1 /*-------------------------------------------------------------------------
4 * Management of "logical tapes" within temporary files.
6 * This module exists to support sorting via multiple merge passes (see
7 * tuplesort.c). Merging is an ideal algorithm for tape devices, but if
8 * we implement it on disk by creating a separate file for each "tape",
9 * there is an annoying problem: the peak space usage is at least twice
10 * the volume of actual data to be sorted. (This must be so because each
11 * datum will appear in both the input and output tapes of the final
12 * merge pass. For seven-tape polyphase merge, which is otherwise a
13 * pretty good algorithm, peak usage is more like 4x actual data volume.)
15 * We can work around this problem by recognizing that any one tape
16 * dataset (with the possible exception of the final output) is written
17 * and read exactly once in a perfectly sequential manner. Therefore,
18 * a datum once read will not be required again, and we can recycle its
19 * space for use by the new tape dataset(s) being generated. In this way,
20 * the total space usage is essentially just the actual data volume, plus
21 * insignificant bookkeeping and start/stop overhead.
23 * Few OSes allow arbitrary parts of a file to be released back to the OS,
24 * so we have to implement this space-recycling ourselves within a single
25 * logical file. logtape.c exists to perform this bookkeeping and provide
26 * the illusion of N independent tape devices to tuplesort.c. Note that
27 * logtape.c itself depends on buffile.c to provide a "logical file" of
28 * larger size than the underlying OS may support.
30 * For simplicity, we allocate and release space in the underlying file
31 * in BLCKSZ-size blocks. Space allocation boils down to keeping track
32 * of which blocks in the underlying file belong to which logical tape,
33 * plus any blocks that are free (recycled and not yet reused). Normally
34 * there are not very many free blocks, so we just keep those in a list.
35 * The blocks in each logical tape are remembered using a method borrowed
36 * from the Unix HFS filesystem: we store data block numbers in an
37 * "indirect block". If an indirect block fills up, we write it out to
38 * the underlying file and remember its location in a second-level indirect
39 * block. In the same way second-level blocks are remembered in third-
40 * level blocks, and so on if necessary (of course we're talking huge
41 * amounts of data here). The topmost indirect block of a given logical
42 * tape is never actually written out to the physical file, but all lower-
43 * level indirect blocks will be.
45 * The initial write pass is guaranteed to fill the underlying file
46 * perfectly sequentially, no matter how data is divided into logical tapes.
47 * Once we begin merge passes, the access pattern becomes considerably
48 * less predictable --- but the seeking involved should be comparable to
49 * what would happen if we kept each logical tape in a separate file,
50 * so there's no serious performance penalty paid to obtain the space
51 * savings of recycling. We try to localize the write accesses by always
52 * writing to the lowest-numbered free block when we have a choice; it's
53 * not clear this helps much, but it can't hurt. (XXX perhaps a LIFO
54 * policy for free blocks would be better?)
56 * Since all the bookkeeping and buffer memory is allocated with palloc(),
57 * and the underlying file(s) are made with OpenTemporaryFile, all resources
58 * for a logical tape set are certain to be cleaned up even if processing
59 * is aborted by ereport(ERROR). To avoid confusion, the caller should take
60 * care that all calls for a single LogicalTapeSet are made in the same
63 * Portions Copyright (c) 1996-2003, PostgreSQL Global Development Group
64 * Portions Copyright (c) 1994, Regents of the University of California
67 * $PostgreSQL: pgsql/src/backend/utils/sort/logtape.c,v 1.13 2003/11/29 19:52:04 pgsql Exp $
69 *-------------------------------------------------------------------------
74 #include "storage/buffile.h"
75 #include "utils/logtape.h"
78 * Block indexes are "long"s, so we can fit this many per indirect block.
79 * NB: we assume this is an exact fit!
