Author: julianfoad
Date: Thu Dec 10 16:48:47 2009
New Revision: 889330
URL: http://svn.apache.org/viewvc?rev=889330&view=rev
Log:
* subversion/libsvn_fs_fs/structure
Expand and clarify the description of FSFS node-revision ids.
Fix a typo.
Modified:
subversion/trunk/subversion/libsvn_fs_fs/structure
Modified: subversion/trunk/subversion/libsvn_fs_fs/structure
URL:
http://svn.apache.org/viewvc/subversion/trunk/subversion/libsvn_fs_fs/structure?rev=889330&r1=889329&r2=889330&view=diff
==============================================================================
--- subversion/trunk/subversion/libsvn_fs_fs/structure (original)
+++ subversion/trunk/subversion/libsvn_fs_fs/structure Thu Dec 10 16:48:47 2009
@@ -224,35 +224,51 @@
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-ID in new and interesting ways.
+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<txnid>".
+
+ 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<rev>/<offset>", to support easy lookup. The <offset> 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<rev>/<offset>" txn-id fields.
+
+ In Format 3 and above, this uniqueness is done by changing a temporary
+ id of "_<base36>" to "<rev>-<base36>". 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.
-Within a revision file, node-revs have a txn-id field of the form
-"r<rev>/<offset>", to support easy lookup. New node-revision IDs
-assigned within a transaction have the txn-id field of "t<txnid>".
-
-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.
-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<rev>/<offset>" txn-id fields.
-
-In Format 3 and above, this uniqueness is done by changing a temporary
-id of "_<base36>" to "<rev>-<base36>". 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.
-
-In Format 2 and below, the "current" file contains global base36
-node-ID and copy-ID counters; during the commit, the counter value is
-added to the transaction-specific base36 ID, and the value in
-"current" is adjusted.
+ In Format 2 and below, the "current" file contains global base36
+ node-ID and copy-ID counters; during the commit, the counter value is
+ added to the transaction-specific base36 ID, and the value in
+ "current" is adjusted.
-(It is legal for Format 3 repositories to contain Format 2-style IDs;
-this just prevents I/O-less node-origin-rev lookup for those nodes.)
+ (It is legal for Format 3 repositories to contain Format 2-style IDs;
+ this just prevents I/O-less node-origin-rev lookup for those nodes.)
The temporary assignment of node-ID and copy-ID fields has
-implications for svn_fs_compare_ids and svn_fs_check_related. The IDs
+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.
