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Sourcecode: postgresql-8.4 version File versions  Download package

buffile.c
/*-------------------------------------------------------------------------
 *
 * buffile.c
 *      Management of large buffered files, primarily temporary files.
 *
 * Portions Copyright (c) 1996-2009, PostgreSQL Global Development Group
 * Portions Copyright (c) 1994, Regents of the University of California
 *
 * IDENTIFICATION
 *      $PostgreSQL: pgsql/src/backend/storage/file/buffile.c,v 1.34 2009/06/11 14:49:01 momjian Exp $
 *
 * NOTES:
 *
 * BufFiles provide a very incomplete emulation of stdio atop virtual Files
 * (as managed by fd.c).  Currently, we only support the buffered-I/O
 * aspect of stdio: a read or write of the low-level File occurs only
 * when the buffer is filled or emptied.  This is an even bigger win
 * for virtual Files than for ordinary kernel files, since reducing the
 * frequency with which a virtual File is touched reduces "thrashing"
 * of opening/closing file descriptors.
 *
 * Note that BufFile structs are allocated with palloc(), and therefore
 * will go away automatically at transaction end.  If the underlying
 * virtual File is made with OpenTemporaryFile, then all resources for
 * the file are certain to be cleaned up even if processing is aborted
 * by ereport(ERROR).   To avoid confusion, the caller should take care that
 * all calls for a single BufFile are made in the same palloc context.
 *
 * BufFile also supports temporary files that exceed the OS file size limit
 * (by opening multiple fd.c temporary files).  This is an essential feature
 * for sorts and hashjoins on large amounts of data.
 *-------------------------------------------------------------------------
 */

#include "postgres.h"

#include "storage/fd.h"
#include "storage/buffile.h"
#include "storage/buf_internals.h"

/*
 * We break BufFiles into gigabyte-sized segments, regardless of RELSEG_SIZE.
 * The reason is that we'd like large temporary BufFiles to be spread across
 * multiple tablespaces when available.
 */
#define MAX_PHYSICAL_FILESIZE 0x40000000
#define BUFFILE_SEG_SIZE            (MAX_PHYSICAL_FILESIZE / BLCKSZ)

/*
 * This data structure represents a buffered file that consists of one or
 * more physical files (each accessed through a virtual file descriptor
 * managed by fd.c).
 */
00054 struct BufFile
{
      int               numFiles;         /* number of physical files in set */
      /* all files except the last have length exactly MAX_PHYSICAL_FILESIZE */
      File     *files;              /* palloc'd array with numFiles entries */
      off_t    *offsets;            /* palloc'd array with numFiles entries */

      /*
       * offsets[i] is the current seek position of files[i].  We use this to
       * avoid making redundant FileSeek calls.
       */

      bool        isTemp;                 /* can only add files if this is TRUE */
      bool        isInterXact;      /* keep open over transactions? */
      bool        dirty;                  /* does buffer need to be written? */

      /*
       * "current pos" is position of start of buffer within the logical file.
       * Position as seen by user of BufFile is (curFile, curOffset + pos).
       */
      int               curFile;          /* file index (0..n) part of current pos */
      off_t       curOffset;        /* offset part of current pos */
      int               pos;              /* next read/write position in buffer */
      int               nbytes;                 /* total # of valid bytes in buffer */
      char        buffer[BLCKSZ];
};

static BufFile *makeBufFile(File firstfile);
static void extendBufFile(BufFile *file);
static void BufFileLoadBuffer(BufFile *file);
static void BufFileDumpBuffer(BufFile *file);
static int  BufFileFlush(BufFile *file);


/*
 * Create a BufFile given the first underlying physical file.
 * NOTE: caller must set isTemp and isInterXact if appropriate.
 */
static BufFile *
makeBufFile(File firstfile)
{
      BufFile    *file = (BufFile *) palloc(sizeof(BufFile));

      file->numFiles = 1;
      file->files = (File *) palloc(sizeof(File));
      file->files[0] = firstfile;
      file->offsets = (off_t *) palloc(sizeof(off_t));
      file->offsets[0] = 0L;
      file->isTemp = false;
      file->isInterXact = false;
      file->dirty = false;
      file->curFile = 0;
      file->curOffset = 0L;
      file->pos = 0;
      file->nbytes = 0;

      return file;
}

/*
 * Add another component temp file.
 */
static void
extendBufFile(BufFile *file)
{
      File        pfile;

