• PostgreSQL File Manager
• Relations as Files
• File Descriptor Pool
• File Manager

❖ PostgreSQL File Manager

PostgreSQL uses the following file organisation ...

❖ 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

❖ Relations as Files

PostgreSQL identifies relation files via their OIDs.

The core data structure for this is RelFileNode:

typedef struct RelFileNode {
    Oid  spcNode;  // tablespace
    Oid  dbNode;   // database
    Oid  relNode;  // relation
} RelFileNode;

Global (shared) tables (e.g. pg_database) have

•   spcNode == GLOBALTABLESPACE_OID
•   dbNode == 0

❖ 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;
}

❖ 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".

❖ 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);
int  FileRead(File file, char *buffer, int amount);
int  FileWrite(File file, char *buffer, int amount);
int  FileSync(File file);
long FileSeek(File file, long offset, int whence);
int  FileTruncate(File file, long offset);

Analogous to Unix syscalls open(), close(), read(), write(), lseek(), ...

❖ 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.

❖ File Descriptor Pool (cont)

Virtual file descriptor records (simplified):

typedef struct vfd
{
    s_short  fd;              // current FD, or VFD_CLOSED if none
    u_short  fdstate;         // 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
    int      fileFlags;       // open(2) flags for (re)opening the file
    int      fileMode;        // mode to pass to open(2)
} Vfd;

❖ File Manager

Reminder: PostgreSQL file organisation

❖ File Manager (cont)

PostgreSQL stores each table

• in the directory PGDATA/pg_database.oid
• often in multiple files (aka forks)

❖ 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)

❖ 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

❖ 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
    ForkNumber  forkNum;  // which fork (of reln)
    BlockNumber blockNum; // which page/block 
} BufferTag;

❖ 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)

❖ 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)
}