What is Reserved MFT Space?

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.

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:

HKEY_LOCAL_MACHINE\System\CurrentControlSet\Control\FileSystem

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