这篇文章主要介绍“PostgreSQL的vacuum过程中lazy_vacuum_heap函数有什么作用”，在日常操作中，相信很多人在PostgreSQL的vacuum过程中lazy_vacuum_heap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的vacuum过程中lazy_vacuum_heap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！
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本节简单介绍了PostgreSQL手工执行vacuum的处理流程,主要分析了ExecVacuum->vacuum->vacuum_rel->heap_vacuum_rel->lazy_scan_heap->lazy_vacuum_heap函数的实现逻辑,该函数访问堆表,标记废弃元组为未使用并在这些元组所在页面上压缩空闲空间。
宏定义
Vacuum和Analyze命令选项
/* ----------------------
* Vacuum and Analyze Statements
* Vacuum和Analyze命令选项
*
* Even though these are nominally two statements, it's convenient to use
* just one node type for both. Note that at least one of VACOPT_VACUUM
* and VACOPT_ANALYZE must be set in options.
* 虽然在这里有两种不同的语句,但只需要使用统一的Node类型即可.
* 注意至少VACOPT_VACUUM/VACOPT_ANALYZE在选项中设置.
* ----------------------
*/
typedef enum VacuumOption
{
VACOPT_VACUUM = 1 << 0, /* do VACUUM */
VACOPT_ANALYZE = 1 << 1, /* do ANALYZE */
VACOPT_VERBOSE = 1 << 2, /* print progress info */
VACOPT_FREEZE = 1 << 3, /* FREEZE option */
VACOPT_FULL = 1 << 4, /* FULL (non-concurrent) vacuum */
VACOPT_SKIP_LOCKED = 1 << 5, /* skip if cannot get lock */
VACOPT_SKIPTOAST = 1 << 6, /* don't process the TOAST table, if any */
VACOPT_DISABLE_PAGE_SKIPPING = 1 << 7 /* don't skip any pages */
} VacuumOption;itemIdSort
PageRepairFragmentation/PageIndexMultiDelete的排序支持
/*
* sorting support for PageRepairFragmentation and PageIndexMultiDelete
* PageRepairFragmentation/PageIndexMultiDelete的排序支持
*/
typedef struct itemIdSortData
{
//行指针数组索引
uint16 offsetindex; /* linp array index */
//item数据页内偏移
int16 itemoff; /* page offset of item data */
//对齐长度
uint16 alignedlen; /* MAXALIGN(item data len) */
} itemIdSortData;
//结构体指针
typedef itemIdSortData *itemIdSort;LVRelStats
typedef struct LVRelStats
{
/* hasindex = true means two-pass strategy; false means one-pass */
//T表示two-pass strategy,F表示one-pass strategy
bool hasindex;
/* Overall statistics about rel */
//rel的全局统计信息
//pg_class.relpages的上一个值
BlockNumber old_rel_pages; /* previous value of pg_class.relpages */
//pages的总数
BlockNumber rel_pages; /* total number of pages */
//扫描的pages
BlockNumber scanned_pages; /* number of pages we examined */
//由于pin跳过的pages
BlockNumber pinskipped_pages; /* # of pages we skipped due to a pin */
//跳过的frozen pages
BlockNumber frozenskipped_pages; /* # of frozen pages we skipped */
//计算其元组的pages
BlockNumber tupcount_pages; /* pages whose tuples we counted */
//pg_class.reltuples的前值
double old_live_tuples; /* previous value of pg_class.reltuples */
//新估算的总元组数
double new_rel_tuples; /* new estimated total # of tuples */
//新估算的存活元组数
double new_live_tuples; /* new estimated total # of live tuples */
//新估算的废弃元组数
double new_dead_tuples; /* new estimated total # of dead tuples */
//已清除的pages
BlockNumber pages_removed;
//已删除的tuples
double tuples_deleted;
//实际上是非空page + 1
BlockNumber nonempty_pages; /* actually, last nonempty page + 1 */
/* List of TIDs of tuples we intend to delete */
/* NB: this list is ordered by TID address */
//将要删除的元组TIDs链表
//注意:该链表已使用TID地址排序
//当前的入口/条目数
int num_dead_tuples; /* current # of entries */
//数组中已分配的slots(最大已废弃元组数)
int max_dead_tuples; /* # slots allocated in array */
//ItemPointer数组
ItemPointer dead_tuples; /* array of ItemPointerData */
//扫描的索引数
int num_index_scans;
//最后被清除的事务ID
TransactionId latestRemovedXid;
//是否存在waiter?
bool lock_waiter_detected;
} LVRelStats;ItemPointer
行指针
typedef struct ItemPointerData
{
BlockIdData ip_blkid;//块号
OffsetNumber ip_posid;//块内偏移
}
typedef ItemPointerData *ItemPointer;lazy_vacuum_heap
lazy_vacuum_heap标记废弃元组为未使用并在这些元组所在页面上压缩空闲空间,在此期间,不会访问lazy_scan_heap标记为存活元组的页面.
