Defragmenters useful for *nix?
Having come from a windows enviroment, I was curious if defragmenters are useful in *nix. More specifically, OS X.
It's a bit of a yes, no answer. Useful in certain circumstances but it's less of an issue than it was with FAT or regular HFS. All filesystems will fragment but newer ones are more resistant to fragmenting so badly.
Speaking for Mac OS X specifically HFS+ does a decent enough job of trying to keep things from being fragmented compared to older systems but it still happens just not on the same scale. The OS itself also defrags "small" (20MB or smaller) files on the fly since 10.3 (Panther).
Fragmenting still happens and you can see performance drop because of it, especially in video editing systems or a workflow that requires the ability to read or write large files quickly to the disk. For your standard user - a near non-issue.
The most popular options for defragmenting a hard drive for OS X I've used and run across are:
Cloning the hard drive to another drive and back. This is done using Carbon Copy Cloner or SuperDuper and requires an extra hard drive. If done as part of a backup routine the time hit may not be terrible but it's free to do it this way.
iDefrag, Drive Genius and a handful of other utilities will all defragment your hard drive as well. Personally I prefer iDefrag.
Yes, they are. People will give you lies like "UNIX filesystems never fragment." They are liars, and you should listen to me instead. Files like sqlite databases, as used by firefox, will quickly fragment as they deliver small writes regularly as you use the browser. At one point my profile had a sqlite database with over three thousand fragments.
These sqlite databases contain the browser history, and are used in places to suggest text strings to you, like URL completion or form autofill. If they are fragmented, you will suffer. Some of this may be masked by OSX's decision to implement POSIX fsync() as a no-op (allowed by the standard, but not very nice). So it's not like you need to edit video to trigger bad conditions, just a large history database that properly calls fsync() on OSX.
On Ubuntu you can check how fragmented a file is with the utility filefrag, in package e2fsprogs. It requires root permissions, but gives you a view of how many non-contiguous regions a file has. As the package name suggests, it is not ext4 aware (yet). Hopefully, ext4's delayed allocation and extents support reduces fragmentation in the wild.
As far as I know, unix file systems like EXT or HFS don't suffer from fragmentation like FAT or NTSF, at least not in the same order of magnitude.
Read more about it here and also check this apple support page about disk maintenance
It depends on what filesystem are you using and most importantly on how are you using it. Most modern filesystem are less prone to fragmentation, but defragmentation is always useful.
You can use xfs_fsr to defragment XFS filesystems. It has some limitations, but it's better than nothing.