freespace.c

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/*-------------------------------------------------------------------------
 *
 * freespace.c
 *    POSTGRES free space map for quickly finding free space in relations
 *
 *
 * Portions Copyright (c) 1996-2013, PostgreSQL Global Development Group
 * Portions Copyright (c) 1994, Regents of the University of California
 *
 * IDENTIFICATION
 *    src/backend/storage/freespace/freespace.c
 *
 *
 * NOTES:
 *
 *  Free Space Map keeps track of the amount of free space on pages, and
 *  allows quickly searching for a page with enough free space. The FSM is
 *  stored in a dedicated relation fork of all heap relations, and those
 *  index access methods that need it (see also indexfsm.c). See README for
 *  more information.
 *
 *-------------------------------------------------------------------------
 */
#include "postgres.h"

#include "access/htup_details.h"
#include "access/xlogutils.h"
#include "miscadmin.h"
#include "storage/freespace.h"
#include "storage/fsm_internals.h"
#include "storage/lmgr.h"
#include "storage/smgr.h"


/*
 * We use just one byte to store the amount of free space on a page, so we
 * divide the amount of free space a page can have into 256 different
 * categories. The highest category, 255, represents a page with at least
 * MaxFSMRequestSize bytes of free space, and the second highest category
 * represents the range from 254 * FSM_CAT_STEP, inclusive, to
 * MaxFSMRequestSize, exclusive.
 *
 * MaxFSMRequestSize depends on the architecture and BLCKSZ, but assuming
 * default 8k BLCKSZ, and that MaxFSMRequestSize is 24 bytes, the categories
 * look like this
 *
 *
 * Range     Category
 * 0    - 31   0
 * 32   - 63   1
 * ...    ...  ...
 * 8096 - 8127 253
 * 8128 - 8163 254
 * 8164 - 8192 255
 *
 * The reason that MaxFSMRequestSize is special is that if MaxFSMRequestSize
 * isn't equal to a range boundary, a page with exactly MaxFSMRequestSize
 * bytes of free space wouldn't satisfy a request for MaxFSMRequestSize
 * bytes. If there isn't more than MaxFSMRequestSize bytes of free space on a
 * completely empty page, that would mean that we could never satisfy a
 * request of exactly MaxFSMRequestSize bytes.
 */
#define FSM_CATEGORIES  256
#define FSM_CAT_STEP    (BLCKSZ / FSM_CATEGORIES)
#define MaxFSMRequestSize   MaxHeapTupleSize

/*
 * Depth of the on-disk tree. We need to be able to address 2^32-1 blocks,
 * and 1626 is the smallest number that satisfies X^3 >= 2^32-1. Likewise,
 * 216 is the smallest number that satisfies X^4 >= 2^32-1. In practice,
 * this means that 4096 bytes is the smallest BLCKSZ that we can get away
 * with a 3-level tree, and 512 is the smallest we support.
 */
#define FSM_TREE_DEPTH  ((SlotsPerFSMPage >= 1626) ? 3 : 4)

#define FSM_ROOT_LEVEL  (FSM_TREE_DEPTH - 1)
#define FSM_BOTTOM_LEVEL 0

/*
 * The internal FSM routines work on a logical addressing scheme. Each
 * level of the tree can be thought of as a separately addressable file.
 */
typedef struct
{
    int         level;          /* level */
    int         logpageno;      /* page number within the level */
} FSMAddress;

/* Address of the root page. */
static const FSMAddress FSM_ROOT_ADDRESS = {FSM_ROOT_LEVEL, 0};

/* functions to navigate the tree */
static FSMAddress fsm_get_child(FSMAddress parent, uint16 slot);
static FSMAddress fsm_get_parent(FSMAddress child, uint16 *slot);
static FSMAddress fsm_get_location(BlockNumber heapblk, uint16 *slot);
static BlockNumber fsm_get_heap_blk(FSMAddress addr, uint16 slot);
static BlockNumber fsm_logical_to_physical(FSMAddress addr);

static Buffer fsm_readbuf(Relation rel, FSMAddress addr, bool extend);
static void fsm_extend(Relation rel, BlockNumber fsm_nblocks);

/* functions to convert amount of free space to a FSM category */
static uint8 fsm_space_avail_to_cat(Size avail);
static uint8 fsm_space_needed_to_cat(Size needed);
static Size fsm_space_cat_to_avail(uint8 cat);