主要处理流程如下:
1.初始化变量
2.遍历vacrelstats->num_dead_tuples行指针数组(ItemPointer)
2.1获取块号/读取块到缓冲区中
2.2加锁,如不成功,则处理下一个元组
2.3调用lazy_vacuum_page释放空间,整理碎片
2.4获取page,获取该page的空闲空间
2.5释放缓冲,记录空闲空间
3.收尾工作
/*
* lazy_vacuum_heap() -- second pass over the heap
* lazy_vacuum_heap() -- 二次访问堆表
*
* This routine marks dead tuples as unused and compacts out free
* space on their pages. Pages not having dead tuples recorded from
* lazy_scan_heap are not visited at all.
* lazy_vacuum_heap标记废弃元组为未使用并在这些元组所在页面上压缩空闲空间.
* 在此期间,不会访问lazy_scan_heap标记没有废弃元组的页面.
*
* Note: the reason for doing this as a second pass is we cannot remove
* the tuples until we've removed their index entries, and we want to
* process index entry removal in batches as large as possible.
* 注意:二次访问堆表的原因是在清除索引条目前不能清除元组,
* 而且我们希望以批量的方式处理索引条目,越大越好.
*/
static void
lazy_vacuum_heap(Relation onerel, LVRelStats *vacrelstats)
{
int tupindex;//元组索引
int npages;//页面数
PGRUsage ru0;
Buffer vmbuffer = InvalidBuffer;//vm缓冲
pg_rusage_init(&ru0);//初始化
npages = 0;
tupindex = 0;
//遍历废弃元组
//vacrelstats->dead_tuples数组中的元素类型ItemPointer
while (tupindex < vacrelstats->num_dead_tuples)
{
BlockNumber tblk;//块号
Buffer buf;//缓冲
Page page;//页面
Size freespace;
vacuum_delay_point();
//获取块号
tblk = ItemPointerGetBlockNumber(&vacrelstats->dead_tuples[tupindex]);
//以扩展方式读取buffer
buf = ReadBufferExtended(onerel, MAIN_FORKNUM, tblk, RBM_NORMAL,
vac_strategy);
//获取锁(不等待)
if (!ConditionalLockBufferForCleanup(buf))
{
//获取不了,释放资源,跳转到下一个元组
ReleaseBuffer(buf);
++tupindex;
continue;
}
//释放page中的废弃元组,并整理碎片
tupindex = lazy_vacuum_page(onerel, tblk, buf, tupindex, vacrelstats,
&vmbuffer);
/* Now that we've compacted the page, record its available space */
//现在已经压缩了页面(释放了空间),记录可用空间
page = BufferGetPage(buf);
freespace = PageGetHeapFreeSpace(page);
UnlockReleaseBuffer(buf);
RecordPageWithFreeSpace(onerel, tblk, freespace);
npages++;
}
if (BufferIsValid(vmbuffer))
{
//释放缓冲区
ReleaseBuffer(vmbuffer);
vmbuffer = InvalidBuffer;
}
ereport(elevel,
(errmsg("\"%s\": removed %d row versions in %d pages",
RelationGetRelationName(onerel),
tupindex, npages),
errdetail_internal("%s", pg_rusage_show(&ru0))));
}lazy_vacuum_page
lazy_vacuum_page释放page中的废弃元组,并整理碎片
主要处理逻辑如下:
1.初始化相关变量
2.遍历废弃元组数组
2.1获取块号,如块号不一致,跳出循环
2.2获取偏移/行指针
2.3标记为未使用,记录偏移
3.调用PageRepairFragmentation整理碎片
3.1判断和检查(严谨的编码!!!)
3.2获取偏移,初始化变量
3.3遍历行指针数组
3.3.1获取行指针lp
3.3.2如ItemId正在使用,记录到itemidbase数组中;否则标记ItemId未被使用
3.4计算数组中存储的元素个数
A.如个数为0,重置page
B.否则调用compactify_tuples压缩页
3.5为PageAddItem方法设置标记位
4.标记buffer为dirty
5.写入WAL Record
6.如all-visible,则设置页面all-visible标记
7.如page为all-visible,设置vm
8.返回下一个page的起始数组编号
/*
* lazy_vacuum_page() -- free dead tuples on a page
* and repair its fragmentation.