81 #define BLOCKS_PER_INDIR_BLOCK ((int) (BLCKSZ / sizeof(long)))
84 * We use a struct like this for each active indirection level of each
85 * logical tape. If the indirect block is not the highest level of its
86 * tape, the "nextup" link points to the next higher level. Only the
87 * "ptrs" array is written out if we have to dump the indirect block to
88 * disk. If "ptrs" is not completely full, we store -1L in the first
89 * unused slot at completion of the write phase for the logical tape.
91 typedef struct IndirectBlock
93 int nextSlot; /* next pointer slot to write or read */
94 struct IndirectBlock *nextup; /* parent indirect level, or NULL
96 long ptrs[BLOCKS_PER_INDIR_BLOCK]; /* indexes of contained
101 * This data structure represents a single "logical tape" within the set
102 * of logical tapes stored in the same file. We must keep track of the
103 * current partially-read-or-written data block as well as the active
104 * indirect block level(s).
106 typedef struct LogicalTape
108 IndirectBlock *indirect; /* bottom of my indirect-block hierarchy */
109 bool writing; /* T while in write phase */
110 bool frozen; /* T if blocks should not be freed when
112 bool dirty; /* does buffer need to be written? */
115 * The total data volume in the logical tape is numFullBlocks * BLCKSZ
116 * + lastBlockBytes. BUT: we do not update lastBlockBytes during
117 * writing, only at completion of a write phase.
119 long numFullBlocks; /* number of complete blocks in log tape */
120 int lastBlockBytes; /* valid bytes in last (incomplete) block */
123 * Buffer for current data block. Note we don't bother to store the
124 * actual file block number of the data block (during the write phase
125 * it hasn't been assigned yet, and during read we don't care
126 * anymore). But we do need the relative block number so we can detect
127 * end-of-tape while reading.
129 long curBlockNumber; /* this block's logical blk# within tape */
130 int pos; /* next read/write position in buffer */
131 int nbytes; /* total # of valid bytes in buffer */
136 * This data structure represents a set of related "logical tapes" sharing
137 * space in a single underlying file. (But that "file" may be multiple files
138 * if needed to escape OS limits on file size; buffile.c handles that for us.)
139 * The number of tapes is fixed at creation.
141 struct LogicalTapeSet
143 BufFile *pfile; /* underlying file for whole tape set */
144 long nFileBlocks; /* # of blocks used in underlying file */
147 * We store the numbers of recycled-and-available blocks in
148 * freeBlocks[]. When there are no such blocks, we extend the
149 * underlying file. Note that the block numbers in freeBlocks are
150 * always in *decreasing* order, so that removing the last entry gives
151 * us the lowest free block.
153 long *freeBlocks; /* resizable array */
154 int nFreeBlocks; /* # of currently free blocks */
155 int freeBlocksLen; /* current allocated length of
159 * tapes[] is declared size 1 since C wants a fixed size, but actually
160 * it is of length nTapes.
162 int nTapes; /* # of logical tapes in set */
163 LogicalTape *tapes[1]; /* must be last in struct! */
166 static void ltsWriteBlock(LogicalTapeSet *lts, long blocknum, void *buffer);
167 static void ltsReadBlock(LogicalTapeSet *lts, long blocknum, void *buffer);
168 static long ltsGetFreeBlock(LogicalTapeSet *lts);
169 static void ltsReleaseBlock(LogicalTapeSet *lts, long blocknum);
170 static void ltsRecordBlockNum(LogicalTapeSet *lts, IndirectBlock *indirect,
172 static long ltsRewindIndirectBlock(LogicalTapeSet *lts,
173 IndirectBlock *indirect,
175 static long ltsRewindFrozenIndirectBlock(LogicalTapeSet *lts,
176 IndirectBlock *indirect);
177 static long ltsRecallNextBlockNum(LogicalTapeSet *lts,
178 IndirectBlock *indirect,
180 static long ltsRecallPrevBlockNum(LogicalTapeSet *lts,
181 IndirectBlock *indirect);
182 static void ltsDumpBuffer(LogicalTapeSet *lts, LogicalTape *lt);
186 * Write a block-sized buffer to the specified block of the underlying file.
