Gentoo Archives: gentoo-user

From: Marc Joliet <marcec@×××.de>
To: gentoo-user@l.g.o
Subject: [gentoo-user] planned btrfs conversion: questions
Date: Tue, 06 May 2014 10:18:51
Message-Id: 20140506121832.678ae781@marcec
1 Hi all,
3 I've become increasingly motivated to convert to btrfs. From what I've seen,
4 it has become increasingly stable; enough so that it is apparently supposed to
5 become the default FS on OpenSuse in 13.2.
7 I am motivated by various reasons:
9 - The experience of insignificant data loss after a reboot after no
10 (visible) problems during normal operations (no similar occurrence since). I
11 suspect this would have been discovered sooner by btrfs' data checksumming.
13 While the data lost was of a small amount, not to mention insignificant
14 (internet pictures) and could be retrieved again easily, I worry that this
15 sort of silent corruption might happen to data that I *do* care about. My
16 backups would become less than useless if I were to discover such corruption
17 too late.
19 - The outright sexy multiple device support :) .
21 This migration will occur in conjunction with a migration of / + /usr to a
22 cheap SSD that I just bought (Crucial M500 120 GB). The overall plan is thus as
23 follows:
25 Replace
27 /boot on /dev/md1 (EXT3, RAID 1)
28 / (with assorted sub-directories, sans /usr) on /dev/md2 (EXT4, RAID 10)
29 the rest on LVM on /dev/md3 (all LVs EXT4, RAID 10)
31 with
33 / + /boot + /usr + swapfile on the SSD (EXT4)
34 the rest (/home, my media partitions) on a btrfs RAID 10
36 (which replaces an older plan to recreate the RAID 10 + LVM to use the whole
37 disks with the current 1.2 metadata format)
39 The goals are, in addition to alleviating my data safety concerns above, to
40 guarantee that I don't need an initramfs at boot (hence the SSD), and to
41 greatly simplify my partitioning scheme (I just have too many separate logical
42 volumes ;-) ). Any added performance is "just" a nice bonus.
44 The reason why I would choose EXT4 for the SSD is that btrfs still lacks support
45 for swap files and I worry about creating a swap partition on the SSD. Is that
46 warranted, or will the wear-levelling of the SSD handle that just fine? Do swap
47 partitions support SSDs specially? Also, does anyone know whether EXT4 goes
48 beyond "merely" supporting TRIM? That is, the btrfs wiki advertises the
49 following:
51 "SSD (Flash storage) awareness (TRIM/Discard for reporting free blocks for
52 reuse) and optimizations (e.g. avoiding unnecessary seek optimizations,
53 sending writes in clusters, even if they are from unrelated files. This
54 results in larger write operations and faster write throughput)"
56 Does EXT4 also implement such optimisations for SSDs?
58 So I guess I want to know: does anybody have any further suggestions to make?
59 Is btrfs a good choice for / after all? And should I be using the most recent
60 kernel versions? (I would go with no, despite the advice from upstream, because
61 the changes in the last two versions don't seem to be particularly user
62 visible, at least to me, from reading
64 I also have a more specific question regarding RAID 10: the btrfs wiki says
65 that you can add devices with different sizes to a multiple device setup, but I
66 don't think it says to which RAID levels this applies and how. From [0] I would
67 say it works with RAID 10 (since that's what the example uses), but thought
68 maybe somebody here knows more details and/or gotchas. From my understanding,
69 this means that I can iteratively upgrade my RAID 10 to larger drives and have
70 btrfs use all of the available space (or at least as much as is possible). This
71 is important to me because I currently have 4 320 GB HDDs + 1 (possibly broken,
72 must check) spare and wish to be able to upgrade without having to buy four
73 HDDs at once.
75 Now to the migration plan: first, partition the SSD and copy all relevant file
76 systems to it; this will be done from a Live-CD (SystemRescueCD). After I have
77 configured the mount options appropriately and can boot from that, I should be
78 able to mount the other file systems (/home and my media partitions) read-only
79 from the actual system and do the RAID + LVM -> btrfs migration from there.
81 Note that I will in general follow the advice from [1-3], and if people
82 recommend btrfs on /, then I will also try to get relevant information from [4].
84 [0]
86 [1]
87 [2]
88 [3]
89 [4]
91 Greetings and thanks in advance for any help given
92 --
93 Marc Joliet
94 --
95 "People who think they know everything really annoy those of us who know we
96 don't" - Bjarne Stroustrup


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Subject Author
[gentoo-user] Re: planned btrfs conversion: questions James <wireless@×××××××××××.com>
Re: [gentoo-user] planned btrfs conversion: questions Marc Joliet <marcec@×××.de>
Re: [gentoo-user] planned btrfs conversion: questions William Kenworthy <billk@×××××××××.au>
[gentoo-user] Re: planned btrfs conversion: questions Jonathan Callen <jcallen@g.o>
[gentoo-user] experience thus far (was: planned btrfs conversion: questions) Marc Joliet <marcec@×××.de>