* lazy_vacuum_page() -- 释放page中的废弃元组,并整理碎片
*
* Caller must hold pin and buffer cleanup lock on the buffer.
* 调用者必须持有buffer的pin和cleanup锁才能执行
*
* tupindex is the index in vacrelstats->dead_tuples of the first dead
* tuple for this page. We assume the rest follow sequentially.
* The return value is the first tupindex after the tuples of this page.
* tupindex是该page中第一个废弃元组在vacrelstats->dead_tuples中的编号,我们假定余下元组是顺序的.
* 返回值是该page中的元组后的第一个编号tupindex.
*/
static int
lazy_vacuum_page(Relation onerel, BlockNumber blkno, Buffer buffer,
int tupindex, LVRelStats *vacrelstats, Buffer *vmbuffer)
{
//获取page
Page page = BufferGetPage(buffer);
OffsetNumber unused[MaxOffsetNumber];//偏移数组
int uncnt = 0;
TransactionId visibility_cutoff_xid;//事务ID
bool all_frozen;//释放全部冻结
pgstat_progress_update_param(PROGRESS_VACUUM_HEAP_BLKS_VACUUMED, blkno);
//进入关键处理部分
START_CRIT_SECTION();
//遍历废弃元组数组
for (; tupindex < vacrelstats->num_dead_tuples; tupindex++)
{
BlockNumber tblk;//块号
OffsetNumber toff;//偏移
ItemId itemid;//行指针
//根据行指针获取块号
tblk = ItemPointerGetBlockNumber(&vacrelstats->dead_tuples[tupindex]);
if (tblk != blkno)
//不是同一个块,跳出循环
break; /* past end of tuples for this block */
//获取偏移
toff = ItemPointerGetOffsetNumber(&vacrelstats->dead_tuples[tupindex]);
//获取行指针
itemid = PageGetItemId(page, toff);
//标记为未使用
ItemIdSetUnused(itemid);
//记录偏移
unused[uncnt++] = toff;
}
//整理碎片
PageRepairFragmentation(page);
/*
* Mark buffer dirty before we write WAL.
* 标记buffer为dirty
*/
MarkBufferDirty(buffer);
/* XLOG stuff */
if (RelationNeedsWAL(onerel))
{
//记录WAL Record
XLogRecPtr recptr;
recptr = log_heap_clean(onerel, buffer,
NULL, 0, NULL, 0,
unused, uncnt,
vacrelstats->latestRemovedXid);
PageSetLSN(page, recptr);
}
/*
* End critical section, so we safely can do visibility tests (which
* possibly need to perform IO and allocate memory!). If we crash now the
* page (including the corresponding vm bit) might not be marked all
* visible, but that's fine. A later vacuum will fix that.
* 结束关键区域,这样我们可以安全的执行可见性检查
* (这可能需要执行IO/分配内存)
* 如果进程崩溃,页面(包括相应的vm位)可能标记为all-visible,但这也没有问题,后续vacuum会修复.
*/
END_CRIT_SECTION();
/*
* Now that we have removed the dead tuples from the page, once again
* check if the page has become all-visible. The page is already marked
* dirty, exclusively locked, and, if needed, a full page image has been
* emitted in the log_heap_clean() above.
* 现在，我们已经从页面中删除了废弃的元组，再次检查页面是否已经全部可见。
* 页面已经被标记为dirty、独占锁定，如需要，还会在log_heap_clean()中记录完整的页面镜像。
*/
if (heap_page_is_all_visible(onerel, buffer, &visibility_cutoff_xid,
&all_frozen))
PageSetAllVisible(page);
/*
* All the changes to the heap page have been done. If the all-visible
* flag is now set, also set the VM all-visible bit (and, if possible, the
* all-frozen bit) unless this has already been done previously.
* 堆页面的所有修改已完成.如果设置了all-visible标记,同时设置VM all-visible位
* (而且,如可能,设置all-frozen位),除非先前已完成.
*/
if (PageIsAllVisible(page))
{
uint8 vm_status = visibilitymap_get_status(onerel, blkno, vmbuffer);
uint8 flags = 0;
/* Set the VM all-frozen bit to flag, if needed */
//如需要,设置VM all-frozen标记位
if ((vm_status & VISIBILITYMAP_ALL_VISIBLE) == 0)
flags |= VISIBILITYMAP_ALL_VISIBLE;
if ((vm_status & VISIBILITYMAP_ALL_FROZEN) == 0 && all_frozen)
flags |= VISIBILITYMAP_ALL_FROZEN;
Assert(BufferIsValid(*vmbuffer));
if (flags != 0)
visibilitymap_set(onerel, blkno, buffer, InvalidXLogRecPtr,
*vmbuffer, visibility_cutoff_xid, flags);
}
return tupindex;
}
/*
* PageRepairFragmentation
*
* Frees fragmented space on a page.