188 * NB: should not attempt to write beyond current end of file (ie, create
189 * "holes" in file), since BufFile doesn't allow that. The first write pass
190 * must write blocks sequentially.
192 * No need for an error return convention; we ereport() on any error.
195 ltsWriteBlock(LogicalTapeSet *lts, long blocknum, void *buffer)
197 if (BufFileSeekBlock(lts->pfile, blocknum) != 0 ||
198 BufFileWrite(lts->pfile, buffer, BLCKSZ) != BLCKSZ)
200 /* XXX is it okay to assume errno is correct? */
201 (errcode_for_file_access(),
202 errmsg("could not write block %ld of temporary file: %m",
204 errhint("Perhaps out of disk space?")));
208 * Read a block-sized buffer from the specified block of the underlying file.
210 * No need for an error return convention; we ereport() on any error. This
211 * module should never attempt to read a block it doesn't know is there.
214 ltsReadBlock(LogicalTapeSet *lts, long blocknum, void *buffer)
216 if (BufFileSeekBlock(lts->pfile, blocknum) != 0 ||
217 BufFileRead(lts->pfile, buffer, BLCKSZ) != BLCKSZ)
219 /* XXX is it okay to assume errno is correct? */
220 (errcode_for_file_access(),
221 errmsg("could not read block %ld of temporary file: %m",
226 * Select a currently unused block for writing to.
228 * NB: should only be called when writer is ready to write immediately,
229 * to ensure that first write pass is sequential.
232 ltsGetFreeBlock(LogicalTapeSet *lts)
235 * If there are multiple free blocks, we select the one appearing last
236 * in freeBlocks[]. If there are none, assign the next block at the
239 if (lts->nFreeBlocks > 0)
240 return lts->freeBlocks[--lts->nFreeBlocks];
242 return lts->nFileBlocks++;
246 * Return a block# to the freelist.
249 ltsReleaseBlock(LogicalTapeSet *lts, long blocknum)
255 * Enlarge freeBlocks array if full.
257 if (lts->nFreeBlocks >= lts->freeBlocksLen)
259 lts->freeBlocksLen *= 2;
260 lts->freeBlocks = (long *) repalloc(lts->freeBlocks,
261 lts->freeBlocksLen * sizeof(long));
265 * Insert blocknum into array, preserving decreasing order (so that
266 * ltsGetFreeBlock returns the lowest available block number). This
267 * could get fairly slow if there were many free blocks, but we don't
268 * expect there to be very many at one time.
270 ndx = lts->nFreeBlocks++;
271 ptr = lts->freeBlocks + ndx;
272 while (ndx > 0 && ptr[-1] < blocknum)
281 * These routines manipulate indirect-block hierarchies. All are recursive
282 * so that they don't have any specific limit on the depth of hierarchy.
286 * Record a data block number in a logical tape's lowest indirect block,
287 * or record an indirect block's number in the next higher indirect level.
290 ltsRecordBlockNum(LogicalTapeSet *lts, IndirectBlock *indirect,
293 if (indirect->nextSlot >= BLOCKS_PER_INDIR_BLOCK)
296 * This indirect block is full, so dump it out and recursively
297 * save its address in the next indirection level. Create a new
298 * indirection level if there wasn't one before.
300 long indirblock = ltsGetFreeBlock(lts);
302 ltsWriteBlock(lts, indirblock, (void *) indirect->ptrs);
303 if (indirect->nextup == NULL)
305 indirect->nextup = (IndirectBlock *) palloc(sizeof(IndirectBlock));
306 indirect->nextup->nextSlot = 0;
307 indirect->nextup->nextup = NULL;
309 ltsRecordBlockNum(lts, indirect->nextup, indirblock);
312 * Reset to fill another indirect block at this level.
314 indirect->nextSlot = 0;
316 indirect->ptrs[indirect->nextSlot++] = blocknum;
320 * Reset a logical tape's indirect-block hierarchy after a write pass
321 * to prepare for reading. We dump out partly-filled blocks except
322 * at the top of the hierarchy, and we rewind each level to the start.
