pg_filenode.map
pg_filenode.map is a special file which maps so-called "mapped relations" (i.e. those relations whose filenode is not stored in pg_class) to their OID.
pg_filenode.map was introduced in PostgreSQL 9.0 (commit b9b8831a).
There is one pg_filenode.map file per database, and one file for the entire cluster.
Currently each pg_filenode.map file is kept to 512 bytes (intended to occupy one standard-sized disk sector) to minimize risk of failed updates; the corresponding Struct is designed to occupy exactly 512 bytes and can contain a maximum of 62 entries (MAX_MAPPINGS) of 8 bytes each:
typedef struct RelMapping
{
Oid mapoid; /* OID of a catalog */
Oid mapfilenode; /* its filenode number */
} RelMapping;
with file metadata bringing the total up to 512:
typedef struct RelMapFile
{
int32 magic; /* always RELMAPPER_FILEMAGIC */
int32 num_mappings; /* number of valid RelMapping entries */
RelMapping mappings[MAX_MAPPINGS];
pg_crc32c crc; /* CRC of all above */
int32 pad; /* to make the struct size be 512 exactly */
} RelMapFile;
For further details, see src/backend/utils/cache/relmapper.c.
Examples
Distribution of pg_filenode.map files in a data directory:
postgres:data$ find . -name pg_filenode.map
./global/pg_filenode.map
./base/1/pg_filenode.map
./base/11787/pg_filenode.map
./base/11788/pg_filenode.map
./base/16385/pg_filenode.map
Useful links
- filenode_map_inspect - an extension to examine the contents of pg_filenode.map files
- PostgreSQL data recovery by Dimitri Fontaine - mentions possibility of recovering a corrupt pg_filenode.map
- Decoding pg_filenode.map files with pg_filenodemapdata - using pg_hexdump