PostgreSQL 可靠性和一致性 代码分析-阿里云开发者社区

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PostgreSQL 可靠性和一致性 代码分析

简介:

PostgreSQL 的数据可靠性是依赖XLOG的实现的,所有的对数据块的变更操作在write到磁盘前,一定是确保这个变更产生的REDO会先写到XLOG,并保证XLOG已落盘。
也就是说流程是这样的:
.1. 首先将需要变更的块从文件读入shared buffer
1
.2. 变更shared buffer中block的内容
2
.3. 将shared buffer中block变更的内容写入XLOG,如果是checkpoint后第一次变更该块,则写full page。(通过参数控制是否要写full page)。
3
.4. 在bgwriter将shared buffer中的脏块write到os dirty page前,会确保它对应的XLOG已经落盘,通过脏块的LSN来确保。
4
所以问题来了,如果用户使用了异步提交,即synchronous_commit=off,会怎样呢?
也没有问题,因为在第四步,一定是会保证造成脏页的XLOG是先落盘的。
所以开启synchronous_commit=off,只会造成丢XLOG,绝对不会造成数据不一致。
确保可靠性和一致性的代码如下:


/*
 * Main entry point for bgwriter process
 *
 * This is invoked from AuxiliaryProcessMain, which has already created the
 * basic execution environment, but not enabled signals yet.
 */
void
BackgroundWriterMain(void)
{
...
        /*
         * Do one cycle of dirty-buffer writing.
         */
        can_hibernate = BgBufferSync();

...



/*
 * BgBufferSync -- Write out some dirty buffers in the pool.
 *
 * This is called periodically by the background writer process.
 *
 * Returns true if it's appropriate for the bgwriter process to go into
 * low-power hibernation mode.  (This happens if the strategy clock sweep
 * has been "lapped" and no buffer allocations have occurred recently,
 * or if the bgwriter has been effectively disabled by setting
 * bgwriter_lru_maxpages to 0.)
 */
bool
BgBufferSync(void)
{
...

    /* Execute the LRU scan */
    while (num_to_scan > 0 && reusable_buffers < upcoming_alloc_est)
    {
        int            buffer_state = SyncOneBuffer(next_to_clean, true);

...


/*
 * SyncOneBuffer -- process a single buffer during syncing.
 *
 * If skip_recently_used is true, we don't write currently-pinned buffers, nor
 * buffers marked recently used, as these are not replacement candidates.
 *
 * Returns a bitmask containing the following flag bits:
 *    BUF_WRITTEN: we wrote the buffer.
 *    BUF_REUSABLE: buffer is available for replacement, ie, it has
 *        pin count 0 and usage count 0.
 *
 * (BUF_WRITTEN could be set in error if FlushBuffers finds the buffer clean
 * after locking it, but we don't care all that much.)
 *
 * Note: caller must have done ResourceOwnerEnlargeBuffers.
 */
static int
SyncOneBuffer(int buf_id, bool skip_recently_used)
{

...

    FlushBuffer(bufHdr, NULL);
...


/*
 * FlushBuffer
 *        Physically write out a shared buffer.
 *
 * NOTE: this actually just passes the buffer contents to the kernel; the
 * real write to disk won't happen until the kernel feels like it.  This
 * is okay from our point of view since we can redo the changes from WAL.
 * However, we will need to force the changes to disk via fsync before
 * we can checkpoint WAL.
 *
 * The caller must hold a pin on the buffer and have share-locked the
 * buffer contents.  (Note: a share-lock does not prevent updates of
 * hint bits in the buffer, so the page could change while the write
 * is in progress, but we assume that that will not invalidate the data
 * written.)
 *
 * If the caller has an smgr reference for the buffer's relation, pass it
 * as the second parameter.  If not, pass NULL.
 */
static void
FlushBuffer(volatile BufferDesc *buf, SMgrRelation reln)
{

...

    /*
     * Force XLOG flush up to buffer's LSN.  This implements the basic WAL
     * rule that log updates must hit disk before any of the data-file changes
     * they describe do.
     *
     * However, this rule does not apply to unlogged relations, which will be
     * lost after a crash anyway.  Most unlogged relation pages do not bear
     * LSNs since we never emit WAL records for them, and therefore flushing
     * up through the buffer LSN would be useless, but harmless.  However,
     * GiST indexes use LSNs internally to track page-splits, and therefore
     * unlogged GiST pages bear "fake" LSNs generated by
     * GetFakeLSNForUnloggedRel.  It is unlikely but possible that the fake
     * LSN counter could advance past the WAL insertion point; and if it did
     * happen, attempting to flush WAL through that location would fail, with
     * disastrous system-wide consequences.  To make sure that can't happen,
     * skip the flush if the buffer isn't permanent.
     */
    if (buf->flags & BM_PERMANENT)
        XLogFlush(recptr);

...