323 * This call returns the first data block number, or -1L if the tape
326 * Unless 'freezing' is true, release indirect blocks to the free pool after
330 ltsRewindIndirectBlock(LogicalTapeSet *lts,
331 IndirectBlock *indirect,
334 /* Insert sentinel if block is not full */
335 if (indirect->nextSlot < BLOCKS_PER_INDIR_BLOCK)
336 indirect->ptrs[indirect->nextSlot] = -1L;
339 * If block is not topmost, write it out, and recurse to obtain
340 * address of first block in this hierarchy level. Read that one in.
342 if (indirect->nextup != NULL)
344 long indirblock = ltsGetFreeBlock(lts);
346 ltsWriteBlock(lts, indirblock, (void *) indirect->ptrs);
347 ltsRecordBlockNum(lts, indirect->nextup, indirblock);
348 indirblock = ltsRewindIndirectBlock(lts, indirect->nextup, freezing);
349 Assert(indirblock != -1L);
350 ltsReadBlock(lts, indirblock, (void *) indirect->ptrs);
352 ltsReleaseBlock(lts, indirblock);
356 * Reset my next-block pointer, and then fetch a block number if any.
358 indirect->nextSlot = 0;
359 if (indirect->ptrs[0] == -1L)
361 return indirect->ptrs[indirect->nextSlot++];
365 * Rewind a previously-frozen indirect-block hierarchy for another read pass.
366 * This call returns the first data block number, or -1L if the tape
370 ltsRewindFrozenIndirectBlock(LogicalTapeSet *lts,
371 IndirectBlock *indirect)
374 * If block is not topmost, recurse to obtain address of first block
375 * in this hierarchy level. Read that one in.
377 if (indirect->nextup != NULL)
381 indirblock = ltsRewindFrozenIndirectBlock(lts, indirect->nextup);
382 Assert(indirblock != -1L);
383 ltsReadBlock(lts, indirblock, (void *) indirect->ptrs);
387 * Reset my next-block pointer, and then fetch a block number if any.
389 indirect->nextSlot = 0;
390 if (indirect->ptrs[0] == -1L)
392 return indirect->ptrs[indirect->nextSlot++];
396 * Obtain next data block number in the forward direction, or -1L if no more.
398 * Unless 'frozen' is true, release indirect blocks to the free pool after
402 ltsRecallNextBlockNum(LogicalTapeSet *lts,
403 IndirectBlock *indirect,
406 if (indirect->nextSlot >= BLOCKS_PER_INDIR_BLOCK ||
407 indirect->ptrs[indirect->nextSlot] == -1L)
411 if (indirect->nextup == NULL)
412 return -1L; /* nothing left at this level */
413 indirblock = ltsRecallNextBlockNum(lts, indirect->nextup, frozen);
414 if (indirblock == -1L)
415 return -1L; /* nothing left at this level */
416 ltsReadBlock(lts, indirblock, (void *) indirect->ptrs);
418 ltsReleaseBlock(lts, indirblock);
419 indirect->nextSlot = 0;
421 if (indirect->ptrs[indirect->nextSlot] == -1L)
423 return indirect->ptrs[indirect->nextSlot++];
427 * Obtain next data block number in the reverse direction, or -1L if no more.
429 * Note this fetches the block# before the one last returned, no matter which
430 * direction of call returned that one. If we fail, no change in state.
432 * This routine can only be used in 'frozen' state, so there's no need to
433 * pass a parameter telling whether to release blocks ... we never do.
436 ltsRecallPrevBlockNum(LogicalTapeSet *lts,
437 IndirectBlock *indirect)
439 if (indirect->nextSlot <= 1)
443 if (indirect->nextup == NULL)
444 return -1L; /* nothing left at this level */
445 indirblock = ltsRecallPrevBlockNum(lts, indirect->nextup);
446 if (indirblock == -1L)
447 return -1L; /* nothing left at this level */
448 ltsReadBlock(lts, indirblock, (void *) indirect->ptrs);
451 * The previous block would only have been written out if full, so
452 * we need not search it for a -1 sentinel.
