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I am writing some code to parse through the MFT on disk in NTFS volumes. This is straightforward, but one particular corner case caught my eye, and I can't find a clear answer anywhere on the internet.
For normal files in NTFS it is possible to have multiple MFT records for a single file, if the file has more attributes than can fit in a single record (for example, many $FILE_NAME attributes if the file has many hard-links, or many $DATA attributes if it has many Alternate Data Streams).
The $MFT file at reference-number 0 holds the data runs for the MFT itself. Normally it is a single record with no children. Is it possible for the $MFT file to have child records? If it were possible, how would you know where to find them? Would those child records have to be stored with very low reference numbers so that you could reliably get to them without having to have parsed the $MFT already to know where they were on disk?
DSIIDSII
1 Answer
There is a special type of attribute called
$ATTRIBUTE_LIST . A file or directory can have up to 65536 attributes and they can't possibly fit into a single MFT entry. It basically contains a list of all the file's attributes except himself. Each entry in the list contains the attribute type and the MFT reference of where to find the attribute. That's what the base file reference field in the file record header is for.
If the list gets too big for a MFT entry, the attribute can become non-resident and the list will be found by interpreting the data run of the attribute.
Because the type of the
$ATTRIBUTE_LIST is 32, it's placed usually right after the $STANDARD_INFORMATION attribute and will contain attributes with greater types (like $FILE_NAME or $DATA ).
When a file becomes very fragmented, the
$DATA attribute run list will not fit in a single MFT entry. This is also a case where $ATTRIBUTE_LIST will be used to store the $DATA attribute in multiple entries.
The
$MFT entry rarely has this problem since the allocation alogrithm is designed to prevent that. But if a $MFT for a volume becomes very fragmented it can have more than one entry to store it's $DATA .
Sebastian-Laurenţiu PlesciucSebastian-Laurenţiu Plesciuc
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I'm examining the NTFS (New Technology File System) and have been stuck in a loop trying to figure out the $ATTRIBUTE_LIST attribute. From this documentation, it is unusual to come across an $ATTRIBUTE_LIST and they're only used if the MFT table is running out of room. However, from looking at the following parsers, I've found they do parse it:
How To View Master File Table
From looking at these, I've come up with the following flowchart:
(There should be a yes to the right of 'Has $ATTRIBUTE_LIST')
I would like to refer to the 2 processes on the right side of the flow chart. Is it correct that:
ub3rst4rub3rst4r
1 Answer
they're only used if the MFT table is running out of room
This is not correct. They are used whenever the MFT entry is too large to hold all the attributes.
The attribute is only parsed if it's FRN is different than the file containing the attribute lists FRN?
It depends on the OS/software, I guess, but it kinda makes sense. While
$ATTRIBUTE_LIST must contain a list of all attributes, you can enumerate 'local' attributes by simply parsing the whole MFT entry. For instance, my software RecuperaBit does it that way.
Conversely, you need the list to figure out in which other MFT entries the 'remote' attributes are stored.
Or, is the FRN listed in the attribute only used for attributes for this file record (and not really a file)?
The MFT entry whose number is contained in the
$ATTRIBUTE_LIST attribute does not contain a $DATA attribute and doesn't have a $FILE_NAME attribute either. It is not a file, it's just an additional MFT entry.
Note: I edited the answer because I was using the word 'resident' in a confusing way to refer to attributes inside the base MFT entry. However, the concept of resident attribute is a different thing.
Andrea LazzarottoAndrea Lazzarotto
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The NTFS file system contains a file called the master file table, or MFT. There is at least one entry in the MFT for every file on an NTFS file system volume, including the MFT itself. All information about a file, including its size, time and date stamps, permissions, and data content, is stored either in MFT entries, or in space outside the MFT that is described by MFT entries.
As files are added to an NTFS file system volume, more entries are added to the MFT and the MFT increases in size. When files are deleted from an NTFS file system volume, their MFT entries are marked as free and may be reused. However, disk space that has been allocated for these entries is not reallocated, and the size of the MFT does not decrease.
The NTFS file system reserves space for the MFT to keep the MFT as contiguous as possible as it grows. The space reserved by the NTFS file system for the MFT in each volume is called the MFT zone. Space for file and directories are also allocated from this space, but only after all of the volume space outside of the MFT zone has been allocated.
How To Find Mft Entry Attributes Chart
Depending on the average file size and other variables, either the reserved MFT zone or the unreserved space on the disk may be allocated first as the disk fills to capacity. Volumes with a small number of relatively large files will allocate the unreserved space first, while volumes with a large number of relatively small files allocate the MFT zone first. In either case, fragmentation of the MFT starts to take place when one region or the other becomes fully allocated. If the unreserved space is completely allocated, space for user files and directories will be allocated from the MFT zone. If the MFT zone is completely allocated, space for new MFT entries will be allocated from the unreserved space.
The MFT itself can be defragmented. To reduce the chance of the MFT zone becoming fully allocated before the defragmentation process is complete, leave as much space at the beginning of the MFT zone as possible before defragmenting the volume. If the MFT zone becomes fully allocated before defragmentation has completed, there must be unallocated space outside of the MFT zone.
The default MFT zone is calculated and reserved by the system when it mounts the volume, and is based on volume size. You can increase the MFT zone by means of the registry entry detailed in Microsoft Knowledge Base Article 174619, but you cannot make the default MFT zone smaller than what is calculated. Increasing the MFT zone does not decrease the disk space that users can use for data files.
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To determine the current size of the MFT, analyze the NTFS file system drive with Disk Defragmenter, then click the View Report button. The drive statistics will be displayed, including the current MFT size, and number of fragments. You can also obtain the size of the MFT by using the FSCTL_GET_NTFS_VOLUME_DATA control code.
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