Gentoo Archives: gentoo-amd64

From: Martin Herrman <martin@×××××××.nl>
To: gentoo-amd64@l.g.o
Subject: Re: [gentoo-amd64] Ext3 to ext4
Date: Fri, 10 Dec 2010 16:03:00
Message-Id: AANLkTim2KUn6H+QdX5yqBoP0BT6baOz7n3oKoM0PkcS+@mail.gmail.com
In Reply to: Re: [gentoo-amd64] Ext3 to ext4 by Florian Philipp
1 2010/10/31 Florian Philipp <lists@f_philipp.fastmail.net>
2 >
3 > See question 9 here:
4 > http://tldp.org/HOWTO/Software-RAID-0.4x-HOWTO-8.html
5 >
6 > and also the calculator here:
7 > http://busybox.net/~aldot/mkfs_stride.html
8 >
9 > There might be more alignment issues with LVM on RAID. I don't think it
10 > will be a problem as long as LVM's physical extent size (default 4MB) is
11 > a multiple of the RAID stripe size (default 64kB?). Someone with more
12 > experience in these setups than me might want to comment on my guess.
13
14 Okay, so after a holiday trip, some hard weeks of labour, I went ill.
15 During recovery I decided to dive into this issue again :-)
16
17 I found out that my current chunk size is 64K:
18
19 martindesktop ~ # mdadm -Q --detail /dev/md3
20 <cut>
21 Chunk Size : 64K
22 <cut>
23 martindesktop ~ #
24
25 and my block size is 4KiB:
26
27 martindesktop ~ # /sbin/dumpe2fs /dev/sda3 | grep 'Block size'
28 <cut>
29 Block size: 4096
30 <cut>
31 martindesktop ~ #
32
33 The calculator output is:
34
35 mkfs.ext3 -b 4096 -E stride=16,stripe-width=32
36
37 The maintained RAID howto at:
38
39 https://raid.wiki.kernel.org/index.php/RAID_setup
40
41 says that stride and stride-width can be changed afterwards by using:
42
43 tune2fs -E stride=n,stripe-width=m /dev/mdx
44
45 So that's what I did:
46
47 martindesktop / # tune2fs -E stride=16,stripe-width=32 /dev/md3
48 tune2fs 1.41.12 (17-May-2010)
49 Setting stride size to 16
50 Setting stripe width to 32
51 martindesktop / #
52
53 (this took less than a second)
54
55 I also read the mdadmin man-page. It looks like one can adjust the
56 chunk size (also for RAID-0) without repartitioning, but I found
57 (using google) no hard evidence that 64K is a wrong choice for a
58 desktop system. I won't perform my own benchmarks (based on what??)
59 and will stick with the 64K.
60
61 Regards,
62
63 Martin