      Assert(file->isTemp);
      pfile = OpenTemporaryFile(file->isInterXact);
      Assert(pfile >= 0);

      file->files = (File *) repalloc(file->files,
                                                      (file->numFiles + 1) * sizeof(File));
      file->offsets = (off_t *) repalloc(file->offsets,
                                                         (file->numFiles + 1) * sizeof(off_t));
      file->files[file->numFiles] = pfile;
      file->offsets[file->numFiles] = 0L;
      file->numFiles++;
}

/*
 * Create a BufFile for a new temporary file (which will expand to become
 * multiple temporary files if more than MAX_PHYSICAL_FILESIZE bytes are
 * written to it).
 *
 * If interXact is true, the temp file will not be automatically deleted
 * at end of transaction.
 *
 * Note: if interXact is true, the caller had better be calling us in a
 * memory context that will survive across transaction boundaries.
 */
BufFile *
BufFileCreateTemp(bool interXact)
{
      BufFile    *file;
      File        pfile;

      pfile = OpenTemporaryFile(interXact);
      Assert(pfile >= 0);

      file = makeBufFile(pfile);
      file->isTemp = true;
      file->isInterXact = interXact;

      return file;
}

#ifdef NOT_USED
/*
 * Create a BufFile and attach it to an already-opened virtual File.
 *
 * This is comparable to fdopen() in stdio.  This is the only way at present
 * to attach a BufFile to a non-temporary file.  Note that BufFiles created
 * in this way CANNOT be expanded into multiple files.
 */
BufFile *
BufFileCreate(File file)
{
      return makeBufFile(file);
}
#endif

/*
 * Close a BufFile
 *
 * Like fclose(), this also implicitly FileCloses the underlying File.
 */
void
BufFileClose(BufFile *file)
{
      int               i;

      /* flush any unwritten data */
      BufFileFlush(file);
      /* close the underlying file(s) (with delete if it's a temp file) */
      for (i = 0; i < file->numFiles; i++)
            FileClose(file->files[i]);
      /* release the buffer space */
      pfree(file->files);
      pfree(file->offsets);
      pfree(file);
}

/*
 * BufFileLoadBuffer
 *
 * Load some data into buffer, if possible, starting from curOffset.
 * At call, must have dirty = false, pos and nbytes = 0.
 * On exit, nbytes is number of bytes loaded.
 */
static void
BufFileLoadBuffer(BufFile *file)
{
      File        thisfile;

      /*
       * Advance to next component file if necessary and possible.
       *
       * This path can only be taken if there is more than one component, so it
       * won't interfere with reading a non-temp file that is over
       * MAX_PHYSICAL_FILESIZE.
       */
      if (file->curOffset >= MAX_PHYSICAL_FILESIZE &&
            file->curFile + 1 < file->numFiles)
      {
            file->curFile++;
            file->curOffset = 0L;
      }

      /*
       * May need to reposition physical file.
       */
      thisfile = file->files[file->curFile];
      if (file->curOffset != file->offsets[file->curFile])
      {
            if (FileSeek(thisfile, file->curOffset, SEEK_SET) != file->curOffset)
                  return;                       /* seek failed, read nothing */
            file->offsets[file->curFile] = file->curOffset;
      }

      /*
       * Read whatever we can get, up to a full bufferload.
       */
      file->nbytes = FileRead(thisfile, file->buffer, sizeof(file->buffer));
      if (file->nbytes < 0)
            file->nbytes = 0;
      file->offsets[file->curFile] += file->nbytes;
      /* we choose not to advance curOffset here */

      BufFileReadCount++;
}

/*
 * BufFileDumpBuffer
 *
 * Dump buffer contents starting at curOffset.
 * At call, should have dirty = true, nbytes > 0.
 * On exit, dirty is cleared if successful write, and curOffset is advanced.
 */
static void
BufFileDumpBuffer(BufFile *file)
{
      int               wpos = 0;
      int               bytestowrite;
      File        thisfile;

      /*
       * Unlike BufFileLoadBuffer, we must dump the whole buffer even if it
       * crosses a component-file boundary; so we need a loop.
       */
      while (wpos < file->nbytes)
      {
            /*
             * Advance to next component file if necessary and possible.
             */
            if (file->curOffset >= MAX_PHYSICAL_FILESIZE && file->isTemp)
            {
                  while (file->curFile + 1 >= file->numFiles)
                        extendBufFile(file);
                  file->curFile++;
                  file->curOffset = 0L;
            }