/* workhorse functions for various operations */
static int fsm_set_and_search(Relation rel, FSMAddress addr, uint16 slot,
                   uint8 newValue, uint8 minValue);
static BlockNumber fsm_search(Relation rel, uint8 min_cat);
static uint8 fsm_vacuum_page(Relation rel, FSMAddress addr, bool *eof);


/******** Public API ********/

/*
 * GetPageWithFreeSpace - try to find a page in the given relation with
 *      at least the specified amount of free space.
 *
 * If successful, return the block number; if not, return InvalidBlockNumber.
 *
 * The caller must be prepared for the possibility that the returned page
 * will turn out to have too little space available by the time the caller
 * gets a lock on it.  In that case, the caller should report the actual
 * amount of free space available on that page and then try again (see
 * RecordAndGetPageWithFreeSpace).  If InvalidBlockNumber is returned,
 * extend the relation.
 */
BlockNumber
GetPageWithFreeSpace(Relation rel, Size spaceNeeded)
{
    uint8       min_cat = fsm_space_needed_to_cat(spaceNeeded);

    return fsm_search(rel, min_cat);
}

/*
 * RecordAndGetPageWithFreeSpace - update info about a page and try again.
 *
 * We provide this combo form to save some locking overhead, compared to
 * separate RecordPageWithFreeSpace + GetPageWithFreeSpace calls. There's
 * also some effort to return a page close to the old page; if there's a
 * page with enough free space on the same FSM page where the old one page
 * is located, it is preferred.
 */
BlockNumber
RecordAndGetPageWithFreeSpace(Relation rel, BlockNumber oldPage,
                              Size oldSpaceAvail, Size spaceNeeded)
{
    int         old_cat = fsm_space_avail_to_cat(oldSpaceAvail);
    int         search_cat = fsm_space_needed_to_cat(spaceNeeded);
    FSMAddress  addr;
    uint16      slot;
    int         search_slot;

    /* Get the location of the FSM byte representing the heap block */
    addr = fsm_get_location(oldPage, &slot);

    search_slot = fsm_set_and_search(rel, addr, slot, old_cat, search_cat);

    /*
     * If fsm_set_and_search found a suitable new block, return that.
     * Otherwise, search as usual.
     */
    if (search_slot != -1)
        return fsm_get_heap_blk(addr, search_slot);
    else
        return fsm_search(rel, search_cat);
}

/*
 * RecordPageWithFreeSpace - update info about a page.
 *
 * Note that if the new spaceAvail value is higher than the old value stored
 * in the FSM, the space might not become visible to searchers until the next
 * FreeSpaceMapVacuum call, which updates the upper level pages.
 */
void
RecordPageWithFreeSpace(Relation rel, BlockNumber heapBlk, Size spaceAvail)
{
    int         new_cat = fsm_space_avail_to_cat(spaceAvail);
    FSMAddress  addr;
    uint16      slot;

    /* Get the location of the FSM byte representing the heap block */
    addr = fsm_get_location(heapBlk, &slot);

    fsm_set_and_search(rel, addr, slot, new_cat, 0);
}

/*
 * XLogRecordPageWithFreeSpace - like RecordPageWithFreeSpace, for use in
 *      WAL replay
 */
void
XLogRecordPageWithFreeSpace(RelFileNode rnode, BlockNumber heapBlk,
                            Size spaceAvail)
{
    int         new_cat = fsm_space_avail_to_cat(spaceAvail);
    FSMAddress  addr;
    uint16      slot;
    BlockNumber blkno;
    Buffer      buf;
    Page        page;

    /* Get the location of the FSM byte representing the heap block */
    addr = fsm_get_location(heapBlk, &slot);
    blkno = fsm_logical_to_physical(addr);

    /* If the page doesn't exist already, extend */
    buf = XLogReadBufferExtended(rnode, FSM_FORKNUM, blkno, RBM_ZERO_ON_ERROR);
    LockBuffer(buf, BUFFER_LOCK_EXCLUSIVE);

    page = BufferGetPage(buf);
    if (PageIsNew(page))
        PageInit(page, BLCKSZ, 0);

    if (fsm_set_avail(page, slot, new_cat))
        MarkBufferDirtyHint(buf);
    UnlockReleaseBuffer(buf);
}

