这篇文章主要介绍“PostgreSQL中ReadBuffer_common函数有什么作用”，在日常操作中，相信很多人在PostgreSQL中ReadBuffer_common函数有什么作用问题上存在疑惑，小编查阅了各式资料，整理出简单好用的操作方法，希望对大家解答”PostgreSQL中ReadBuffer_common函数有什么作用”的疑惑有所帮助！接下来，请跟着小编一起来学习吧！
一、数据结构
BufferDesc
共享缓冲区的共享描述符(状态)数据
//buffer header锁定
#define BM_LOCKED (1U << 22)
//数据需要写入(标记为DIRTY)
#define BM_DIRTY (1U << 23)
//数据是有效的
#define BM_VALID (1U << 24)
//已分配buffer tag
#define BM_TAG_VALID (1U << 25)
//正在R/W
#define BM_IO_IN_PROGRESS (1U << 26)
//上一个I/O出现错误
#define BM_IO_ERROR (1U << 27)
//开始写则变DIRTY
#define BM_JUST_DIRTIED (1U << 28)
//存在等待sole pin的其他进程
#define BM_PIN_COUNT_WAITER (1U << 29)
//checkpoint发生,必须刷到磁盘上
#define BM_CHECKPOINT_NEEDED (1U << 30)
//持久化buffer(不是unlogged或者初始化fork)
#define BM_PERMANENT (1U << 31)
typedef struct BufferDesc
{
//buffer tag
BufferTag tag;
//buffer索引编号(0开始)
int buf_id;
//tag状态,包括flags/refcount和usagecount
pg_atomic_uint32 state;
//pin-count等待进程ID
int wait_backend_pid;
//空闲链表链中下一个空闲的buffer
int freeNext;
//缓冲区内容锁
LWLock content_lock;
} BufferDesc;BufferTag
Buffer tag标记了buffer存储的是磁盘中哪个block
typedef struct buftag
{
//物理relation标识符
RelFileNode rnode;
ForkNumber forkNum;
//相对于relation起始的块号
BlockNumber blockNum;
} BufferTag;二、源码解读
ReadBuffer_common函数是所有ReadBuffer相关的通用逻辑,其实现逻辑如下:
1.初始化相关变量和执行相关判断(是否扩展isExtend?是否临时表isLocalBuf?)
2.如为临时表,则调用LocalBufferAlloc获取描述符;否则调用BufferAlloc获取描述符;
同时,设置是否在缓存命中的标记(变量found)
3.如在缓存中命中
3.1如非扩展buffer,更新统计信息,如有需要,锁定buffer并返回
3.2如为扩展buffer,则获取block
3.2.1如PageIsNew返回F,则报错
3.2.2如为本地buffer(临时表),则调整标记
3.2.3如非本地buffer,则清除BM_VALID标记
4.没有在缓存中命中,则获取block
4.1如为扩展buffer,通过填充0初始化buffer,调用smgrextend扩展
4.2如为普通buffer
4.2.1如模式为RBM_ZERO_AND_LOCK/RBM_ZERO_AND_CLEANUP_LOCK,填充0
4.2.2否则,通过smgr(存储管理器)读取block,如需要,则跟踪I/O时间,同时检查垃圾数据
5.已扩展了buffer或者已读取了block
5.1如需要,锁定buffer
5.2如为临时表,则调整标记;否则设置BM_VALID,中断IO,唤醒等待的进程
5.3更新统计信息
5.4返回buffer
static Buffer
ReadBuffer_common(SMgrRelation smgr, char relpersistence, ForkNumber forkNum,
BlockNumber blockNum, ReadBufferMode mode,
BufferAccessStrategy strategy, bool *hit)
{
BufferDesc *bufHdr;//buffer描述符
Block bufBlock;//相应的block
bool found;//是否命中?
bool isExtend;//扩展?
bool isLocalBuf = SmgrIsTemp(smgr);//本地buffer?
*hit = false;
//确保有空间存储buffer pin
ResourceOwnerEnlargeBuffers(CurrentResourceOwner);
//如为P_NEW,则需扩展
isExtend = (blockNum == P_NEW);
//跟踪
TRACE_POSTGRESQL_BUFFER_READ_START(forkNum, blockNum,
smgr->smgr_rnode.node.spcNode,
smgr->smgr_rnode.node.dbNode,
smgr->smgr_rnode.node.relNode,
smgr->smgr_rnode.backend,
isExtend);
//如调用方要求P_NEW，则替换适当的块号
if (isExtend)
blockNum = smgrnblocks(smgr, forkNum);
if (isLocalBuf)
{
//本地buffer(临时表)
bufHdr = LocalBufferAlloc(smgr, forkNum, blockNum, &found);
if (found)
pgBufferUsage.local_blks_hit++;
else if (isExtend)
pgBufferUsage.local_blks_written++;
else if (mode == RBM_NORMAL || mode == RBM_NORMAL_NO_LOG ||
mode == RBM_ZERO_ON_ERROR)
pgBufferUsage.local_blks_read++;
}
else
{
//非临时表
//获取buffer描述符
bufHdr = BufferAlloc(smgr, relpersistence, forkNum, blockNum,
strategy, &found);
if (found)
//在内存中命中
pgBufferUsage.shared_blks_hit++;
else if (isExtend)
//新的buffer
pgBufferUsage.shared_blks_written++;
else if (mode == RBM_NORMAL || mode == RBM_NORMAL_NO_LOG ||
mode == RBM_ZERO_ON_ERROR)
//读取block
pgBufferUsage.shared_blks_read++;
}
//这时候,我们还没有持有任何锁.
