Re: How come Linux/Unix filesystems don't seem to get fragmented?
- From: Robert Heller <heller@xxxxxxxxxxxx>
- Date: Mon, 03 Dec 2007 22:58:33 +0100
At Mon, 03 Dec 2007 15:13:40 -0600 Chris Cox <ccox_nopenothis@xxxxxxxxxxx> wrote:
On Sat, 2007-12-01 at 15:30 +0000, Grant Edwards wrote:
On 2007-12-01, Ramon F Herrera <ramon@xxxxxxxxxxx> wrote:
Well, the point is that in some 15 years being in charge of
Unix (Solaris, IBM AIX, HP-UX, Linux, even friggin' SCO) I
don't recall ever having problems caused by fragmentation.
Linux:
ext2/3, fragments. Not very fast, but they will fragment. A long
running filesystem with a lot of activity can be over 20%
fragmented. There is a defragmenter (defrag) for ext2/3.
jfs (Linux), fragments. I hear this one gets pretty bad when
disk gets full. There used to be a jfs defragmenter, but I think
there isn't a maintained one anymore.
Reiserfs, doesn't fragment much. The most I've ever seen is
about 9%, generally less than 5%.
Regardless though, the bigger issue has to do with size of
fragments and so in general, it's never a serious problem performance
wise.
The only time fragmentation is a serious issue with Linux (and UNIX in
general) file systems is when they are close to being full (eg like 90%
full).
Because of the uniform way multiple [logical] disks can be mounted under
Linux/UNIX, most of the fragmentation / full disk issues can be isolated
and/or avoided. Those file systems which has fullness and/or fragmentation
issues (news spools, mail spools, temp/scratch directories, etc.) can be
set aside as separate partitions and problems that might arise can be
dealt with without impacting access or performance. For example, a busy
news server could have two news spool partitions/disks and once a month,
the 'offline' disk can be freshly formatted, the news server daemon can
be throttled, a disk-to-disk backup made, and the disks can be swaped
(fun with mount/umount). The rest of the system's file systems (eg
/usr, etc.) are not effected by the (special case) of the news spool.
With all of that said, Windows (NTFS, pick your version) fragments
a lot. Just applying updates is enough to frament NTFS horribly.
And, unlike the filesystems above, you WILL feel it performance wise.
And FAT file systems are even worse (DOS-based MS-Windows).
So... it's not that the Linux filesystem don't fragment, because
they all do... it's just that they do it so slowly that it appears
that they don't and even when they do, it doesn't impact performance
(that much) like it does with NTFS fragmentation.
--
Robert Heller -- Get the Deepwoods Software FireFox Toolbar!
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