454 indirect->nextSlot = BLOCKS_PER_INDIR_BLOCK + 1;
456 indirect->nextSlot--;
457 return indirect->ptrs[indirect->nextSlot - 1];
462 * Create a set of logical tapes in a temporary underlying file.
464 * Each tape is initialized in write state.
467 LogicalTapeSetCreate(int ntapes)
474 * Create top-level struct. First LogicalTape pointer is already
475 * counted in sizeof(LogicalTapeSet).
478 lts = (LogicalTapeSet *) palloc(sizeof(LogicalTapeSet) +
479 (ntapes - 1) *sizeof(LogicalTape *));
480 lts->pfile = BufFileCreateTemp(false);
481 lts->nFileBlocks = 0L;
482 lts->freeBlocksLen = 32; /* reasonable initial guess */
483 lts->freeBlocks = (long *) palloc(lts->freeBlocksLen * sizeof(long));
484 lts->nFreeBlocks = 0;
485 lts->nTapes = ntapes;
488 * Create per-tape structs, including first-level indirect blocks.
490 for (i = 0; i < ntapes; i++)
492 lt = (LogicalTape *) palloc(sizeof(LogicalTape));
494 lt->indirect = (IndirectBlock *) palloc(sizeof(IndirectBlock));
495 lt->indirect->nextSlot = 0;
496 lt->indirect->nextup = NULL;
500 lt->numFullBlocks = 0L;
501 lt->lastBlockBytes = 0;
502 lt->curBlockNumber = 0L;
510 * Close a logical tape set and release all resources.
513 LogicalTapeSetClose(LogicalTapeSet *lts)
520 BufFileClose(lts->pfile);
521 for (i = 0; i < lts->nTapes; i++)
524 for (ib = lt->indirect; ib != NULL; ib = nextib)
531 pfree(lts->freeBlocks);
536 * Dump the dirty buffer of a logical tape.
539 ltsDumpBuffer(LogicalTapeSet *lts, LogicalTape *lt)
541 long datablock = ltsGetFreeBlock(lts);
544 ltsWriteBlock(lts, datablock, (void *) lt->buffer);
545 ltsRecordBlockNum(lts, lt->indirect, datablock);
547 /* Caller must do other state update as needed */
551 * Write to a logical tape.
553 * There are no error returns; we ereport() on failure.
556 LogicalTapeWrite(LogicalTapeSet *lts, int tapenum,
557 void *ptr, size_t size)
562 Assert(tapenum >= 0 && tapenum < lts->nTapes);
563 lt = lts->tapes[tapenum];
568 if (lt->pos >= BLCKSZ)
570 /* Buffer full, dump it out */
572 ltsDumpBuffer(lts, lt);
575 /* Hmm, went directly from reading to writing? */
576 elog(ERROR, "invalid logtape state: should be dirty");
579 lt->curBlockNumber++;
584 nthistime = BLCKSZ - lt->pos;
585 if (nthistime > size)
587 Assert(nthistime > 0);
589 memcpy(lt->buffer + lt->pos, ptr, nthistime);
592 lt->pos += nthistime;
593 if (lt->nbytes < lt->pos)
594 lt->nbytes = lt->pos;
595 ptr = (void *) ((char *) ptr + nthistime);
601 * Rewind logical tape and switch from writing to reading or vice versa.
603 * Unless the tape has been "frozen" in read state, forWrite must be the
604 * opposite of the previous tape state.
607 LogicalTapeRewind(LogicalTapeSet *lts, int tapenum, bool forWrite)
612 Assert(tapenum >= 0 && tapenum < lts->nTapes);
613 lt = lts->tapes[tapenum];
620 * Completion of a write phase. Flush last partial data
621 * block, flush any partial indirect blocks, rewind for normal
622 * (destructive) read.