//---------- 如果buffer已在换冲池中,工作已完成
if (found)
{
//------------- buffer已在缓冲池中
//已在换冲池中
if (!isExtend)
{
//非扩展buffer
//在退出前,更新统计信息
*hit = true;
VacuumPageHit++;
if (VacuumCostActive)
VacuumCostBalance += VacuumCostPageHit;
TRACE_POSTGRESQL_BUFFER_READ_DONE(forkNum, blockNum,
smgr->smgr_rnode.node.spcNode,
smgr->smgr_rnode.node.dbNode,
smgr->smgr_rnode.node.relNode,
smgr->smgr_rnode.backend,
isExtend,
found);
if (!isLocalBuf)
{
//非临时表buffer
if (mode == RBM_ZERO_AND_LOCK)
LWLockAcquire(BufferDescriptorGetContentLock(bufHdr),
LW_EXCLUSIVE);
else if (mode == RBM_ZERO_AND_CLEANUP_LOCK)
LockBufferForCleanup(BufferDescriptorGetBuffer(bufHdr));
}
//根据buffer描述符读取buffer并返回buffer
//#define BufferDescriptorGetBuffer(bdesc) ((bdesc)->buf_id + 1)
return BufferDescriptorGetBuffer(bufHdr);
}
//获取block
bufBlock = isLocalBuf ? LocalBufHdrGetBlock(bufHdr) : BufHdrGetBlock(bufHdr);
if (!PageIsNew((Page) bufBlock))
//不是PageIsNew,则报错
ereport(ERROR,
(errmsg("unexpected data beyond EOF in block %u of relation %s",
blockNum, relpath(smgr->smgr_rnode, forkNum)),
errhint("This has been seen to occur with buggy kernels; consider updating your system.")));
if (isLocalBuf)
{
//临时表
//只需要调整标记
uint32 buf_state = pg_atomic_read_u32(&bufHdr->state);
Assert(buf_state & BM_VALID);
buf_state &= ~BM_VALID;
pg_atomic_unlocked_write_u32(&bufHdr->state, buf_state);
}
else
{
//非临时表
do
{
uint32 buf_state = LockBufHdr(bufHdr);
Assert(buf_state & BM_VALID);
//清除BM_VALID标记
buf_state &= ~BM_VALID;
UnlockBufHdr(bufHdr, buf_state);
} while (!StartBufferIO(bufHdr, true));
}
}
//------------- buffer不在缓冲池中
//验证
Assert(!(pg_atomic_read_u32(&bufHdr->state) & BM_VALID));
//获取block
bufBlock = isLocalBuf ? LocalBufHdrGetBlock(bufHdr) : BufHdrGetBlock(bufHdr);
if (isExtend)
{
//-------- 扩展block
//新buffers使用0填充
MemSet((char *) bufBlock, 0, BLCKSZ);
//对于使用全0填充的page,不要设置checksum
smgrextend(smgr, forkNum, blockNum, (char *) bufBlock, false);
}
else
{
//-------- 普通block
if (mode == RBM_ZERO_AND_LOCK || mode == RBM_ZERO_AND_CLEANUP_LOCK)
//如为RBM_ZERO_AND_LOCK或者RBM_ZERO_AND_CLEANUP_LOCK模式,初始化为0
MemSet((char *) bufBlock, 0, BLCKSZ);
else
{
//其他模式
instr_time io_start,//io的起止时间
io_time;
if (track_io_timing)
INSTR_TIME_SET_CURRENT(io_start);
//smgr(存储管理器)读取block
smgrread(smgr, forkNum, blockNum, (char *) bufBlock);
if (track_io_timing)
{
//需要跟踪io时间
INSTR_TIME_SET_CURRENT(io_time);
INSTR_TIME_SUBTRACT(io_time, io_start);
pgstat_count_buffer_read_time(INSTR_TIME_GET_MICROSEC(io_time));
INSTR_TIME_ADD(pgBufferUsage.blk_read_time, io_time);
}
//检查垃圾数据
if (!PageIsVerified((Page) bufBlock, blockNum))
{
//如果page为通过验证
if (mode == RBM_ZERO_ON_ERROR || zero_damaged_pages)
{
//出错,则初始化
ereport(WARNING,
(errcode(ERRCODE_DATA_CORRUPTED),
