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When I look at the drive map of Defraggler, I see a large section that I would estimate is about 4 Gigs that is Reserved MFT space.

I am running XP Home. This is an older machine and I am running out of disk space. I will be forced to upgrade this machine at some point, but for now, what is this used for and is there a way to reclaim this?

As a side note that I think might be related, I also have about 4 Gigs dedicated to the swap file on this same partition.

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Note that a "visual block" will show up as "Reserved MFT Space" if any of it is. So you probably don't have 4GB allocated to this. IIRC the number of blocks changes when you resize the window? It's not meant to be an accurate measurement, but a rough visual guide (which is why half the drive might show up coloured as fragmented when only a few % fragmented - because a tiny bit of each block is affected). – DMA57361 Sep 9 '10 at 14:17
I understand that it isn't meant to be an accurate measurement. But I disagree with you slightly. For one thing, the MFT space is all contiguous, it has no fragmentation at all. Secondly, in Defraggler when you click on a block it shows you the files stored in that block. With the MFT space it doesn't do this which makes me believe that those blocks contain the MFT only. I know that this assumption is potentially not based on solid facts, but I am looking for all the clues I can get! – Icode4food Sep 9 '10 at 14:24
Reserved MFT space doesn't "consume" space btw, so don't worry about this eating your HD up - it is justspace that is used last, when no other free space remains. This MS KB article might be worth a read. – DMA57361 Sep 9 '10 at 14:42
Ok, that is interesting. If that is the case then my whole concern is basically mute. – Icode4food Sep 9 '10 at 14:51
Well, unless your MFT itself is 4GB big. I'm not sure if Defraggler shows the reserved space and used space for the MFT seperately? Obviously the MFT itself consumes space. You know what, I might actually post an answer as well... – DMA57361 Sep 9 '10 at 14:54
up vote 6 down vote accepted

The MFT (Master File Table) proper is the space on your hard drive the records where every file is stored on the drive.

Since it is generally a bad thing when the MFT becomes fragmented you will typically allocate a chuck of space much bigger than it needs so that it can grow without suffering from fragmentation - this is what the Reserved MFT space refers to.

It is not actually "used" space, and will be used as a last resort for your data. Taken from to this Microsoft KB Article:

The MFT Zone is not subtracted from available (free) drive space used for user data files, it is only space that is used last. When the MFT needs to increase in size, for example, you created new files and directories, it is taken from the MFT Zone first, thus decreasing MFT fragmentation and optimizing MFT performance.

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Extract from this forum

What is Reserved MFT Space?

The MFT is the heart of an NTFS partition. There is at least one entry in the MFT for every file on an NTFS volume. All the information about a file, including its' size, time and date stamps, permissions, data content, etc. are stored in the MFT (or in space described by the MFT).

To prevent fragmentation of the MFT, NTFS reserves space for the MFT in an effort to keep it as contiguous as it grows. This is important because defraggers can not move MFT records and fragmentation of the MFT can severely impact performance.

When you add files to an NTFS volume, entries are added to the MFT. When files are deleted from an NTFS volume, their MFT entries are marked as free and may be reused, but the MFT does not shrink. Thus, space used by these entries is not reclaimed from the disk.

NTFS reserves a percentage of the volume for exclusive use of the MFT. Space for files and directories will not be allocated from this MFT zone until all other space is allocated first. Depending on the average file size and other variables, either the reserved MFT zone or the unreserved space on the disk may be filled first. Volumes with a few large files will exhaust the unreserved space first, while volumes with a large number of small files will exhaust the MFT zone space first. When either the MFT zone or the unreserved space fills, fragmentation of the MFT starts. If the unreserved space becomes full, space for user files and directories will be allocated from the MFT zone. If the MFT zone becomes full, space for new MFT entries will be allocated from the remainder of the disk.

You can impact the amount of space NTFS reserves for the MFT by editing:


Add Value name NtfsMftZoneReservation as a type REG_DWORD and set the data value. The valid range is 1 - 4.

Value: 1 12.5% of free space, default

Value: 2 25% of free space

Value: 3 37.5% of free space

Value: 4 50% of free space

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I hope this provides some information rather than aiming at answering the question – subanki Sep 9 '10 at 15:04

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