/*
 * GetRecordedFreePage - return the amount of free space on a particular page,
 *      according to the FSM.
 */
Size
GetRecordedFreeSpace(Relation rel, BlockNumber heapBlk)
{
    FSMAddress  addr;
    uint16      slot;
    Buffer      buf;
    uint8       cat;

    /* Get the location of the FSM byte representing the heap block */
    addr = fsm_get_location(heapBlk, &slot);

    buf = fsm_readbuf(rel, addr, false);
    if (!BufferIsValid(buf))
        return 0;
    cat = fsm_get_avail(BufferGetPage(buf), slot);
    ReleaseBuffer(buf);

    return fsm_space_cat_to_avail(cat);
}

/*
 * FreeSpaceMapTruncateRel - adjust for truncation of a relation.
 *
 * The caller must hold AccessExclusiveLock on the relation, to ensure that
 * other backends receive the smgr invalidation event that this function sends
 * before they access the FSM again.
 *
 * nblocks is the new size of the heap.
 */
void
FreeSpaceMapTruncateRel(Relation rel, BlockNumber nblocks)
{
    BlockNumber new_nfsmblocks;
    FSMAddress  first_removed_address;
    uint16      first_removed_slot;
    Buffer      buf;

    RelationOpenSmgr(rel);

    /*
     * If no FSM has been created yet for this relation, there's nothing to
     * truncate.
     */
    if (!smgrexists(rel->rd_smgr, FSM_FORKNUM))
        return;

    /* Get the location in the FSM of the first removed heap block */
    first_removed_address = fsm_get_location(nblocks, &first_removed_slot);

    /*
     * Zero out the tail of the last remaining FSM page. If the slot
     * representing the first removed heap block is at a page boundary, as the
     * first slot on the FSM page that first_removed_address points to, we can
     * just truncate that page altogether.
     */
    if (first_removed_slot > 0)
    {
        buf = fsm_readbuf(rel, first_removed_address, false);
        if (!BufferIsValid(buf))
            return;             /* nothing to do; the FSM was already smaller */
        LockBuffer(buf, BUFFER_LOCK_EXCLUSIVE);
        fsm_truncate_avail(BufferGetPage(buf), first_removed_slot);
        MarkBufferDirtyHint(buf);
        UnlockReleaseBuffer(buf);

        new_nfsmblocks = fsm_logical_to_physical(first_removed_address) + 1;
    }
    else
    {
        new_nfsmblocks = fsm_logical_to_physical(first_removed_address);
        if (smgrnblocks(rel->rd_smgr, FSM_FORKNUM) <= new_nfsmblocks)
            return;             /* nothing to do; the FSM was already smaller */
    }

    /* Truncate the unused FSM pages, and send smgr inval message */
    smgrtruncate(rel->rd_smgr, FSM_FORKNUM, new_nfsmblocks);

    /*
     * We might as well update the local smgr_fsm_nblocks setting.
     * smgrtruncate sent an smgr cache inval message, which will cause other
     * backends to invalidate their copy of smgr_fsm_nblocks, and this one too
     * at the next command boundary.  But this ensures it isn't outright wrong
     * until then.
     */
    if (rel->rd_smgr)
        rel->rd_smgr->smgr_fsm_nblocks = new_nfsmblocks;
}

/*
 * FreeSpaceMapVacuum - scan and fix any inconsistencies in the FSM
 */
void
FreeSpaceMapVacuum(Relation rel)
{
    bool        dummy;

    /*
     * Traverse the tree in depth-first order. The tree is stored physically
     * in depth-first order, so this should be pretty I/O efficient.
     */
    fsm_vacuum_page(rel, FSM_ROOT_ADDRESS, &dummy);
}

/******** Internal routines ********/

/*
 * Return category corresponding x bytes of free space
 */
static uint8
fsm_space_avail_to_cat(Size avail)
{
    int         cat;

    Assert(avail < BLCKSZ);

    if (avail >= MaxFSMRequestSize)
        return 255;

    cat = avail / FSM_CAT_STEP;

