Changes by: antona
Update of /cvsroot/linux-ntfs/linux-ntfs/doc
In directory usw-pr-cvs1:/tmp/cvs-serv9378
Added Files:
attributes.txt
Log Message:
Some documentation I wrote on how attributes work in NTFS.
--- NEW FILE ---
Notes on the storage of attributes.
===================================
Resident attributes
===================
Resident attributes are small enough to be stored in the mft record itself.
When an attribute becomes too big to fir in the mft record or when the number
of attributes grows so that there is no more space in the mft record, the
largest attribute(s) is(are) made non-resident.
Non-resident attributes
=======================
Non-resident attributes ontain only their non-resident attribute header in the
mft record and the actual attribute value is stored anywhere on the volume
(but not in other mft records!).
The value of the attribute is saved as a sequence of clusters, named virtual
clusters, beginning at virtual cluster number (vcn) zero. Each vcn corresponds
to a cluster on the volume, or a logical cluster number (lcn). The lcns on the
volume begin with lcn zero for the very first cluster on the volume.
The location of the attribute value is described by the mapping pairs array
present in the non-resident attribute header structure. The mapping pairs array
contains, in compressed form, the mapping between the attribute's vcns and the
corresponding lcns on the volume.
When the mapping pairs array of a non-resident attribute becomes too large to
fit in the mft record or when there are so many attributes in the mft record
that even though all attributes that can be non-resident have been made
non-resident they still do not fit in the mft record, the larger non-resident
attributes are moved away from the mft record to make space.
This is done by moving the whole non-resident attribute header structures to
another mft record.
Attribute list attribute
========================
The attribute list attribute is then added to the base mft record to describe
the location of each attribute.
The attribute list attribute lists all attributes that belong to this mft
record (with the exception of itself). Each entry in the attribute list
describes the attribute listed and in which mft record it's attribute header can
be found. For resident attributes this will be the same number as the base mft
record in which the attribute list attribute is located itself. For non-resident
attributes this will be another mft record, called an extension mft record.
Naturally, all extension mft records point back to their base mft record.
Should the mapping pairs array of an attribute become so large as to not fit
into an extenstion mft record, even though it's attribute is the only attribute
in this extension record, then the attribute is splitt into several extents.
The first extent starts at vcn 0 and has it's lowest vcn value set to zero and
continues up to it's highest vcn value. This is determined by splitting up the
mapping pairs array into chunks which just fit into an extension mft record
each. Thus the first mapping pairs array chunk will determined the value of
highest vcn for the first extens. The attribute list will contain an entry for
this extent. Then, a second extent is created which has it's lowest vcn value
set to the highest vcn of the previous extent + 1 and the next chunk of the
mapping pairs array is inserted into this extent. Again, an entry for this
extent is placed into the attribute list and so on until the whole non-resident
attribute's mapping pairs array has been accomodated.
Should the attribute list become too big to fit inside the base mft record it
is made non-resident. However, it's maximum value size is determined by the
Windows cache manager to be 256kb. Also, the mapping pairs array of the
attribute list has to fit inside the base mft record as an attribute list can't
be described by itself and attribute lists are not allowed to be nested.
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