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Why does Windows XP require a minimum space of 15% of the HDD to complete a defrag of a NTFS drive? Is there a better utility which doesn't have this limitation and yet does the job?

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Use MyDefrag (new version of Jkdefrag)

MyDefrag does not require a minimum free disk space, but:

Windows reserves some space on NTFS disks for expansion of the MFT, default is 12.5% of the volume size. This space is counted by Windows as free space because it can/will be used for regular files when the rest of the volume is full. MyDefrag cannot move files into this space, only out of (see ReclaimNtfsReservedAreas).

On very full harddisks MyDefrag will take more time to shuffle files around, and in some cases will have to throw the towel in the ring and leave some files fragmented or not optimized. A file can only be defragmented if there is a gap on disk big enough to hold the entire file, depending on the script that you have chosen. There may be plenty of free space, but what is needed is a single big gap.

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Ok..i understand that..i got no space on C:\ which I want to defrag but lots on D:\ . Can i specify by a command line param to use D:\ for moving stuff if it has a cmd tools? Quoting my other comment again "Its just amazing that these third party guys understand the NTFS file system just better than MS themselves or maybe its just a business decision to help make sure ISVs survive in the ecosystem!!! Maybe I'm wrong.." – iceman Nov 26 '09 at 16:04
I don't think you can let it use D:\ especially if it's not on the same physical drive (which isn't specified). But trust me: set Mydefrag as your screensaver and it will defragment just fine, unless you keep DVD iso's on your drive... – Ivo Flipse♦ Nov 26 '09 at 17:12
9000+ points. Well done! – alex Nov 30 '09 at 10:43
Merci ;-) Now for the 10k and get evil mod rights! – Ivo Flipse♦ Nov 30 '09 at 11:01
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There needs to be some free space so that the defragmenter can copy files from their current location either directly to a new location or via a temporary location so that they end up contiguous.

I don't know where the 15% figure comes from, but I'm guessing that it was settled upon after testing and analysis as it will cope with most situations.

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According to their site, PerfectDisk 10 will defragment with 1% free. I've used PerfectDisk and it an excellent piece of software. Very powerful.

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have you used perfectdisk when MS complained that going ahead with less than 15% space may impact your performance? Its just amazing that these third party guys understand the NTFS file system just better than MS themselves or maybe its just a business decision to help make sure ISVs survive in the ecosystem!!! Maybe I'm wrong.. – iceman Nov 26 '09 at 15:28
I've not used PerfectDisk with less that 15% of space, but my understanding is that when you do a defrag with an insufficient amount of space, you can actually degrade performance by pushing files that need to grow right up against another file... so fragmentation is bound to occur. I believe PD is aware of these issues and has multiple modes of defragmentation some of which account for performance. My feeling is that both companies understand NTFS very well, but PD has spent more time developing defragmentation technology. – Jason R. Coombs Nov 27 '09 at 16:30
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Diskeeper will work with less than the 15% and can be set to do much more than the MS tool. Check out their info at diskeeper.com.

I have found it has little imapct on system performance even when running all the time to eliminate fragmentation. We run it on a number of servers and it has helped a gret deal.

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