Gentoo Archives: gentoo-user

From: Daniel Iliev <daniel.iliev@×××××.com>
To: gentoo-user@l.g.o
Subject: Re: [gentoo-user] [OT] Fragmentation of my drives. Curious mostly
Date: Sun, 30 Nov 2008 09:50:20
Message-Id: ae107db90811300150i28c734f1n60b48e69e09a7a79@mail.gmail.com
1 On Sat, 29 Nov 2008 12:23:23 +0200
2 Alan McKinnon <alan.mckinnon@×××××.com> wrote:
3
4 > On Saturday 29 November 2008 11:19:47 Daniel Iliev wrote:
5 > > Now let's put the assumptions aside and do a test.
6 > >
7 > > root@localhost # test $ cat /usr/portage/packages/All/* > test1
8 > > root@localhost # test $ cp test1 test2
9 > > root@localhost # test $ ls -lah
10 > > total 2.3G
11 > > drwxr-xr-x 2 root users 4.0K 2008-11-29 01:38 .
12 > > drwxr-xr-x 44 root users 4.0K 2008-11-29 01:36 ..
13 > > -rw-r--r-- 1 root users 1.2G 2008-11-29 01:38 test1
14 > > -rw-r--r-- 1 root users 1.2G 2008-11-29 01:40 test2
15 > > localhost test # filefrag *
16 > > test1: 1125 extents found, perfection would be 10 extents
17 > > test2: 1923 extents found, perfection would be 10 extents
18 > > localhost test # time cat test1 > /dev/null
19 > >
20 > > real 0m26.747s
21 > > user 0m2.110s
22 > > sys 0m1.450s
23 > > localhost test # time cat test2 > /dev/null
24 > >
25 > > real 0m29.825s
26 > > user 0m1.780s
27 > > sys 0m1.690s
28 >
29 > This is not a test unfortunately. You did one run on one file and one
30 > run on another file. We do not know what else the machine was doing
31 > at that time, and that unknown is a considerable one.
32 >
33
34
35 This result is from the last of three repetitions. Its values were in
36 the middle (not average). The deviations were in the range 1 to 1.5
37 seconds. Every time reading the more fragmented was slower.
38
39 The system was idle in level 3, no X running. I used iostat before
40 the tests and it registered several tens of KBs written for 1 min before
41 the test. Compared to the size of the test files and the speed of the
42 disk it is insignificant.
43
44
45 > Repeat your test eliminating this factor. Preferably, remount the
46 > filesystems after each run and repeat 1000 times. Then analyze the
47 > statistical distribution of your results. This should eliminate most
48 > random factors and give a more realistic real-world view.
49 >
50
51 I'm not willing to waste time for 1000 repetitions, but why don't you
52 do the test yourself just a couple of times and see if there will be a
53 case when the more fragmented file gets read faster?
54
55 Actually my results are a little lower than what I expected but enough
56 for me to say that fragmentation matters. At least until proved
57 otherwise. Fragmentation leads to seeks. The average seek time on
58 modern disks is several milliseconds. Yes, there are algorithms
59 reordering the I/O requests to minimize the seek penalty, but still
60 seeks are there and they hurt performance.
61
62
63 --
64 Best regards,
65 Daniel

Replies

Subject Author
Re: [gentoo-user] [OT] Fragmentation of my drives. Curious mostly Alan McKinnon <alan.mckinnon@×××××.com>