    /*
     * The highest category, 255, is reserved for MaxFSMRequestSize bytes or
     * more.
     */
    if (cat > 254)
        cat = 254;

    return (uint8) cat;
}

/*
 * Return the lower bound of the range of free space represented by given
 * category.
 */
static Size
fsm_space_cat_to_avail(uint8 cat)
{
    /* The highest category represents exactly MaxFSMRequestSize bytes. */
    if (cat == 255)
        return MaxFSMRequestSize;
    else
        return cat * FSM_CAT_STEP;
}

/*
 * Which category does a page need to have, to accommodate x bytes of data?
 * While fsm_size_to_avail_cat() rounds down, this needs to round up.
 */
static uint8
fsm_space_needed_to_cat(Size needed)
{
    int         cat;

    /* Can't ask for more space than the highest category represents */
    if (needed > MaxFSMRequestSize)
        elog(ERROR, "invalid FSM request size %lu",
             (unsigned long) needed);

    if (needed == 0)
        return 1;

    cat = (needed + FSM_CAT_STEP - 1) / FSM_CAT_STEP;

    if (cat > 255)
        cat = 255;

    return (uint8) cat;
}

/*
 * Returns the physical block number an FSM page
 */
static BlockNumber
fsm_logical_to_physical(FSMAddress addr)
{
    BlockNumber pages;
    int         leafno;
    int         l;

    /*
     * Calculate the logical page number of the first leaf page below the
     * given page.
     */
    leafno = addr.logpageno;
    for (l = 0; l < addr.level; l++)
        leafno *= SlotsPerFSMPage;

    /* Count upper level nodes required to address the leaf page */
    pages = 0;
    for (l = 0; l < FSM_TREE_DEPTH; l++)
    {
        pages += leafno + 1;
        leafno /= SlotsPerFSMPage;
    }

    /*
     * If the page we were asked for wasn't at the bottom level, subtract the
     * additional lower level pages we counted above.
     */
    pages -= addr.level;

    /* Turn the page count into 0-based block number */
    return pages - 1;
}

/*
 * Return the FSM location corresponding to given heap block.
 */
static FSMAddress
fsm_get_location(BlockNumber heapblk, uint16 *slot)
{
    FSMAddress  addr;

    addr.level = FSM_BOTTOM_LEVEL;
    addr.logpageno = heapblk / SlotsPerFSMPage;
    *slot = heapblk % SlotsPerFSMPage;

    return addr;
}

/*
 * Return the heap block number corresponding to given location in the FSM.
 */
static BlockNumber
fsm_get_heap_blk(FSMAddress addr, uint16 slot)
{
    Assert(addr.level == FSM_BOTTOM_LEVEL);
    return ((unsigned int) addr.logpageno) * SlotsPerFSMPage + slot;
}

/*
 * Given a logical address of a child page, get the logical page number of
 * the parent, and the slot within the parent corresponding to the child.
 */
static FSMAddress
fsm_get_parent(FSMAddress child, uint16 *slot)
{
    FSMAddress  parent;

    Assert(child.level < FSM_ROOT_LEVEL);

    parent.level = child.level + 1;
    parent.logpageno = child.logpageno / SlotsPerFSMPage;
    *slot = child.logpageno % SlotsPerFSMPage;

    return parent;
}

/*
 * Given a logical address of a parent page, and a slot number get the
 * logical address of the corresponding child page.
 */
static FSMAddress
fsm_get_child(FSMAddress parent, uint16 slot)
{
    FSMAddress  child;

    Assert(parent.level > FSM_BOTTOM_LEVEL);

    child.level = parent.level - 1;
    child.logpageno = parent.logpageno * SlotsPerFSMPage + slot;

    return child;
}

/*
 * Read a FSM page.
 *
 * If the page doesn't exist, InvalidBuffer is returned, or if 'extend' is
 * true, the FSM file is extended.
 */
static Buffer
fsm_readbuf(Relation rel, FSMAddress addr, bool extend)
{
    BlockNumber blkno = fsm_logical_to_physical(addr);
    Buffer      buf;

    RelationOpenSmgr(rel);

    /*
     * If we haven't cached the size of the FSM yet, check it first.  Also
     * recheck if the requested block seems to be past end, since our cached
     * value might be stale.  (We send smgr inval messages on truncation, but
     * not on extension.)
     */
    if (rel->rd_smgr->smgr_fsm_nblocks == InvalidBlockNumber ||
        blkno >= rel->rd_smgr->smgr_fsm_nblocks)
    {
        if (smgrexists(rel->rd_smgr, FSM_FORKNUM))
            rel->rd_smgr->smgr_fsm_nblocks = smgrnblocks(rel->rd_smgr,
                                                         FSM_FORKNUM);
        else
            rel->rd_smgr->smgr_fsm_nblocks = 0;
    }

