolerating a single disk
7 >> > failure.
8 >>
9 >> btrfs raid5 is still fairly experimental (though now it supports
10 >> recovery) and works more-or-less how you'd expect raid5 to work.
11 >> Raid1 on btrfs gives you the capacity of n/2 and not n-1 disks,
12 >
13 > You're right, I was clearly confused (an oxymoron?) when I wrote that.
14 >
15 > So RAID1 gives less capacity than RAID5 on BTRFS, but it is stable (in
16 > btrfs terms).
17 >
18
19 Correct.
20
21 For drives of identical size and not using compression, I'd expect
22 space use on btrfs to be equivalent to the same raid level on
23 mdadm+lvm+ext4. With mixed drives you will potentially get more space
24 on btrfs, and compression will of course get you more space.
25
26 As far as data security goes there is a tradeoff. Btrfs is still
27 immature and I seem to have issues with it 1-2 times per year (but
28 I've yet to have unrecoverable data loss). On the other hand, btrfs
29 does do full data checksumming which means you're less likely to lose
30 data due to issues with the physical storage than with mdadm - as with
31 zfs it always checks the checksum and will recover from another disk
32 if possible, and in the event of raid disparity it always knows which
33 (if any) of the copies is right.
34
35 I'm hopeful that at some point I'll be able to recommend it without
36 reservation. Right now, that isn't entirely the case. I'm still
37 patching the 3.18 kernel series so that it actually mounts my root
38 partition when whatever is causing it to panic (probably also btrfs)
39 does so (the patch is in the queue, but hasn't made it to 3.18 yet for
40 some reason - I believe it has been in the other stable series for a
41 release or two now).
42
43 --
44 Rich

Replies

Subject Author
Re: [gentoo-user] Hard drive storage questions Neil Bothwick <neil@××××××××××.uk>