625 ltsDumpBuffer(lts, lt);
626 lt->lastBlockBytes = lt->nbytes;
628 datablocknum = ltsRewindIndirectBlock(lts, lt->indirect, false);
633 * This is only OK if tape is frozen; we rewind for (another)
637 datablocknum = ltsRewindFrozenIndirectBlock(lts, lt->indirect);
639 /* Read the first block, or reset if tape is empty */
640 lt->curBlockNumber = 0L;
643 if (datablocknum != -1L)
645 ltsReadBlock(lts, datablocknum, (void *) lt->buffer);
647 ltsReleaseBlock(lts, datablocknum);
648 lt->nbytes = (lt->curBlockNumber < lt->numFullBlocks) ?
649 BLCKSZ : lt->lastBlockBytes;
655 * Completion of a read phase. Rewind and prepare for write.
657 * NOTE: we assume the caller has read the tape to the end; otherwise
658 * untouched data and indirect blocks will not have been freed. We
659 * could add more code to free any unread blocks, but in current
660 * usage of this module it'd be useless code.
665 Assert(!lt->writing && !lt->frozen);
666 /* Must truncate the indirect-block hierarchy down to one level. */
667 for (ib = lt->indirect->nextup; ib != NULL; ib = nextib)
672 lt->indirect->nextSlot = 0;
673 lt->indirect->nextup = NULL;
676 lt->numFullBlocks = 0L;
677 lt->lastBlockBytes = 0;
678 lt->curBlockNumber = 0L;
685 * Read from a logical tape.
687 * Early EOF is indicated by return value less than #bytes requested.
690 LogicalTapeRead(LogicalTapeSet *lts, int tapenum,
691 void *ptr, size_t size)
697 Assert(tapenum >= 0 && tapenum < lts->nTapes);
698 lt = lts->tapes[tapenum];
699 Assert(!lt->writing);
703 if (lt->pos >= lt->nbytes)
705 /* Try to load more data into buffer. */
706 long datablocknum = ltsRecallNextBlockNum(lts, lt->indirect,
709 if (datablocknum == -1L)
711 lt->curBlockNumber++;
713 ltsReadBlock(lts, datablocknum, (void *) lt->buffer);
715 ltsReleaseBlock(lts, datablocknum);
716 lt->nbytes = (lt->curBlockNumber < lt->numFullBlocks) ?
717 BLCKSZ : lt->lastBlockBytes;
719 break; /* EOF (possible here?) */
722 nthistime = lt->nbytes - lt->pos;
723 if (nthistime > size)
725 Assert(nthistime > 0);
727 memcpy(ptr, lt->buffer + lt->pos, nthistime);
729 lt->pos += nthistime;
730 ptr = (void *) ((char *) ptr + nthistime);
739 * "Freeze" the contents of a tape so that it can be read multiple times
740 * and/or read backwards. Once a tape is frozen, its contents will not
741 * be released until the LogicalTapeSet is destroyed. This is expected
742 * to be used only for the final output pass of a merge.
744 * This *must* be called just at the end of a write pass, before the
745 * tape is rewound (after rewind is too late!). It performs a rewind
746 * and switch to read mode "for free". An immediately following rewind-
747 * for-read call is OK but not necessary.
750 LogicalTapeFreeze(LogicalTapeSet *lts, int tapenum)
755 Assert(tapenum >= 0 && tapenum < lts->nTapes);
756 lt = lts->tapes[tapenum];
760 * Completion of a write phase. Flush last partial data block, flush
761 * any partial indirect blocks, rewind for nondestructive read.
764 ltsDumpBuffer(lts, lt);
765 lt->lastBlockBytes = lt->nbytes;
768 datablocknum = ltsRewindIndirectBlock(lts, lt->indirect, true);
769 /* Read the first block, or reset if tape is empty */
770 lt->curBlockNumber = 0L;
773 if (datablocknum != -1L)
775 ltsReadBlock(lts, datablocknum, (void *) lt->buffer);
776 lt->nbytes = (lt->curBlockNumber < lt->numFullBlocks) ?
777 BLCKSZ : lt->lastBlockBytes;
782 * Backspace the tape a given number of bytes. (We also support a more
783 * general seek interface, see below.)
