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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
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