* 释放页面上的碎片空间.
*
* It doesn't remove unused line pointers! Please don't change this.
* 该方法不会清楚未使用的行指针!因此,不要修改它.
*
* This routine is usable for heap pages only, but see PageIndexMultiDelete.
* 该方法只用于堆页面,但注意参考PageIndexMultiDelete.
*
* As a side effect, the page's PD_HAS_FREE_LINES hint bit is updated.
* 该方法在处理的时候,页面的PD_HAS_FREE_LINES标记位会被更新.
*
*/
void
PageRepairFragmentation(Page page)
{
Offset pd_lower = ((PageHeader) page)->pd_lower;
Offset pd_upper = ((PageHeader) page)->pd_upper;
Offset pd_special = ((PageHeader) page)->pd_special;
itemIdSortData itemidbase[MaxHeapTuplesPerPage];//存储数据
itemIdSort itemidptr;
ItemId lp;
int nline,
nstorage,
nunused;
int i;
Size totallen;
/*
* It's worth the trouble to be more paranoid here than in most places,
* because we are about to reshuffle data in (what is usually) a shared
* disk buffer. If we aren't careful then corrupted pointers, lengths,
* etc could cause us to clobber adjacent disk buffers, spreading the data
* loss further. So, check everything.
* 在这里比在其他地方执行更多的检查是值得的，因为我们将在(通常是)共享磁盘缓冲区中重新洗牌数据。
* 如果我们不小心，那么损坏的行指针、数据长度等可能会导致与相邻磁盘缓冲区冲突，
* 如果错误进一步传播会导致数据丢失。因此,需要仔细检查。
*/
if (pd_lower < SizeOfPageHeaderData ||
pd_lower > pd_upper ||
pd_upper > pd_special ||
pd_special > BLCKSZ ||
pd_special != MAXALIGN(pd_special))
ereport(ERROR,
(errcode(ERRCODE_DATA_CORRUPTED),
errmsg("corrupted page pointers: lower = %u, upper = %u, special = %u",
pd_lower, pd_upper, pd_special)));
/*
* Run through the line pointer array and collect data about live items.
* 遍历行指针数组,收集存活的条目.
*/
nline = PageGetMaxOffsetNumber(page);//获取最大的偏移
itemidptr = itemidbase;//
nunused = totallen = 0;
for (i = FirstOffsetNumber; i <= nline; i++)
{
//---------- 遍历行指针数组
//获取line pointer
lp = PageGetItemId(page, i);
if (ItemIdIsUsed(lp))
{
//如果ItemId在使用
if (ItemIdHasStorage(lp))
{
//如ItemID与存储相关,判断条件:((itemId)->lp_len != 0)
itemidptr->offsetindex = i - 1;
itemidptr->itemoff = ItemIdGetOffset(lp);
//执行判断
if (unlikely(itemidptr->itemoff < (int) pd_upper ||
itemidptr->itemoff >= (int) pd_special))
ereport(ERROR,
(errcode(ERRCODE_DATA_CORRUPTED),
errmsg("corrupted item pointer: %u",
itemidptr->itemoff)));
//对齐长度
itemidptr->alignedlen = MAXALIGN(ItemIdGetLength(lp));
totallen += itemidptr->alignedlen;
itemidptr++;//数组下一个元素
}
}
else
{
/* Unused entries should have lp_len = 0, but make sure */
//未使用的ItemId
ItemIdSetUnused(lp);
nunused++;
}
}
//数组中存储的元素个数
nstorage = itemidptr - itemidbase;
if (nstorage == 0)
{
/* Page is completely empty, so just reset it quickly */
//page完全是空的,重置page
((PageHeader) page)->pd_upper = pd_special;
}
else
{
/* Need to compact the page the hard way */
//page非空,压缩页
if (totallen > (Size) (pd_special - pd_lower))
ereport(ERROR,
(errcode(ERRCODE_DATA_CORRUPTED),
errmsg("corrupted item lengths: total %u, available space %u",
(unsigned int) totallen, pd_special - pd_lower)));
compactify_tuples(itemidbase, nstorage, page);
}
/* Set hint bit for PageAddItem */
//为PageAddItem方法设置标记位
if (nunused > 0)
//存在未使用的空位,设置标记
PageSetHasFreeLinePointers(page);
else
//清除标记
PageClearHasFreeLinePointers(page);
}
/*
* After removing or marking some line pointers unused, move the tuples to
* remove the gaps caused by the removed items.
