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Diffstat (limited to 'src/backend/storage/smgr/README')
| -rw-r--r-- | src/backend/storage/smgr/README | 19 |
1 files changed, 18 insertions, 1 deletions
diff --git a/src/backend/storage/smgr/README b/src/backend/storage/smgr/README index 5d79ef54161..cc798533f2e 100644 --- a/src/backend/storage/smgr/README +++ b/src/backend/storage/smgr/README @@ -1,4 +1,4 @@ -$PostgreSQL: pgsql/src/backend/storage/smgr/README,v 1.5 2008/03/21 13:23:28 momjian Exp $ +$PostgreSQL: pgsql/src/backend/storage/smgr/README,v 1.6 2008/08/11 11:05:11 heikki Exp $ Storage Manager =============== @@ -32,3 +32,20 @@ The files in this directory, and their contents, are md.c The magnetic disk storage manager. Note that md.c in turn relies on src/backend/storage/file/fd.c. + +Relation Forks +============== + +Since 8.4, a single smgr relation can be comprised of multiple physical +files, called relation forks. This allows storing additional metadata like +Free Space information in additional forks, which can be grown and truncated +independently of the main data file, while still treating it all as a single +physical relation in system catalogs. + +It is assumed that the main fork, fork number 0 or MAIN_FORKNUM, always +exists. Fork numbers are assigned in src/include/storage/relfilenode.h. +Functions in smgr.c and md.c take an extra fork number argument, in addition +to relfilenode and block number, to identify which relation fork you want to +access. Since most code wants to access the main fork, a shortcut version of +ReadBuffer that accesses MAIN_FORKNUM is provided in the buffer manager for +convenience. |