785 * *Only* a frozen-for-read tape can be backed up; we don't support
786 * random access during write, and an unfrozen read tape may have
787 * already discarded the desired data!
789 * Return value is TRUE if seek successful, FALSE if there isn't that much
790 * data before the current point (in which case there's no state change).
793 LogicalTapeBackspace(LogicalTapeSet *lts, int tapenum, size_t size)
799 Assert(tapenum >= 0 && tapenum < lts->nTapes);
800 lt = lts->tapes[tapenum];
804 * Easy case for seek within current block.
806 if (size <= (size_t) lt->pos)
808 lt->pos -= (int) size;
813 * Not-so-easy case. Figure out whether it's possible at all.
815 size -= (size_t) lt->pos; /* part within this block */
816 nblocks = size / BLCKSZ;
817 size = size % BLCKSZ;
821 newpos = (int) (BLCKSZ - size);
825 if (nblocks > lt->curBlockNumber)
826 return false; /* a seek too far... */
829 * OK, we need to back up nblocks blocks. This implementation would
830 * be pretty inefficient for long seeks, but we really aren't
831 * expecting that (a seek over one tuple is typical).
833 while (nblocks-- > 0)
835 long datablocknum = ltsRecallPrevBlockNum(lts, lt->indirect);
837 if (datablocknum == -1L)
838 elog(ERROR, "unexpected end of tape");
839 lt->curBlockNumber--;
842 ltsReadBlock(lts, datablocknum, (void *) lt->buffer);
851 * Seek to an arbitrary position in a logical tape.
853 * *Only* a frozen-for-read tape can be seeked.
855 * Return value is TRUE if seek successful, FALSE if there isn't that much
856 * data in the tape (in which case there's no state change).
859 LogicalTapeSeek(LogicalTapeSet *lts, int tapenum,
860 long blocknum, int offset)
864 Assert(tapenum >= 0 && tapenum < lts->nTapes);
865 lt = lts->tapes[tapenum];
867 Assert(offset >= 0 && offset <= BLCKSZ);
870 * Easy case for seek within current block.
872 if (blocknum == lt->curBlockNumber && offset <= lt->nbytes)
879 * Not-so-easy case. Figure out whether it's possible at all.
881 if (blocknum < 0 || blocknum > lt->numFullBlocks ||
882 (blocknum == lt->numFullBlocks && offset > lt->lastBlockBytes))
886 * OK, advance or back up to the target block. This implementation
887 * would be pretty inefficient for long seeks, but we really aren't
888 * expecting that (a seek over one tuple is typical).
890 while (lt->curBlockNumber > blocknum)
892 long datablocknum = ltsRecallPrevBlockNum(lts, lt->indirect);
894 if (datablocknum == -1L)
895 elog(ERROR, "unexpected end of tape");
896 if (--lt->curBlockNumber == blocknum)
897 ltsReadBlock(lts, datablocknum, (void *) lt->buffer);
899 while (lt->curBlockNumber < blocknum)
901 long datablocknum = ltsRecallNextBlockNum(lts, lt->indirect,
904 if (datablocknum == -1L)
905 elog(ERROR, "unexpected end of tape");
906 if (++lt->curBlockNumber == blocknum)
907 ltsReadBlock(lts, datablocknum, (void *) lt->buffer);
909 lt->nbytes = (lt->curBlockNumber < lt->numFullBlocks) ?
910 BLCKSZ : lt->lastBlockBytes;
916 * Obtain current position in a form suitable for a later LogicalTapeSeek.
918 * NOTE: it'd be OK to do this during write phase with intention of using
919 * the position for a seek after freezing. Not clear if anyone needs that.
922 LogicalTapeTell(LogicalTapeSet *lts, int tapenum,
923 long *blocknum, int *offset)
927 Assert(tapenum >= 0 && tapenum < lts->nTapes);
928 lt = lts->tapes[tapenum];
929 *blocknum = lt->curBlockNumber;