PostgreSQL의 실제 데이터 쓰기 시작 위치
19512 단어 PostgreSQL
Background WriterMain 순환 중 BgBuffer Sync () -->SyncOneBuffer -->FlushBuffer -->smgrwrite 호출
코드 보기:
/*
* 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)
{
……
/*
* Loop forever
*/
for (;;)
{
……
/*
* Do one cycle of dirty-buffer writing.
*/
can_hibernate = BgBufferSync();
……
}
} 다시 보기:
/*
* 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);
if (++next_to_clean >= NBuffers)
{
next_to_clean = 0;
next_passes++;
}
num_to_scan--;
if (buffer_state & BUF_WRITTEN)
{
reusable_buffers++;
if (++num_written >= bgwriter_lru_maxpages)
{
BgWriterStats.m_maxwritten_clean++;
break;
}
}
else if (buffer_state & BUF_REUSABLE)
reusable_buffers++;
}
……
} 다시 보기:
/*
* 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)
{
volatile BufferDesc *bufHdr = &BufferDescriptors[buf_id];
int result = 0;
/*
* Check whether buffer needs writing.
*
* We can make this check without taking the buffer content lock so long
* as we mark pages dirty in access methods *before* logging changes with
* XLogInsert(): if someone marks the buffer dirty just after our check we
* don't worry because our checkpoint.redo points before log record for
* upcoming changes and so we are not required to write such dirty buffer.
*/
LockBufHdr(bufHdr);
if (bufHdr->refcount == 0 && bufHdr->usage_count == 0)
result |= BUF_REUSABLE;
else if (skip_recently_used)
{
/* Caller told us not to write recently-used buffers */
UnlockBufHdr(bufHdr);
return result;
}
if (!(bufHdr->flags & BM_VALID) || !(bufHdr->flags & BM_DIRTY))
{
/* It's clean, so nothing to do */
UnlockBufHdr(bufHdr);
return result;
}
/*
* Pin it, share-lock it, write it. (FlushBuffer will do nothing if the
* buffer is clean by the time we've locked it.)
*/
PinBuffer_Locked(bufHdr);
LWLockAcquire(bufHdr->content_lock, LW_SHARED);
FlushBuffer(bufHdr, NULL);
LWLockRelease(bufHdr->content_lock);
UnpinBuffer(bufHdr, true);
return result | BUF_WRITTEN;
} 다시 보기:
/*
* 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. In the latter case, the
* relation will be marked as "transient" so that the corresponding
* kernel-level file descriptors are closed when the current transaction ends,
* if any.
*/
static void
FlushBuffer(volatile BufferDesc *buf, SMgrRelation reln)
{
XLogRecPtr recptr;
ErrorContextCallback errcontext;
instr_time io_start,
io_time;
/*
* Acquire the buffer's io_in_progress lock. If StartBufferIO returns
* false, then someone else flushed the buffer before we could, so we need
* not do anything.
*/
if (!StartBufferIO(buf, false))
return;
/* Setup error traceback support for ereport() */
errcontext.callback = shared_buffer_write_error_callback;
errcontext.arg = (void *) buf;
errcontext.previous = error_context_stack;
error_context_stack = &errcontext;
/* Find smgr relation for buffer, and mark it as transient */
if (reln == NULL)
{
reln = smgropen(buf->tag.rnode, InvalidBackendId);
smgrsettransient(reln);
}
TRACE_POSTGRESQL_BUFFER_FLUSH_START(buf->tag.forkNum,
buf->tag.blockNum,
reln->smgr_rnode.node.spcNode,
reln->smgr_rnode.node.dbNode,
reln->smgr_rnode.node.relNode);
/*
* 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.
*/
recptr = BufferGetLSN(buf);
XLogFlush(recptr);
/*
* Now it's safe to write buffer to disk. Note that no one else should
* have been able to write it while we were busy with log flushing because
* we have the io_in_progress lock.
*/
/* To check if block content changes while flushing. - vadim 01/17/97 */
LockBufHdr(buf);
buf->flags &= ~BM_JUST_DIRTIED;
UnlockBufHdr(buf);
if (track_io_timing)
INSTR_TIME_SET_CURRENT(io_start);
smgrwrite(reln,
buf->tag.forkNum,
buf->tag.blockNum,
(char *) BufHdrGetBlock(buf),
false);
if (track_io_timing)
{
INSTR_TIME_SET_CURRENT(io_time);
INSTR_TIME_SUBTRACT(io_time, io_start);
pgstat_count_buffer_write_time(INSTR_TIME_GET_MICROSEC(io_time));
INSTR_TIME_ADD(pgBufferUsage.blk_write_time, io_time);
}
pgBufferUsage.shared_blks_written++;
/*
* Mark the buffer as clean (unless BM_JUST_DIRTIED has become set) and
* end the io_in_progress state.
*/
TerminateBufferIO(buf, true, 0);
TRACE_POSTGRESQL_BUFFER_FLUSH_DONE(buf->tag.forkNum,
buf->tag.blockNum,
reln->smgr_rnode.node.spcNode,
reln->smgr_rnode.node.dbNode,
reln->smgr_rnode.node.relNode);
/* Pop the error context stack */
error_context_stack = errcontext.previous;
} 순환 안에 버퍼를 한 번씩 써요. 와, 이상하지 않아요?어쩌면 조금만 썼을지도 몰라, 디자이너가 일부러 그런 거야?
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