Gentoo Archives: gentoo-user

From: David W Noon <dwnoon@××××××××.com>
To: gentoo-user@l.g.o
Subject: Re: [gentoo-user] migrating disks (from mounts to disklabels
Date: Sat, 20 Nov 2010 13:27:44
Message-Id: 20101120132603.56b13ccd@karnak.local
1 On Sat, 20 Nov 2010 00:10:02 +0100, Mick wrote about Re: [gentoo-user]
2 migrating disks (from mounts to disklabels:
3
4 >On Friday 19 November 2010 19:19:34 David W Noon wrote:
5 >> On Fri, 19 Nov 2010 17:00:04 +0100, Peter Humphrey wrote about Re:
6 >>
7 >> [gentoo-user] migrating disks (from mounts to disklabels:
8 >> >On Friday 19 November 2010 14:42:23 Neil Bothwick wrote:
9 >> >> On Fri, 19 Nov 2010 10:52:50 +0000, Mick wrote:
10 >> >> > Also primary partitions which he does not seem to be using at
11 >> >> > all have a slight edge over logical.
12 >> >>
13 >> >> Do you have any data on this? I generally use all logical
14 >> >> partitions but could be persuaded to rethink.
15 >> >
16 >> >Well there must be one level of indirection on first access, since
17 >> >the start of the logical partition has to be looked up in a
18 >> >"primary" partition, but I can't imagine that being needed more
19 >> >than once per reboot.
20 >>
21 >> Correct.
22 >>
23 >> The same applies to LVM2 or EVMS logical volumes: a small "lookup"
24 >> penalty (a few milliseconds) when the filesystem is first
25 >> activated/mounted, and as fast as the drive itself thereafter.
26 >
27 >Short of measuring the latency with some system (which I wouldn't know
28 >how) I have experimented with setting the /boot partition on primary
29 >and logical partitions and the difference (on a stopwatch) was
30 >measurable in seconds betweeen having said partition on a primary and
31 >having it on a logical. Furthermore, sda7 was slower than sda5.
32
33 Unless you have the mother of all initrd's or initramfs's, you cannot
34 have /boot on a logical partition -- only a primary partition, as BIOS
35 interrupts will only access raw drives and primary partitions. If you do
36 put /boot on a logical partition, you will pay the "lookup" overhead
37 repeatedly as part of the early bootstrap process. Since you won't have
38 a kernel running at that time. no caching, including device mapping,
39 will be in force. It will be dog slow if /boot is not in the primary
40 partition table. I always make my /boot partition /dev/sda1, which is
41 the first primary on the first hard drive.
42
43 >I haven't measured latencies for first mount and subsequent look ups.
44 >I thought that it would be the same every time a partition fs is being
45 >accessed, no?
46
47 No.
48
49 The absolute seek addresses of all partitions and logical volumes are
50 cached by the kernel. Later accesses will always use the cached extent
51 details. Resizing a logical volume rebuilds that part of the cache and,
52 if you resize with the filesystem mounted, forces the filesystem driver
53 to reread the cached extent information.
54
55 --
56 Regards,
57
58 Dave [RLU #314465]
59 *-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*
60 dwnoon@××××××××.com (David W Noon)
61 *-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*

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Replies

Subject Author
Re: [gentoo-user] migrating disks (from mounts to disklabels Neil Bothwick <neil@××××××××××.uk>
Re: [gentoo-user] migrating disks (from mounts to disklabels Mick <michaelkintzios@×××××.com>