[brokenhead]

Originally Posted by saikee

It is just one of the many design aspects that makes Linux or any Unix-like system a more stable and secure. For example an user never needs to defrag a Linux partition because a swap is used.

I've seen it claimed that defrag is either unnecessary or harmful or both. Saikee said the above in one thread, but I'm not sure I follow the logic. Windows also uses a swap file and has at least since 3.x, yet its performance is markedly improved by regular defrags. I know a pagefile in Windows is memory emulation, i.e., virtual memory. I was under the impression that is pecisely what a Linux swap is.

I use ext3 fs on my Ubuntu 7.10 install. I love it. I keep it updated. But constant updating just has to cause fragmentation. I know there are defrag utilities packaged with Ubuntu, at least for ext2 and ext3.

So far I have been frightened out of trying to run any defrag. But could someone please explain why it is either harmful or unnecessary?

[bearqst]

A good description of the need / no need to defrag is at

http://www.linuxquestions.org/questi...-linux-331862/

Post #10

[lotuseclat79]

Hi brokenhead,

Linux ext2 and ext3 file systems calculate and allocate disk space on the fly to allow contiguous writing of files. This prevents files being written in pieces all across the disk, rather, they get written linearly.

There is no online ext3 defragmentation tool working on the file system level. An offline ext2 defragmenter, e2defrag, exists but requires that the ext3 filesystem be converted back to ext2 first. But depending on the feature bits turned on the filesystem, e2defrag may destroy data; it does not know how to treat many of the newer ext3 features.

There are userspace defragmentation tools like Shake and defrag, which work by copying each file and hoping the newly allocated file was not fragmented. However this only works if the filesystem is reasonably empty, and such filesystems are not usually fragmented. A true defragmentation tool does not exist for ext3.

The Linux System Administrator Guide states, "Modern Linux filesystem(s) keep fragmentation at a minimum by keeping all blocks in a file close together, even if they can't be stored in consecutive sectors. Some filesystems, like ext3, effectively allocate the free block that is nearest to other blocks in a file. Therefore it is not necessary to worry about fragmentation in a Linux system.

-- Tom

[brokenhead]

Thanks for the responses! You have convinced me that fragmentation is nothing to be concerned about with my Linux install. One less thing to worry about is always welcome!

[RobLinux]

The file system lay out is clever using cylinder groups, and allocations for extensions of files is in the cylinder group. ext4 is looking at extent based allocation to improve large file performance, ext3 & ext2 try to be fairly intelligtent about it. Fragmentation is however inevitable, *BUT* it doesn't matter, as the disk i/o prioritisation minimises the effect of fragmentation, so long as a file has runs of blocks, in the same cylinder group, then moves to a different one every few megabytes, performance will stay good.

Windows degrades badly due to fragmentation, but "defragging" actually tends to make the situation worse, as subsequent deletes, and extends of files cause a vast increase in fragmentation, which would be avoided by using a sparser (more scattered) lay out.

Of course partitioning the disk into a number of file systems, reduces seek times, and fragmenation of heavily re-written areas (like /var which should be mostly empty) and reduces times on fsck(8) when that becomes necessary.

To defrag, copying out, deleting and restoring files should prove perfectly adequete. In my experience this has tended only to be necessary where large amount of small synchronous writes were performed (NFS v2 access to file system).

[RootbeaR]

Here is a link to an article that tells why windows needs it.
http://forums.techguy.org/random-dis...ml#post5581672