    /* Handle requests beyond EOF */
    if (blkno >= rel->rd_smgr->smgr_fsm_nblocks)
    {
        if (extend)
            fsm_extend(rel, blkno + 1);
        else
            return InvalidBuffer;
    }

    /*
     * Use ZERO_ON_ERROR mode, and initialize the page if necessary. The FSM
     * information is not accurate anyway, so it's better to clear corrupt
     * pages than error out. Since the FSM changes are not WAL-logged, the
     * so-called torn page problem on crash can lead to pages with corrupt
     * headers, for example.
     */
    buf = ReadBufferExtended(rel, FSM_FORKNUM, blkno, RBM_ZERO_ON_ERROR, NULL);
    if (PageIsNew(BufferGetPage(buf)))
        PageInit(BufferGetPage(buf), BLCKSZ, 0);
    return buf;
}

/*
 * Ensure that the FSM fork is at least fsm_nblocks long, extending
 * it if necessary with empty pages. And by empty, I mean pages filled
 * with zeros, meaning there's no free space.
 */
static void
fsm_extend(Relation rel, BlockNumber fsm_nblocks)
{
    BlockNumber fsm_nblocks_now;
    Page        pg;

    pg = (Page) palloc(BLCKSZ);
    PageInit(pg, BLCKSZ, 0);

    /*
     * We use the relation extension lock to lock out other backends trying to
     * extend the FSM at the same time. It also locks out extension of the
     * main fork, unnecessarily, but extending the FSM happens seldom enough
     * that it doesn't seem worthwhile to have a separate lock tag type for
     * it.
     *
     * Note that another backend might have extended or created the relation
     * by the time we get the lock.
     */
    LockRelationForExtension(rel, ExclusiveLock);

    /* Might have to re-open if a cache flush happened */
    RelationOpenSmgr(rel);

    /*
     * Create the FSM file first if it doesn't exist.  If smgr_fsm_nblocks is
     * positive then it must exist, no need for an smgrexists call.
     */
    if ((rel->rd_smgr->smgr_fsm_nblocks == 0 ||
         rel->rd_smgr->smgr_fsm_nblocks == InvalidBlockNumber) &&
        !smgrexists(rel->rd_smgr, FSM_FORKNUM))
        smgrcreate(rel->rd_smgr, FSM_FORKNUM, false);

    fsm_nblocks_now = smgrnblocks(rel->rd_smgr, FSM_FORKNUM);

    while (fsm_nblocks_now < fsm_nblocks)
    {
        PageSetChecksumInplace(pg, fsm_nblocks_now);

        smgrextend(rel->rd_smgr, FSM_FORKNUM, fsm_nblocks_now,
                   (char *) pg, false);
        fsm_nblocks_now++;
    }

    /* Update local cache with the up-to-date size */
    rel->rd_smgr->smgr_fsm_nblocks = fsm_nblocks_now;

    UnlockRelationForExtension(rel, ExclusiveLock);

    pfree(pg);
}

/*
 * Set value in given FSM page and slot.
 *
 * If minValue > 0, the updated page is also searched for a page with at
 * least minValue of free space. If one is found, its slot number is
 * returned, -1 otherwise.
 */
static int
fsm_set_and_search(Relation rel, FSMAddress addr, uint16 slot,
                   uint8 newValue, uint8 minValue)
{
    Buffer      buf;
    Page        page;
    int         newslot = -1;

    buf = fsm_readbuf(rel, addr, true);
    LockBuffer(buf, BUFFER_LOCK_EXCLUSIVE);

    page = BufferGetPage(buf);

    if (fsm_set_avail(page, slot, newValue))
        MarkBufferDirtyHint(buf);

    if (minValue != 0)
    {
        /* Search while we still hold the lock */
        newslot = fsm_search_avail(buf, minValue,
                                   addr.level == FSM_BOTTOM_LEVEL,
                                   true);
    }

