This file will describe the design, layouts, and file formats of a libsvn_fs_x repository. Since FSX is still in a very early phase of its development, all sections either subject to major change or simply "TBD". Design ------ TBD. Similar to FSFS format 7 but using a radically different on-disk format. In FSFS, each committed revision is represented as an immutable file containing the new node-revisions, contents, and changed-path information for the revision, plus a second, changeable file containing the revision properties. To reduce the size of the on-disk representation, revision data gets packed, i.e. multiple revision files get combined into a single pack file of smaller total size. The same strategy is applied to revprops. In-progress transactions are represented with a prototype rev file containing only the new text representations of files (appended to as changed file contents come in), along with a separate file for each node-revision, directory representation, or property representation which has been changed or added in the transaction. During the final stage of the commit, these separate files are marshalled onto the end of the prototype rev file to form the immutable revision file. Layout of the FS directory -------------------------- The layout of the FS directory (the "db" subdirectory of the repository) is: revs/ Subdirectory containing revs / Shard directory, if sharding is in use (see below) File containing rev .pack/ Pack directory, if the repo has been packed (see below) pack Pack file, if the repository has been packed (see below) manifest Pack manifest file, if a pack file exists (see below) revprops/ Subdirectory containing rev-props / Shard directory, if sharding is in use (see below) File containing rev-props for .pack/ Pack directory, if the repo has been packed (see below) . Pack file, if the repository has been packed (see below) manifest Pack manifest file, if a pack file exists (see below) revprops.db SQLite database of the packed revision properties transactions/ Subdirectory containing transactions .txn/ Directory containing transaction txn-protorevs/ Subdirectory containing transaction proto-revision files .rev Proto-revision file for transaction .rev-lock Write lock for proto-rev file txn-current File containing the next transaction key locks/ Subdirectory containing locks / Subdirectory named for first 3 letters of an MD5 digest File containing locks/children for path with current File specifying current revision and next node/copy id fs-type File identifying this filesystem as an FSFS filesystem write-lock Empty file, locked to serialise writers pack-lock Empty file, locked to serialise 'svnadmin pack' (f. 7+) txn-current-lock Empty file, locked to serialise 'txn-current' uuid File containing the UUID of the repository format File containing the format number of this filesystem fsx.conf Configuration file min-unpacked-rev File containing the oldest revision not in a pack file min-unpacked-revprop File containing the oldest revision of unpacked revprop rep-cache.db SQLite database mapping rep checksums to locations Files in the revprops directory are in the hash dump format used by svn_hash_write. The format of the "current" file is a single line of the form "\n" giving the youngest revision for the repository. The "write-lock" file is an empty file which is locked before the final stage of a commit and unlocked after the new "current" file has been moved into place to indicate that a new revision is present. It is also locked during a revprop propchange while the revprop file is read in, mutated, and written out again. Furthermore, it will be used to serialize the repository structure changes during 'svnadmin pack' (see also next section). Note that readers are never blocked by any operation - writers must ensure that the filesystem is always in a consistent state. The "pack-lock" file is an empty file which is locked before an 'svnadmin pack' operation commences. Thus, only one process may attempt to modify the repository structure at a time while other processes may still read and write (commit) to the repository during most of the pack procedure. It is only available with format 7 and newer repositories. Older formats use the global write-lock instead which disables commits completely for the duration of the pack process. The "txn-current" file is a file with a single line of text that contains only a base-36 number. The current value will be used in the next transaction name, along with the revision number the transaction is based on. This sequence number ensures that transaction names are not reused, even if the transaction is aborted and a new transaction based on the same revision is begun. The only operation that FSFS performs on this file is "get and increment"; the "txn-current-lock" file is locked during this operation. "fsx.conf" is a configuration file in the standard Subversion/Python config format. It is automatically generated when you create a new repository; read the generated file for details on what it controls. When representation sharing is enabled, the filesystem tracks representation checksum and location mappings using a SQLite database in "rep-cache.db". The database has a single table, which stores the sha1 hash text as the primary key, mapped to the representation revision, offset, size and expanded size. This file is only consulted during writes and never during reads. Consequently, it is not required, and may be removed at an arbitrary time, with the subsequent loss of rep-sharing capabilities for revisions written thereafter. Filesystem formats ------------------ TBD. The "format" file defines what features are permitted within the filesystem, and indicates changes that are not backward-compatible. It serves the same purpose as the repository file of the same name. So far, there is only format 1. Node-revision IDs ----------------- A node-rev ID consists of the following three fields: node_revision_id ::= node_id '.' copy_id '.' txn_id At this level, the form of the ID is the same as for BDB - see the section called "ID's" in <../libsvn_fs_base/notes/structure>. In order to support efficient lookup of node-revisions by their IDs and to simplify the allocation of fresh node-IDs during a transaction, we treat the fields of a node-rev ID in new and interesting ways. Within a new transaction: New node-revision IDs assigned within a transaction have a txn-id field of the form "t". When a new node-id or copy-id is assigned in a transaction, the ID used is a "_" followed by a base36 number unique to the transaction. Within a revision: Within a revision file, node-revs have a txn-id field of the form "r/", to support easy lookup. The is the (ASCII decimal) number of bytes from the start of the revision file to the start of the node-rev. During the final phase of a commit, node-revision IDs are rewritten to have repository-wide unique node-ID and copy-ID fields, and to have "r/" txn-id fields. This uniqueness is done by changing a temporary id of "_" to "-". Note that this means that the originating revision of a line of history or a copy can be determined by looking at the node ID. The temporary assignment of node-ID and copy-ID fields has implications for svn_fs_compare_ids and svn_fs_check_related. The ID _1.0.t1 is not related to the ID _1.0.t2 even though they have the same node-ID, because temporary node-IDs are restricted in scope to the transactions they belong to. Copy-IDs and copy roots ----------------------- Copy-IDs are assigned in the same manner as they are in the BDB implementation: * A node-rev resulting from a creation operation (with no copy history) receives the copy-ID of its parent directory. * A node-rev resulting from a copy operation receives a fresh copy-ID, as one would expect. * A node-rev resulting from a modification operation receives a copy-ID depending on whether its predecessor derives from a copy operation or whether it derives from a creation operation with no intervening copies: - If the predecessor does not derive from a copy, the new node-rev receives the copy-ID of its parent directory. If the node-rev is being modified through its created-path, this will be the same copy-ID as the predecessor node-rev has; however, if the node-rev is being modified through a copied ancestor directory (i.e. we are performing a "lazy copy"), this will be a different copy-ID. - If the predecessor derives from a copy and the node-rev is being modified through its created-path, the new node-rev receives the copy-ID of the predecessor. - If the predecessor derives from a copy and the node-rev is not being modified through its created path, the new node-rev receives a fresh copy-ID. This is called a "soft copy" operation, as distinct from a "true copy" operation which was actually requested through the svn_fs interface. Soft copies exist to ensure that the same pair is not used twice within a transaction. Unlike the BDB implementation, we do not have a "copies" table. Instead, each node-revision record contains a "copyroot" field identifying the node-rev resulting from the true copy operation most proximal to the node-rev. If the node-rev does not itself derive from a copy operation, then the copyroot field identifies the copy of an ancestor directory; if no ancestor directories derive from a copy operation, then the copyroot field identifies the root directory of rev 0. Revision file format -------------------- TBD A revision file contains a concatenation of various kinds of data: * Text and property representations * Node-revisions * The changed-path data That data is aggregated in compressed containers with a binary on-disk representation. Transaction layout ------------------ A transaction directory has the following layout: props Transaction props props-final Final transaction props (optional) next-ids Next temporary node-ID and copy-ID changes Changed-path information so far node.. New node-rev data for node node...props Props for new node-rev, if changed node...children Directory contents for node-rev Text representation of that sha1 txn-protorevs/rev Prototype rev file with new text reps txn-protorevs/rev-lock Lockfile for writing to the above The prototype rev file is used to store the text representations as they are received from the client. To ensure that only one client is writing to the file at a given time, the "rev-lock" file is locked for the duration of each write. The three kinds of props files are all in hash dump format. The "props" file will always be present. The "node...props" file will only be present if the node-rev properties have been changed. The "props-final" only exists while converting the transaction into a revision. The files' content is that of text rep references: " " They will be written for text reps in the current transaction and be used to eliminate duplicate reps within that transaction. The "next-ids" file contains a single line " \n" giving the next temporary node-ID and copy-ID assignments (without the leading underscores). The next node-ID is also used as a uniquifier for representations which may share the same underlying rep. The "children" file for a node-revision begins with a copy of the hash dump representation of the directory entries from the old node-rev (or a dump of the empty hash for new directories), and then an incremental hash dump entry for each change made to the directory. The "changes" file contains changed-path entries in the same form as the changed-path entries in a rev file, except that and may both be "reset" (in which case and are both always "false") to indicate that all changes to a path should be considered undone. Reset entries are only used during the final merge phase of a transaction. Actions in the "changes" file always contain a node kind. The node-rev files have the same format as node-revs in a revision file, except that the "text" and "props" fields are augmented as follows: * The "props" field may have the value "-1" if properties have been changed and are contained in a "props" file within the node-rev subdirectory. * For directory node-revs, the "text" field may have the value "-1" if entries have been changed and are contained in a "contents" file in the node-rev subdirectory. * For the directory node-rev representing the root of the transaction, the "is-fresh-txn-root" field indicates that it has not been made mutable yet (see Issue #2608). * For file node-revs, the "text" field may have the value "-1 " if the text representation is within the prototype rev file. * The "copyroot" field may have the value "-1 " if the copy root of the node-rev is part of the transaction in process. Locks layout ------------ Locks in FSX are stored in serialized hash format in files whose names are MD5 digests of the FS path which the lock is associated with. For the purposes of keeping directory inode usage down, these digest files live in subdirectories of the main lock directory whose names are the first 3 characters of the digest filename. Also stored in the digest file for a given FS path are pointers to other digest files which contain information associated with other FS paths that are beneath our path (an immediate child thereof, or a grandchild, or a great-grandchild, ...). To answer the question, "Does path FOO have a lock associated with it?", one need only generate the MD5 digest of FOO's absolute-in-the-FS path (say, 3b1b011fed614a263986b5c4869604e8), look for a file located like so: /path/to/repos/locks/3b1/3b1b011fed614a263986b5c4869604e8 And then see if that file contains lock information. To inquire about locks on children of the path FOO, you would reference the same path as above, but look for a list of children in that file (instead of lock information). Children are listed as MD5 digests, too, so you would simply iterate over those digests and consult the files they reference for lock information.