            /*
             * Enforce per-file size limit only for temp files, else just try to
             * write as much as asked...
             */
            bytestowrite = file->nbytes - wpos;
            if (file->isTemp)
            {
                  off_t       availbytes = MAX_PHYSICAL_FILESIZE - file->curOffset;

                  if ((off_t) bytestowrite > availbytes)
                        bytestowrite = (int) availbytes;
            }

            /*
             * May need to reposition physical file.
             */
            thisfile = file->files[file->curFile];
            if (file->curOffset != file->offsets[file->curFile])
            {
                  if (FileSeek(thisfile, file->curOffset, SEEK_SET) != file->curOffset)
                        return;                 /* seek failed, give up */
                  file->offsets[file->curFile] = file->curOffset;
            }
            bytestowrite = FileWrite(thisfile, file->buffer + wpos, bytestowrite);
            if (bytestowrite <= 0)
                  return;                       /* failed to write */
            file->offsets[file->curFile] += bytestowrite;
            file->curOffset += bytestowrite;
            wpos += bytestowrite;

            BufFileWriteCount++;
      }
      file->dirty = false;

      /*
       * At this point, curOffset has been advanced to the end of the buffer,
       * ie, its original value + nbytes.  We need to make it point to the
       * logical file position, ie, original value + pos, in case that is less
       * (as could happen due to a small backwards seek in a dirty buffer!)
       */
      file->curOffset -= (file->nbytes - file->pos);
      if (file->curOffset < 0)      /* handle possible segment crossing */
      {
            file->curFile--;
            Assert(file->curFile >= 0);
            file->curOffset += MAX_PHYSICAL_FILESIZE;
      }

      /*
       * Now we can set the buffer empty without changing the logical position
       */
      file->pos = 0;
      file->nbytes = 0;
}

/*
 * BufFileRead
 *
 * Like fread() except we assume 1-byte element size.
 */
size_t
BufFileRead(BufFile *file, void *ptr, size_t size)
{
      size_t            nread = 0;
      size_t            nthistime;

      if (file->dirty)
      {
            if (BufFileFlush(file) != 0)
                  return 0;               /* could not flush... */
            Assert(!file->dirty);
      }

      while (size > 0)
      {
            if (file->pos >= file->nbytes)
            {
                  /* Try to load more data into buffer. */
                  file->curOffset += file->pos;
                  file->pos = 0;
                  file->nbytes = 0;
                  BufFileLoadBuffer(file);
                  if (file->nbytes <= 0)
                        break;                  /* no more data available */
            }

            nthistime = file->nbytes - file->pos;
            if (nthistime > size)
                  nthistime = size;
            Assert(nthistime > 0);

            memcpy(ptr, file->buffer + file->pos, nthistime);

            file->pos += nthistime;
            ptr = (void *) ((char *) ptr + nthistime);
            size -= nthistime;
            nread += nthistime;
      }

      return nread;
}

/*
 * BufFileWrite
 *
 * Like fwrite() except we assume 1-byte element size.
 */
size_t
BufFileWrite(BufFile *file, void *ptr, size_t size)
{
      size_t            nwritten = 0;
      size_t            nthistime;

      while (size > 0)
      {
            if (file->pos >= BLCKSZ)
            {
                  /* Buffer full, dump it out */
                  if (file->dirty)
                  {
                        BufFileDumpBuffer(file);
                        if (file->dirty)
                              break;            /* I/O error */
                  }
                  else
                  {
                        /* Hmm, went directly from reading to writing? */
                        file->curOffset += file->pos;
                        file->pos = 0;
                        file->nbytes = 0;
                  }
            }

            nthistime = BLCKSZ - file->pos;
            if (nthistime > size)
                  nthistime = size;
            Assert(nthistime > 0);

            memcpy(file->buffer + file->pos, ptr, nthistime);

            file->dirty = true;
            file->pos += nthistime;
            if (file->nbytes < file->pos)
                  file->nbytes = file->pos;
            ptr = (void *) ((char *) ptr + nthistime);
            size -= nthistime;
            nwritten += nthistime;
      }