/*
 * Ensure that all XLOG data through the given position is flushed to disk.
 *
 * NOTE: this differs from XLogWrite mainly in that the WALWriteLock is not
 * already held, and we try to avoid acquiring it if possible.
 */
void
XLogFlush(XLogRecPtr record)
{
    XLogRecPtr    WriteRqstPtr;
    XLogwrtRqst WriteRqst;

...
        XLogWrite(WriteRqst, false);

...



/*
 * Write and/or fsync the log at least as far as WriteRqst indicates.
 *
 * If flexible == TRUE, we don't have to write as far as WriteRqst, but
 * may stop at any convenient boundary (such as a cache or logfile boundary).
 * This option allows us to avoid uselessly issuing multiple writes when a
 * single one would do.
 *
 * Must be called with WALWriteLock held. WaitXLogInsertionsToFinish(WriteRqst)
 * must be called before grabbing the lock, to make sure the data is ready to
 * write.
 */
static void
XLogWrite(XLogwrtRqst WriteRqst, bool flexible)
{

...
    /*
     * If asked to flush, do so
     */
    if (LogwrtResult.Flush < WriteRqst.Flush &&
        LogwrtResult.Flush < LogwrtResult.Write)

    {
        /*
         * Could get here without iterating above loop, in which case we might
         * have no open file or the wrong one.  However, we do not need to
         * fsync more than one file.
         */
        if (sync_method != SYNC_METHOD_OPEN &&
            sync_method != SYNC_METHOD_OPEN_DSYNC)
        {
            if (openLogFile >= 0 &&
                !XLByteInPrevSeg(LogwrtResult.Write, openLogSegNo))
                XLogFileClose();
            if (openLogFile < 0)
            {
                XLByteToPrevSeg(LogwrtResult.Write, openLogSegNo);
                openLogFile = XLogFileOpen(openLogSegNo);
                openLogOff = 0;
            }

            issue_xlog_fsync(openLogFile, openLogSegNo);
        }

        /* signal that we need to wakeup walsenders later */
        WalSndWakeupRequest();

        LogwrtResult.Flush = LogwrtResult.Write;
    }
...

异步提交代码如下

/*
     * Check if we want to commit asynchronously.  We can allow the XLOG flush
     * to happen asynchronously if synchronous_commit=off, or if the current
     * transaction has not performed any WAL-logged operation or didn't assign
     * a xid.  The transaction can end up not writing any WAL, even if it has
     * a xid, if it only wrote to temporary and/or unlogged tables.  It can
     * end up having written WAL without an xid if it did HOT pruning.  In
     * case of a crash, the loss of such a transaction will be irrelevant;
     * temp tables will be lost anyway, unlogged tables will be truncated and
     * HOT pruning will be done again later. (Given the foregoing, you might
     * think that it would be unnecessary to emit the XLOG record at all in
     * this case, but we don't currently try to do that.  It would certainly
     * cause problems at least in Hot Standby mode, where the
     * KnownAssignedXids machinery requires tracking every XID assignment.  It
     * might be OK to skip it only when wal_level < hot_standby, but for now
     * we don't.)
     *
     * However, if we're doing cleanup of any non-temp rels or committing any
     * command that wanted to force sync commit, then we must flush XLOG
     * immediately.  (We must not allow asynchronous commit if there are any
     * non-temp tables to be deleted, because we might delete the files before
     * the COMMIT record is flushed to disk.  We do allow asynchronous commit
     * if all to-be-deleted tables are temporary though, since they are lost
     * anyway if we crash.)
     */
    if ((wrote_xlog && markXidCommitted &&
         synchronous_commit > SYNCHRONOUS_COMMIT_OFF) ||
        forceSyncCommit || nrels > 0)
    {
        XLogFlush(XactLastRecEnd);

        /*
         * Now we may update the CLOG, if we wrote a COMMIT record above
         */
        if (markXidCommitted)
            TransactionIdCommitTree(xid, nchildren, children);
    }
    else
    {
        /*
         * Asynchronous commit case:
         *
         * This enables possible committed transaction loss in the case of a
         * postmaster crash because WAL buffers are left unwritten. Ideally we
         * could issue the WAL write without the fsync, but some
         * wal_sync_methods do not allow separate write/fsync.
         *
         * Report the latest async commit LSN, so that the WAL writer knows to
         * flush this commit.
         */
        XLogSetAsyncXactLSN(XactLastRecEnd);

        /*
         * We must not immediately update the CLOG, since we didn't flush the
         * XLOG. Instead, we store the LSN up to which the XLOG must be
         * flushed before the CLOG may be updated.
         */
        if (markXidCommitted)
            TransactionIdAsyncCommitTree(xid, nchildren, children, XactLastRecEnd);
    }

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