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+$Id$
+
+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 fit in the mft record or when the number
+of attributes grows so large that there is no more space in the mft record, the
+largest attribute(s) is(are) made non-resident. (Note, that some attributes
+cannot be non-resident.)
+
+Non-resident attributes
+=======================
+
+Non-resident attributes contain 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 whole non-resident attribute header structures to other
+mft records and/or by splitting the mapping pairs array of attributes into
+several non-resident attribute headers, each containing a chunk of the
+original mapping pairs array. These non-resident attribute headers each
+describing the same attribute value (but different parts of it) are called
+extents.
+
+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.
+
+Only one attribute is stored in an extension mft record, even if the attribute
+is very small. At least this is the case with Windows NT4 SP6a driver.
+
+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 determine the value of
+the highest vcn for the first extent. 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 (the size of a VACB). 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.
+
+Compressed attributes
+=====================
+
+The attribute value of each $DATA attribute can be compressed to save space.
+If this is the case, the ATTR_IS_COMPRESSED flag is set.
+
+See the discussion on compression in include/attrib.h for more details.
+FIXME: The discussion belongs here! (AIA)
+
+Sparse attributes (NTFS 3.0+)
+=============================
+
+The attribute value of each $DATA attribute can be sparse to save space.
+If this is the case, the ATTR_IS_SPARSE flag is set. Note, that compressed
+attributes are by definition sparse, as well as compressed, without having the
+ATTR_IS_SPARSE flag set.
+
+See the discussion on compression in include/attrib.h for more details.
+FIXME: The discussion belongs here! (AIA)
+
+Encrypted attributes (NTFS 3.0+)
+================================
+
+Since NTFS 3.0, the attribute value of each $DATA attribute can be encrypted,
+to protect the contents from spying eyes. If this is the case, the
+ATTR_IS_ENCRYPTED flag is set.
+
+FIXME: Write notes on attribute encryption. The descussions from the articles
+"Inside the Encrypting File System" in Windows NT magazine (?) are very good
+starting points. (AIA)
+