>>
PostgreSQL File Manager
•PostgreSQL File Manager
•Relations as Files
•File Descriptor Pool
•File Manager
COMP9315 21T1 ♢ PG File Manager ♢ [0/15]
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❖ PostgreSQL File Manager
PostgreSQL uses the following file organisation …

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❖ PostgreSQL File Manager (cont)
Components of storage subsystem:
•mapping from relations to files (RelFileNode)
•abstraction for open relation pool (storage/smgr)
•functions for managing files (storage/smgr/md.c)
•file-descriptor pool (storage/file)
PostgreSQL has two basic kinds of files:
•heap files containing data (tuples)
•index files containing index entries
Note: smgr designed for many storage devices; only disk handler provided
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❖ Relations as Files
PostgreSQL identifies relation files via their OIDs.
The core data structure for this is RelFileNode:
typedef struct RelFileNode {
OidspcNode;// tablespace
OiddbNode; // database
OidrelNode;// relation
} RelFileNode;
Global (shared) tables (e.g. pg_database) have
• spcNode == GLOBALTABLESPACE_OID
• dbNode == 0
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❖ Relations as Files (cont)
The relpath function maps RelFileNode to file:
char *relpath(RelFileNode r)// simplified
{
char *path = malloc(ENOUGH_SPACE);
if (r.spcNode == GLOBALTABLESPACE_OID) {
/* Shared system relations live in PGDATA/global */
Assert(r.dbNode == 0);
sprintf(path, “%s/global/%u”,
DataDir, r.relNode);
}
else if (r.spcNode == DEFAULTTABLESPACE_OID) {
/* The default tablespace is PGDATA/base */
sprintf(path, “%s/base/%u/%u”,
DataDir, r.dbNode, r.relNode);
}
else {
/* All other tablespaces accessed via symlinks */
sprintf(path, “%s/pg_tblspc/%u/%u/%u”, DataDir
r.spcNode, r.dbNode, r.relNode);
}
return path;
}
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❖ File Descriptor Pool
Unix has limits on the number of concurrently open files.
PostgreSQL maintains a pool of open file descriptors:
•to hide this limitation from higher level functions
•to minimise expensive open() operations
File names are simply strings: typedef char *FileName
Open files are referenced via: typedef int File
A File is an index into a table of “virtual file descriptors”.
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❖ File Descriptor Pool (cont)
Interface to file descriptor (pool):
File FileNameOpenFile(FileName fileName,
int fileFlags, int fileMode);
// open a file in the database directory ($PGDATA/base/…)
File OpenTemporaryFile(bool interXact);
// open temp file; flag: close at end of transaction?
void FileClose(File file);
void FileUnlink(File file);
intFileRead(File file, char *buffer, int amount);
intFileWrite(File file, char *buffer, int amount);
intFileSync(File file);
long FileSeek(File file, long offset, int whence);
intFileTruncate(File file, long offset);
Analogous to Unix syscalls open(), close(), read(), write(), lseek(), …
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❖ File Descriptor Pool (cont)
Virtual file descriptors (Vfd)
•physically stored in dynamically-allocated array

•also arranged into list by recency-of-use

VfdCache[0] holds list head/tail pointers.
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❖ File Descriptor Pool (cont)
Virtual file descriptor records (simplified):
typedef struct vfd
{
s_shortfd;// current FD, or VFD_CLOSED if none
u_shortfdstate; // bitflags for VFD’s state
File nextFree;// link to next free VFD, if in freelist
File lruMoreRecently; // doubly linked recency-of-use list
File lruLessRecently;
long seekPos; // current logical file position
char *fileName; // name of file, or NULL for unused VFD
// NB: fileName is malloc’d, and must be free’d when closing the VFD
intfileFlags; // open(2) flags for (re)opening the file
intfileMode;// mode to pass to open(2)
} Vfd;
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❖ File Manager
Reminder: PostgreSQL file organisation

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❖ File Manager (cont)
PostgreSQL stores each table
•in the directory PGDATA/pg_database.oid
•often in multiple files (aka forks)

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❖ File Manager (cont)
Data files (Oid, Oid.1, …):
•sequence of fixed-size blocks/pages (typically 8KB)
•each page contains tuple data and admin data (see later)
•max size of data files 1GB (Unix limitation)

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❖ File Manager (cont)
Free space map (Oid_fsm):
•indicates where free space is in data pages
•“free” space is only free after VACUUM
(DELETE simply marks tuples as no longer in use xmax)
Visibility map (Oid_vm):
•indicates pages where all tuples are “visible”
(visible = accessible to all currently active transactions)
•such pages can be ignored by VACUUM
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❖ File Manager (cont)
The “magnetic disk storage manager” (storage/smgr/md.c)
•manages its own pool of open file descriptors (Vfd’s)
•may use several Vfd’s to access data, if several forks
•manages mapping from PageID to file+offset.
PostgreSQL PageID values are structured:
typedef struct
{
RelFileNode rnode;// which relation/file
ForkNumberforkNum;// which fork (of reln)
BlockNumber blockNum; // which page/block
} BufferTag;
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❖ File Manager (cont)
Access to a block of data proceeds (roughly) as follows:
// pageID set from pg_catalog tables
// buffer obtained from Buffer pool
getBlock(BufferTag pageID, Buffer buf)
{
Vfd vf;off_t offset;
(vf, offset) = findBlock(pageID)
lseek(vf.fd, offset, SEEK_SET)
vf.seekPos = offset;
nread = read(vf.fd, buf, BLOCKSIZE)
if (nread < BLOCKSIZE) … we have a problem}BLOCKSIZE is a global configurable constant (default: 8192) COMP9315 21T1 ♢ PG File Manager ♢ [14/15]<< ∧ ❖ File Manager (cont)findBlock(BufferTag pageID) returns (Vfd, off_t){ offset = pageID.blockNum * BLOCKSIZE fileName = relpath(pageID.rnode) if (pageID.forkNum > 0)
fileName = fileName+”.”+pageID.forkNum
if (fileName is not in Vfd pool)
fd = allocate new Vfd for fileName
else
fd = use Vfd from pool
if (pageID.forkNum > 0) {
offset = offset – (pageID.forkNum*MAXFILESIZE)
}
return (fd, offset)
}
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Produced: 28 Feb 2021
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