Compression - Concept - NTFS Documentation
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# Concept - Compression

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## Overview

here's a short summary of the mechanism: data. These are compressed using a modified LZ77 algorithm. The basic idea is that substrings of the block which have been seen before are compressed by referencing the string rather than mentioning it again. For example, Consider the Plain text

        #include <ntfs.h>\n
        #include <stdio.h>\n

This is compressed to #include <ntfs.h>\n (-18,10)stdio(-17,4)

So the algorithm recognizes that -18 bytes from the current position, it has already seen the text '#include <'. Then, stdio is new, but '.h>\n' has been seen before.

The interesting details are in the question? How to encode the pair (-18,10), and how to mix this with plain-text strings. The first thing to understand is that such a pair is recorded in two bytes. Because a back-reference takes two bytes, there is no point in back-referencing one- or two-byte substrings. This means the shortest possible substring is 3. This means that length values of 0, 1, and 2 are not possible. So you can subtract 3 of the length before encoding it. Also, the references are always backward, and never 0. So you can store them as positive numbers, and subtract one. The first back-reference is stored as (17,7), and the second one as (16,1).

Given that a block is 4096 in size, you might need 12 bits to encode the back reference. This means that you have only for bits left to encode the length, allowing for a maximum length of 19. This is not desirable as it limits to compression ratio to 1:19. OTOH, if the current offset is, say, 123, a back reference of -512 is not possible. Some clever MS engineer decided to dynamically allocate more bits for the back-reference and less for the length. The exact split can be written as a table, or as

        for(i=clear_pos-1,lmask=0xFFF,dshift=12;i>=0x10;i>>=1){
                lmask >>= 1; /* bit mask for length */
                dshift--;    /* shift width for delta */
        }

Now that we can encode a (offset,length) pair as two bytes, we still have to know whether a token is a back-reference, or plain-text. This is one bit per token. Eight tokens are grouped together and preceded with the tags byte. So the group

        >\n(18,10)stdio

would be encoded as

        00000100 > \n 0A 90 s t d i o

(the 1 bit indicates the back reference). As an extreme case, a block of all space (' ') is compressed as

        00000010 ' ' FC 0F

or ' ' (-1,4095). This works because you always read data you just stored. As a compression unit consists of 16 clusters, it usually contains more than one of these blocks. If you want to access the second block, it would be a waste of time to decompress the first one. Instead, each block is preceded by a 2-byte length. The lower twelve bits are the length, the higher 4 bits are of unknown purpose.

        FIXME: Compression unit's size 2^4 in attribute header.
        The compression method is based on independently compressing blocks of X
        clusters, where X is determined from the compression_unit value found in the
        non-resident attribute record header (more precisely: X = 2^compression_unit
        clusters). On Windows NT/2k, X always is 16 clusters (compression_unit = 4).

          1) The data in the block is all zero (a sparse block):
            This is stored as a sparse block in the run list, i.e. the run list
            entry has length = X and lcn = -1. The mapping pairs array actually
            uses a delta_lcn value length of 0, i.e. delta_lcn is not present at
            all, which is then interpreted by the driver as lcn = -1.
            NOTE: Even uncompressed files can be sparse on NTFS 3.0 volumes, then
            the same principles apply as above, except that the length is not
            restricted to being any particular value.

          2) The data in the block is not compressed:
            This happens when compression doesn't reduce the size of the block
            in clusters. I.e. if compression has a small effect so that the
            compressed data still occupies X clusters, then the uncompressed data
            is stored in the block.
            This case is recognised by the fact that the run list entry has
            length = X and lcn >= 0. The mapping pairs array stores this as
            normal with a run length of X and some specific delta_lcn, i.e.
            delta_lcn has to be present.

          3) The data in the block is compressed:
            The common case. This case is recognised by the fact that the run
            list entry has length L < X and lcn >= 0. The mapping pairs array
            stores this as normal with a run length of X and some specific
            delta_lcn, i.e. delta_lcn has to be present. This run list entry is
            immediately followed by a sparse entry with length = X - L and
            lcn = -1. The latter entry is to make up the vcn counting to the
            full compression block size X.

        In fact, life is more complicated because adjacent entries of the same type
        can be coalesced. This means that one has to keep track of the number of
        clusters handled and work on a basis of X clusters at a time being one
        block. An example: if length L > X this means that this particular run list
        entry contains a block of length X and part of one or more blocks of length
        L - X. Another example: if length L < X, this does not necessarily mean that
        the block is compressed as it might be that the lcn changes inside the block
        and hence the following run list entry describes the continuation of the
        potentially compressed block. The block would be compressed if the
        following run list entry describes at least X - L sparse clusters, thus
        making up the compression block length as described in point 3 above. (Of
        course, there can be several run list entries with small lengths so that the
        sparse entry does not follow the first data containing entry with
        length < X.)

        NOTE: At the end of the compressed attribute value, there most likely is not
        just the right amount of data to make up a compression block, thus this data
        is not even attempted to be compressed. It is just stored as is.

If you look at the algorithm, you will notice that it will not always reduce the data size. If there are no back references, each byte plain-text will remain as-is. However, every 8 bytes, a tag bit is inserted, which then will be zero. So, in the worst case, a block might grow to 4610 bytes (counting the length of the block). If the block grows in size, it will be stored uncompressed. A length of exactly 4095 is used to indicate this case. It might be still possible that the following block will compress well, reducing the total size of the chunk. If it doesn't, the entire chunk is stored uncompressed, which is indicated in the run list.

> each block is preceded by a 2-byte length. The lower twelve bits are the > length, the higher 4 bits are of unknown purpose.#

Bit 0x8000 is the flag specifying that the block is compressed. The compression code OR's in the value 0xB000 (if its compressed), but the decompression code only looks at bit 0x8000.

Also, the length is actually stored as (n-3) in the low 12 bits. Actually, (n-1) if you don't count the two bytes used to store the length itself. So for an uncompressed block the length is stored as 0xFFF, meaning the length is 4096 + 2 more bytes holding the length itself.

A 0x1000 length block compressed to length 0x500 would require 0x502 bytes, and have an advertised length of 0x4FF.

What I don't know is whether a 16 cluster file that doesn't compress at all requires 17 clusters to store, in order to accommodate the extra 2 bytes per block.

I believe it will take only 16 clusters. The fact that it is not compressed will be expressed in the run list. For example, the compressed file will look like

        (1000 A) (0 6)       //(rel.VCN length)

whereas the uncompressable file will look like

        (1000 10)

or

        (1000 A) (1040 6)

IOW, if you don't have any runs with VCN==0 in the 16 clusters, the chunk is entirely uncompressed and plain. Given the compression algorithm, it is fairly easy to create such a file:

        s=""
        for i in range(0,16):   #adjust to clusters >512 if necessary

        s=s+chr(i)+chr(j)
        open("uncompressable","w").write(s)

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