    UnlockReleaseBuffer(buf);

    return newslot;
}

/*
 * Search the tree for a heap page with at least min_cat of free space
 */
static BlockNumber
fsm_search(Relation rel, uint8 min_cat)
{
    int         restarts = 0;
    FSMAddress  addr = FSM_ROOT_ADDRESS;

    for (;;)
    {
        int         slot;
        Buffer      buf;
        uint8       max_avail = 0;

        /* Read the FSM page. */
        buf = fsm_readbuf(rel, addr, false);

        /* Search within the page */
        if (BufferIsValid(buf))
        {
            LockBuffer(buf, BUFFER_LOCK_SHARE);
            slot = fsm_search_avail(buf, min_cat,
                                    (addr.level == FSM_BOTTOM_LEVEL),
                                    false);
            if (slot == -1)
                max_avail = fsm_get_max_avail(BufferGetPage(buf));
            UnlockReleaseBuffer(buf);
        }
        else
            slot = -1;

        if (slot != -1)
        {
            /*
             * Descend the tree, or return the found block if we're at the
             * bottom.
             */
            if (addr.level == FSM_BOTTOM_LEVEL)
                return fsm_get_heap_blk(addr, slot);

            addr = fsm_get_child(addr, slot);
        }
        else if (addr.level == FSM_ROOT_LEVEL)
        {
            /*
             * At the root, failure means there's no page with enough free
             * space in the FSM. Give up.
             */
            return InvalidBlockNumber;
        }
        else
        {
            uint16      parentslot;
            FSMAddress  parent;

            /*
             * At lower level, failure can happen if the value in the upper-
             * level node didn't reflect the value on the lower page. Update
             * the upper node, to avoid falling into the same trap again, and
             * start over.
             *
             * There's a race condition here, if another backend updates this
             * page right after we release it, and gets the lock on the parent
             * page before us. We'll then update the parent page with the now
             * stale information we had. It's OK, because it should happen
             * rarely, and will be fixed by the next vacuum.
             */
            parent = fsm_get_parent(addr, &parentslot);
            fsm_set_and_search(rel, parent, parentslot, max_avail, 0);

            /*
             * If the upper pages are badly out of date, we might need to loop
             * quite a few times, updating them as we go. Any inconsistencies
             * should eventually be corrected and the loop should end. Looping
             * indefinitely is nevertheless scary, so provide an emergency
             * valve.
             */
            if (restarts++ > 10000)
                return InvalidBlockNumber;

            /* Start search all over from the root */
            addr = FSM_ROOT_ADDRESS;
        }
    }
}


/*
 * Recursive guts of FreeSpaceMapVacuum
 */
static uint8
fsm_vacuum_page(Relation rel, FSMAddress addr, bool *eof_p)
{
    Buffer      buf;
    Page        page;
    uint8       max_avail;

    /* Read the page if it exists, or return EOF */
    buf = fsm_readbuf(rel, addr, false);
    if (!BufferIsValid(buf))
    {
        *eof_p = true;
        return 0;
    }
    else
        *eof_p = false;

    page = BufferGetPage(buf);

    /*
     * Recurse into children, and fix the information stored about them at
     * this level.
     */
    if (addr.level > FSM_BOTTOM_LEVEL)
    {
        int         slot;
        bool        eof = false;

        for (slot = 0; slot < SlotsPerFSMPage; slot++)
        {
            int         child_avail;

            CHECK_FOR_INTERRUPTS();

            /* After we hit end-of-file, just clear the rest of the slots */
            if (!eof)
                child_avail = fsm_vacuum_page(rel, fsm_get_child(addr, slot), &eof);
            else
                child_avail = 0;

            /* Update information about the child */
            if (fsm_get_avail(page, slot) != child_avail)
            {
                LockBuffer(buf, BUFFER_LOCK_EXCLUSIVE);
                fsm_set_avail(BufferGetPage(buf), slot, child_avail);
                MarkBufferDirtyHint(buf);
                LockBuffer(buf, BUFFER_LOCK_UNLOCK);
            }
        }
    }

    max_avail = fsm_get_max_avail(BufferGetPage(buf));

    /*
     * Reset the next slot pointer. This encourages the use of low-numbered
     * pages, increasing the chances that a later vacuum can truncate the
     * relation.
     */
    ((FSMPage) PageGetContents(page))->fp_next_slot = 0;

    ReleaseBuffer(buf);

    return max_avail;
}