errmsg("invalid page in block %u of relation %s; zeroing out page",
blockNum,
relpath(smgr->smgr_rnode, forkNum))));
//初始化
MemSet((char *) bufBlock, 0, BLCKSZ);
}
else
//出错,报错
ereport(ERROR,
(errcode(ERRCODE_DATA_CORRUPTED),
errmsg("invalid page in block %u of relation %s",
blockNum,
relpath(smgr->smgr_rnode, forkNum))));
}
}
}
//--------- 已扩展了buffer或者已读取了block
if ((mode == RBM_ZERO_AND_LOCK || mode == RBM_ZERO_AND_CLEANUP_LOCK) &&
!isLocalBuf)
{
//锁定
LWLockAcquire(BufferDescriptorGetContentLock(bufHdr), LW_EXCLUSIVE);
}
if (isLocalBuf)
{
//临时表
//只需要调整标记
uint32 buf_state = pg_atomic_read_u32(&bufHdr->state);
buf_state |= BM_VALID;
pg_atomic_unlocked_write_u32(&bufHdr->state, buf_state);
}
else
{
//普通表
//设置BM_VALID,中断IO,唤醒等待的进程
TerminateBufferIO(bufHdr, false, BM_VALID);
}
//更新统计信息
VacuumPageMiss++;
if (VacuumCostActive)
VacuumCostBalance += VacuumCostPageMiss;
//跟踪
TRACE_POSTGRESQL_BUFFER_READ_DONE(forkNum, blockNum,
smgr->smgr_rnode.node.spcNode,
smgr->smgr_rnode.node.dbNode,
smgr->smgr_rnode.node.relNode,
smgr->smgr_rnode.backend,
isExtend,
found);
//返回buffer
//#define BufferDescriptorGetBuffer(bdesc) ((bdesc)->buf_id + 1)
return BufferDescriptorGetBuffer(bufHdr);
}三、跟踪分析
测试场景一:Block不在缓冲区中
脚本:
16:42:48 (xdb@[local]:5432)testdb=# select * from t1 limit 10;启动gdb,设置断点
(gdb) b ReadBuffer_common
Breakpoint 1 at 0x876e28: file bufmgr.c, line 711.
(gdb) c
Continuing.
Breakpoint 1, ReadBuffer_common (smgr=0x2b7cce0, relpersistence=112 'p', forkNum=MAIN_FORKNUM, blockNum=0, mode=RBM_NORMAL,
strategy=0x0, hit=0x7ffc7761dfab) at bufmgr.c:711
711 bool isLocalBuf = SmgrIsTemp(smgr);
(gdb)1.初始化相关变量和执行相关判断(是否扩展isExtend?是否临时表isLocalBuf?)
(gdb) n
713 *hit = false;
(gdb)
716 ResourceOwnerEnlargeBuffers(CurrentResourceOwner);
(gdb)
718 isExtend = (blockNum == P_NEW);
(gdb)
720 TRACE_POSTGRESQL_BUFFER_READ_START(forkNum, blockNum,
(gdb)
728 if (isExtend)
(gdb)
731 if (isLocalBuf)
(gdb)
745 bufHdr = BufferAlloc(smgr, relpersistence, forkNum, blockNum,
(gdb)2.调用BufferAlloc获取buffer描述符
(gdb)
747 if (found)
(gdb) p *bufHdr
$1 = {tag = {rnode = {spcNode = 1663, dbNode = 16402, relNode = 51439}, forkNum = MAIN_FORKNUM, blockNum = 0},
buf_id = 108, state = {value = 2248409089}, wait_backend_pid = 0, freeNext = -2, content_lock = {tranche = 54, state = {
value = 536870912}, waiters = {head = 2147483647, tail = 2147483647}}}
(gdb) p found
$2 = false
(gdb)
(gdb) n
750 pgBufferUsage.shared_blks_read++; --> 更新统计信息
(gdb)4.没有在缓存中命中,则获取block
756 if (found)
(gdb)
856 Assert(!(pg_atomic_read_u32(&bufHdr->state) & BM_VALID));
(gdb)
858 bufBlock = isLocalBuf ? LocalBufHdrGetBlock(bufHdr) : BufHdrGetBlock(bufHdr);
(gdb)
860 if (isExtend)
(gdb) p bufBlock
$4 = (Block) 0x7fe8c240e3804.2如为普通buffer
4.2.1如模式为RBM_ZERO_AND_LOCK/RBM_ZERO_AND_CLEANUP_LOCK,填充0