      return nwritten;
}

/*
 * BufFileFlush
 *
 * Like fflush()
 */
static int
BufFileFlush(BufFile *file)
{
      if (file->dirty)
      {
            BufFileDumpBuffer(file);
            if (file->dirty)
                  return EOF;
      }

      return 0;
}

/*
 * BufFileSeek
 *
 * Like fseek(), except that target position needs two values in order to
 * work when logical filesize exceeds maximum value representable by long.
 * We do not support relative seeks across more than LONG_MAX, however.
 *
 * Result is 0 if OK, EOF if not.  Logical position is not moved if an
 * impossible seek is attempted.
 */
int
BufFileSeek(BufFile *file, int fileno, off_t offset, int whence)
{
      int               newFile;
      off_t       newOffset;

      switch (whence)
      {
            case SEEK_SET:
                  if (fileno < 0)
                        return EOF;
                  newFile = fileno;
                  newOffset = offset;
                  break;
            case SEEK_CUR:

                  /*
                   * Relative seek considers only the signed offset, ignoring
                   * fileno. Note that large offsets (> 1 gig) risk overflow in this
                   * add, unless we have 64-bit off_t.
                   */
                  newFile = file->curFile;
                  newOffset = (file->curOffset + file->pos) + offset;
                  break;
#ifdef NOT_USED
            case SEEK_END:
                  /* could be implemented, not needed currently */
                  break;
#endif
            default:
                  elog(ERROR, "invalid whence: %d", whence);
                  return EOF;
      }
      while (newOffset < 0)
      {
            if (--newFile < 0)
                  return EOF;
            newOffset += MAX_PHYSICAL_FILESIZE;
      }
      if (newFile == file->curFile &&
            newOffset >= file->curOffset &&
            newOffset <= file->curOffset + file->nbytes)
      {
            /*
             * Seek is to a point within existing buffer; we can just adjust
             * pos-within-buffer, without flushing buffer.  Note this is OK
             * whether reading or writing, but buffer remains dirty if we were
             * writing.
             */
            file->pos = (int) (newOffset - file->curOffset);
            return 0;
      }
      /* Otherwise, must reposition buffer, so flush any dirty data */
      if (BufFileFlush(file) != 0)
            return EOF;

      /*
       * At this point and no sooner, check for seek past last segment. The
       * above flush could have created a new segment, so checking sooner would
       * not work (at least not with this code).
       */
      if (file->isTemp)
      {
            /* convert seek to "start of next seg" to "end of last seg" */
            if (newFile == file->numFiles && newOffset == 0)
            {
                  newFile--;
                  newOffset = MAX_PHYSICAL_FILESIZE;
            }
            while (newOffset > MAX_PHYSICAL_FILESIZE)
            {
                  if (++newFile >= file->numFiles)
                        return EOF;
                  newOffset -= MAX_PHYSICAL_FILESIZE;
            }
      }
      if (newFile >= file->numFiles)
            return EOF;
      /* Seek is OK! */
      file->curFile = newFile;
      file->curOffset = newOffset;
      file->pos = 0;
      file->nbytes = 0;
      return 0;
}

void
BufFileTell(BufFile *file, int *fileno, off_t *offset)
{
      *fileno = file->curFile;
      *offset = file->curOffset + file->pos;
}

/*
 * BufFileSeekBlock --- block-oriented seek
 *
 * Performs absolute seek to the start of the n'th BLCKSZ-sized block of
 * the file.  Note that users of this interface will fail if their files
 * exceed BLCKSZ * LONG_MAX bytes, but that is quite a lot; we don't work
 * with tables bigger than that, either...
 *
 * Result is 0 if OK, EOF if not.  Logical position is not moved if an
 * impossible seek is attempted.
 */
int
BufFileSeekBlock(BufFile *file, long blknum)
{
      return BufFileSeek(file,
                                 (int) (blknum / BUFFILE_SEG_SIZE),
                                 (off_t) (blknum % BUFFILE_SEG_SIZE) * BLCKSZ,
                                 SEEK_SET);
}

#ifdef NOT_USED
/*
 * BufFileTellBlock --- block-oriented tell
 *
 * Any fractional part of a block in the current seek position is ignored.
 */
long
BufFileTellBlock(BufFile *file)
{
      long        blknum;

      blknum = (file->curOffset + file->pos) / BLCKSZ;
      blknum += file->curFile * BUFFILE_SEG_SIZE;
      return blknum;
}

#endif

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