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diff --git a/subversion/libsvn_fs_fs/structure-indexes b/subversion/libsvn_fs_fs/structure-indexes new file mode 100644 index 0000000..25490c7 --- /dev/null +++ b/subversion/libsvn_fs_fs/structure-indexes @@ -0,0 +1,352 @@ +This file describes the design, data model, and storage formats of FSFS +index data. + + +Design +====== + +Each pack and each rev file using logical addressing contains exactly +two index sections. One, the log-to-phys index, maps the (rev, item_index) +pairs to absolute file offsets. The other, phys-to-log, is a reverse +index that gives basic information on any file location. This is enough +to read and cache any data without traversing DAGs. + +Rev and pack files are immutable, so the same is true for index data. +During a transaction or while packing a file, a proto index file gets +written (actually, one log-to-phys and one phys-to-log). Its format is +a simple concatenation of runtime structs and as such, an implementation +detail subject to change. A proto index basically aggregates all the +information that must later be transformed into the final index. + + +General design concerns +----------------------- + +In Subversion, there is no limit to the size of a revision; even practical +limits are in the order of millions of changes at least. Index data for +these would be multiple megabytes in size with pack file indexes possibly +approaching 1 GB. To ensure we still get roughly O(1) access time, we +need a hierarchical data structure. + +Therefore, the indexes will start with a header containing an array of +references to sub-sections or pages. The length of these pages varies +but is limited to a size configurable in fsfs.conf. + +Finally, it is assumed that whole pages can be cached efficiently and +with a high cache hit rate. So, although a page may have a thousand or +more entries, the access time is still amortized O(1) in many scenarios. + + +Items and item types +-------------------- + +The index implementation treats item_index and item type as simple ints, +except for SVN_FS_FS__ITEM_INDEX_UNUSED and SVN_FS_FS__ITEM_TYPE_UNUSED. +Since they have been defined as 0, the code may test for "used" etc. +by simply comparing with 0. + +See section "addressing modes" in structure to a list of item types +and pre-defined item_index values. + + +Encoding +-------- + +The final index data format is tuned for space and decoding efficiency. +Indexes are stored as a sequence of variable integers. The encoding is +as follows: + +* Unsigned integers are stored in little endian order with a variable + length 7b/8b encoding. If most significant bit a byte has been set, + the next byte has also belongs to the same value. + + 0x00 .. 0x7f -> 0x00 .. 0x7f ( 7 bits stored in 8 bits) + 0x80 .. 0xff -> 0x80 0x01 .. 0xff 0x01 (14 bits stored in 16 bits) + 0x100 .. 0x3fff -> 0x80 0x02 .. 0xff 0x7f (14 bits stored in 16 bits) + 0x100000000 -> 0x80 0x80 0x80 0x80 0x10 (35 bits stored in 40 bits) + + Technically, we can represent integers of arbitrary lengths. Currently, + we only generate and parse up to 64 bits. + +* Signed integers are mapped onto the unsigned value space as follows: + + x >= 0 -> 2 * x + x < 0 -> -2 * x - 1 + + Again, we can represent arbitrary length numbers that way but the code + is currently restricted to 64 bits. + +Most data is unsigned by nature but will be stored differentially using +signed integers. + + +Encoding in proto-index files +----------------------------- + +These have a much simpler encoding. Throughout the files, all records have +the same length (but different between L2P and P2L). All records contain +unsigned 64 bit integers only, stored in little endian byte order. + + +Log-to-phys index +================= + +This index has to map (rev, item_index) -> offset. It assumes that the +item_index values per revision are dense and start at 0. There may be +unused item_index values, though; the data structure simply gets less +space-efficient when the more sparse the value space gets. + + +Index data model +---------------- + +hierarchy: + + header -> per-revision info -> page -> offset + + There is one entry per revision in the header. Per revision there are + one or more pages (exclusive to that revision) containing up to a known, + fixed limit of entries (= page size). The total access path is: + + pages = header->pages[revision]; + offsets = page = pages[item_index / page_size]; + offset = offsets[item_index % page_size]; + + Different log-to-phys indexes in the same repository may have different + page sizes but within any given index, the page size is the same and + immutable. + +header: + + <first revision> ... first revision covered by this index + <revision count> ... number of revision covered by this index + <page size> ... maximum number of entries per page + <page table index> ... array, for each revision containing the index in + <page table> of the first page that belongs to + this revision. This has <revision count>+1 + entries to terminate the last revision. + <page table> ... array of page headers. It has + <page table index>[<revision count>] entries. + +page table: + + <offset> ... absolute position of the page contents within the + index + <entry count> ... number of offset entries in the page. + Must match <header>.<page size> unless this is + the last page for the respective revision. + <size> ... length in bytes of the on-disk page description. + Note that this is redundant with the <offset>. + +page: + + <entry count> ... number of offset entries in the page. + Must match <header>.<page size> unless this is + the last page for the respective revision. + Redundant with <page table>.<entry count> + <offsets> ... array of absolute file positions within the rev / + pack file. This has <entry count> entries. + + +Index on-disk format +-------------------- + + index := "L2P-INDEX\n" header revisions pages offsets + + header := u(<header>.<first revision>) \ + u(<header>.<page size>) \ + u(<header>.<revision count>) \ + u(s(<header>.<page table>)) + + revisions := u( <header>.<page table index>[k+1] + - <header>.<page table index>[k]), + for k in 0 .. <header>.<revision count>-1 + + pages := u(<header>.<page table>[k].<size>) \ + u(<header>.<page table>[k].<entry count>), + for k in 0 .. s(<header>.<page table>)-1 + + offsets := page(k), + for k in 0 .. s(<header>.<page table>)-1 + + page(k) := i(<header>.<page table>[k].<offsets>[0]) \ + i( <header>.<page table>[k].<offsets>[l] \ + - <header>.<page table>[k].<offsets>[l - 1]), + for l in 1 .. s(<header>.<page table>[k].<entry count>)-1 + + u(x) ... unsigned int x in 7b/8b encoding + i(x) ... signed int x in 7b/8b encoding + s(x) ... number of entries in array x + + +Proto index file format +----------------------- + +The index will be created from a "revision-less" proto index file +containing (<offset><item_index>) pairs only. + +All mappings belonging to the same revision must be written in one go but +there is no restriction on the order of those entries. To signify that +a new revision begins, a (0, 0) mapping must be written. A (0, 0) entry +at the beginning of the file is optional and will be ignored. + + <bof> /* begin of proto index file for revision r and following */ + (0, 0) /* mark start of revision r, optional for first rev */ + (off, item)* /* zero to many mappings in random order */ + (0, 0) /* mark start of revision r + 1 */ + (off, item)* /* zero to many mappings in random order */ + (0, 0) /* mark start of revision r + 2 */ + (off, item)* /* zero to many mappings in random order */ + ... + <eof> /* end of file. */ + +All entries are pairs of 64 bit unsigned integers in little endian order. + + +Phys-to-log index +================= + +This index has to map offset -> (rev, item_index, type, len, checksum). + + +Index data model +---------------- + +hierarchy: + + header -> page -> item info + + Logically, the index splits up index rev / pack file into pages of a + fixed size. That page size may differ from the FS's block size. The + index will describe each rev / pack file page with one index page. + + page = header->pages[offset % page_size]; + item info = binary_search(page.data, offset) + + Note that the current implementation will not return any data if the + offset is does not match any actual item start. To simplify the lookup, + the last index page will have an "unused item" entry for the section + behind EOF. Holes aren't allowed as well, i.e. every byte of the rev / + pack is expected to be covered by the index. + + Also, there may be items stretching across page borders or even over + multiple pages. The data model solves this issue by storing the item + descriptions as a "primary array" and then representing the pages as + ranges within that array. Thus multiple pages may reference the same + item description. + +header: + + <first revision> ... first revision covered by this index + <file size> ... size of the rev / pack file in bytes + <page size> ... number of bytes in the rev / pack file covered by + each index page + <page count> ... number of pages + <offsets> ... array of page offsets, i.e. locations the page + data within the index. + This array has <page count> + 1 entries. + +page: + + <entries> ... array of item descriptions, ordered by offset. + First and last entry may cross page boundaries. + +entry: + + <offset> ... absolute position in the pack / rev file + <size> ... on-disk size of the item in the pack / rev file + <type> ... item type + <FNV checksum> ... modified 32 bit FNV-1a checksum of that section + of the pack / rev file (see below). 0 for empty + or zero-length items + <revision> ... revision that this item belongs to + <item_index> ... item_index within that revision + + +Index on-disk format +-------------------- + + index := "P2L-INDEX\n" header pages items + + header := u(<header>.<first revision>) \ + u(<header>.<file size>) \ + u(<header>.<page size>) \ + u(<header>.<page count>) + + pages := u(<header>.<offsets>[k+1] - <header>.<offsets>[k]), + for k in 0 .. <header>.<page count> -1 + + items := u(<items in page k>[0].<offset>) \ + u(<items in page k>[l].<size>) \ + i(c(<items in page k>[l]) - c(<items of page k>[l-1])) \ + i( <items in page k>[l].<revision> + - <items in page k>[l-1].<revision>), \ + u(FNV checksum) + for l in 0 .. s(<items in page k>)-1, + for k in 0 .. <header>.<page count>-1 + + u(x) ... unsigned int x in 7b/8b encoding + i(x) ... signed int x in 7b/8b encoding + s(x) ... number of entries in collection x + c(x) ... compound function := x.<item_index> * 8 + x.<type> + + Access to negative indexes gives a 0 value. + + <Items in page k> are in strict ascending offset order. Items that + started after the begin of a given page and overlap with the next page + will not be stored in the start page. The runtime representation will + duplicate items overlapping page boundaries; the on-disk representation + will not. Thus, pages inside large items will have zero entries on disk. + + +Proto index file format +----------------------- + +The index will be created from a proto index file containing simple +instances of svn_fs_fs__p2l_entry_t with the following element order: + + item offset as uint64 + item size as uint64 + item type as uint64 + modified FNV1a checksum as uint64 + revision as uint64, with SVN_INVALID_REVNUM mapped to 0 + and revisions >= 0 stored as rev+1 + item index as uint64 + +All values are stored in little endian order. + +Page table and header information, except start revision and page size, +can easily be derived from that information. + +All entries must be written in strict offset order. Overlapping entries +are not allowed; zero-length items are. + +In transactions, the final revision number may not be known when writing +the proto index file (e.g. while still writing the proto rev file). Items +with revision set to SVN_INVALID_REVNUM will therefore be automatically +updated when creating the final index. This is possible in conjunction +with rev files but not for pack files. + + +FNV checksum +------------ + +FNV-1a can be found here: http://www.isthe.com/chongo/tech/comp/fnv/ +For performance reasons we use a modified version: + +* split the input byte stream [b0 .. bN] into 4 sub-streams of equal + length and up to 3 remnants: + + [b0 b4 b8 ..], [b1 b5 b9 ..], [b2 b6 b10 ..], [b3 b7 b11 ..], [remnant] + +* calculate 32 bit FNV-1a checksums for the 4 substreams: + + h0 = fnv_1a([b0 b4 b8 ..]), ..., h3 = fnv_1a([b3 b7 b11 ..]) + +* combine the big endian representation of these checksums plus the + remnant of the original stream into a 12 to 15 byte long intermediate + + [i0 .. iK], 12 <= K+1 <= 15 + +* FNV checksum = fnv_1a([i0 .. iK]) in big endian representation + |