4.2.2否则,通过smgr(存储管理器)读取block,如需要,则跟踪I/O时间,同时检查垃圾数据
(gdb) p mode
$5 = RBM_NORMAL
(gdb)
(gdb) n
880 if (mode == RBM_ZERO_AND_LOCK || mode == RBM_ZERO_AND_CLEANUP_LOCK)
(gdb)
887 if (track_io_timing)
(gdb)
890 smgrread(smgr, forkNum, blockNum, (char *) bufBlock);
(gdb)
892 if (track_io_timing)
(gdb) p *smgr
$6 = {smgr_rnode = {node = {spcNode = 1663, dbNode = 16402, relNode = 51439}, backend = -1}, smgr_owner = 0x7fe8ee2bc7b8,
smgr_targblock = 4294967295, smgr_fsm_nblocks = 4294967295, smgr_vm_nblocks = 4294967295, smgr_which = 0,
md_num_open_segs = {1, 0, 0, 0}, md_seg_fds = {0x2b0dd78, 0x0, 0x0, 0x0}, next_unowned_reln = 0x0}
(gdb) p forkNum
$7 = MAIN_FORKNUM
(gdb) p blockNum
$8 = 0
(gdb) p (char *) bufBlock
$9 = 0x7fe8c240e380 "\001"
(gdb)5.已扩展了buffer或者已读取了block
5.1如需要,锁定buffer
5.2如为临时表,则调整标记;否则设置BM_VALID,中断IO,唤醒等待的进程
(gdb) n
901 if (!PageIsVerified((Page) bufBlock, blockNum))
(gdb)
932 if ((mode == RBM_ZERO_AND_LOCK || mode == RBM_ZERO_AND_CLEANUP_LOCK) &&
(gdb) n
938 if (isLocalBuf)
(gdb)
949 TerminateBufferIO(bufHdr, false, BM_VALID);
(gdb)5.3更新统计信息
5.4返回buffer
(gdb)
952 VacuumPageMiss++;
(gdb)
953 if (VacuumCostActive)
(gdb)
956 TRACE_POSTGRESQL_BUFFER_READ_DONE(forkNum, blockNum,
(gdb)
964 return BufferDescriptorGetBuffer(bufHdr);
(gdb)
965 }
(gdb)buf为109
(gdb)
ReadBufferExtended (reln=0x7fe8ee2bc7a8, forkNum=MAIN_FORKNUM, blockNum=0, mode=RBM_NORMAL, strategy=0x0) at bufmgr.c:666
666 if (hit)
(gdb)
668 return buf;
(gdb) p buf
$10 = 109
(gdb)测试场景二:Block已在缓冲区中
再次执行上面的SQL语句,这时候相应的block已读入到buffer中
(gdb) del
Delete all breakpoints? (y or n) y
(gdb) c
Continuing.
^C
Program received signal SIGINT, Interrupt.
0x00007fe8ec448903 in __epoll_wait_nocancel () at ../sysdeps/unix/syscall-template.S:81
81 T_PSEUDO (SYSCALL_SYMBOL, SYSCALL_NAME, SYSCALL_NARGS)
(gdb) b ReadBuffer_common
Breakpoint 2 at 0x876e28: file bufmgr.c, line 711.
(gdb)found变量为T
...
(gdb)
745 bufHdr = BufferAlloc(smgr, relpersistence, forkNum, blockNum,
(gdb)
747 if (found)
(gdb) p found
$11 = true
(gdb)
(gdb) n
748 pgBufferUsage.shared_blks_hit++;
(gdb)进入相应的逻辑
3.如在缓存中命中
3.1如非扩展buffer,更新统计信息,如有需要,锁定buffer并返回
3.2如为扩展buffer,则获取block
3.2.1如PageIsNew返回F,则报错
3.2.2如为本地buffer(临时表),则调整标记
3.2.3如非本地buffer,则清除BM_VALID标记
(gdb)
756 if (found)
(gdb)
758 if (!isExtend)
(gdb)
761 *hit = true;
(gdb)
762 VacuumPageHit++;
(gdb)
764 if (VacuumCostActive)
(gdb)
767 TRACE_POSTGRESQL_BUFFER_READ_DONE(forkNum, blockNum,
(gdb)
779 if (!isLocalBuf)
(gdb)
781 if (mode == RBM_ZERO_AND_LOCK)
(gdb)
784 else if (mode == RBM_ZERO_AND_CLEANUP_LOCK)
(gdb)
788 return BufferDescriptorGetBuffer(bufHdr);
(gdb)
965 }
(gdb)到此，关于“PostgreSQL中ReadBuffer_common函数有什么作用”的学习就结束了，希望能够解决大家的疑惑。理论与实践的搭配能更好的帮助大家学习，快去试试吧！若想继续学习更多相关知识，请继续关注亿速云网站，小编会继续努力为大家带来更多实用的文章！