* 在清除或者标记某些行指针为没有使用后,移动元组以消除已删除元组之间的鸿沟
*/
static void
compactify_tuples(itemIdSort itemidbase, int nitems, Page page)
{
PageHeader phdr = (PageHeader) page;
Offset upper;
int i;
/* sort itemIdSortData array into decreasing itemoff order */
//以itemoff降序的方式排序itemIdSortData数组
qsort((char *) itemidbase, nitems, sizeof(itemIdSortData),
itemoffcompare);
//重整page
upper = phdr->pd_special;
for (i = 0; i < nitems; i++)
{
itemIdSort itemidptr = &itemidbase[i];
ItemId lp;
lp = PageGetItemId(page, itemidptr->offsetindex + 1);
upper -= itemidptr->alignedlen;
memmove((char *) page + upper,
(char *) page + itemidptr->itemoff,
itemidptr->alignedlen);
lp->lp_off = upper;
}
phdr->pd_upper = upper;
}
/*
* ItemIdSetUnused
* Set the item identifier to be UNUSED, with no storage.
* Beware of multiple evaluations of itemId!
* 设置ItemId为未使用.
*/
#define ItemIdSetUnused(itemId) \
( \
(itemId)->lp_flags = LP_UNUSED, \
(itemId)->lp_off = 0, \
(itemId)->lp_len = 0 \
)测试脚本 : 删除数据,执行vacuum
11:04:59 (xdb@[local]:5432)testdb=# delete from t1 where id < 600; DELETE 100 14:26:16 (xdb@[local]:5432)testdb=# checkpoint; CHECKPOINT 11:18:29 (xdb@[local]:5432)testdb=# vacuum verbose t1;
lazy_vacuum_heap
启动gdb,设置断点
(gdb) b lazy_vacuum_heap Breakpoint 7 at 0x6bdf2e: file vacuumlazy.c, line 1472. (gdb) c Continuing. Breakpoint 7, lazy_vacuum_heap (onerel=0x7f4c70d96688, vacrelstats=0x1873928) at vacuumlazy.c:1472 1472 Buffer vmbuffer = InvalidBuffer; (gdb)
输入参数
1-relation
(gdb) p *onerel
$14 = {rd_node = {spcNode = 1663, dbNode = 16402, relNode = 50820}, rd_smgr = 0x18362e0, rd_refcnt = 1, rd_backend = -1,
rd_islocaltemp = false, rd_isnailed = false, rd_isvalid = true, rd_indexvalid = 1 '\001', rd_statvalid = false,
rd_createSubid = 0, rd_newRelfilenodeSubid = 0, rd_rel = 0x7f4c70d95bb8, rd_att = 0x7f4c70d95cd0, rd_id = 50820,
rd_lockInfo = {lockRelId = {relId = 50820, dbId = 16402}}, rd_rules = 0x0, rd_rulescxt = 0x0, trigdesc = 0x0,
rd_rsdesc = 0x0, rd_fkeylist = 0x0, rd_fkeyvalid = false, rd_partkeycxt = 0x0, rd_partkey = 0x0, rd_pdcxt = 0x0,
rd_partdesc = 0x0, rd_partcheck = 0x0, rd_indexlist = 0x7f4c70d94820, rd_oidindex = 0, rd_pkindex = 0,
rd_replidindex = 0, rd_statlist = 0x0, rd_indexattr = 0x0, rd_projindexattr = 0x0, rd_keyattr = 0x0, rd_pkattr = 0x0,
rd_idattr = 0x0, rd_projidx = 0x0, rd_pubactions = 0x0, rd_options = 0x0, rd_index = 0x0, rd_indextuple = 0x0,
rd_amhandler = 0, rd_indexcxt = 0x0, rd_amroutine = 0x0, rd_opfamily = 0x0, rd_opcintype = 0x0, rd_support = 0x0,
rd_supportinfo = 0x0, rd_indoption = 0x0, rd_indexprs = 0x0, rd_indpred = 0x0, rd_exclops = 0x0, rd_exclprocs = 0x0,
rd_exclstrats = 0x0, rd_amcache = 0x0, rd_indcollation = 0x0, rd_fdwroutine = 0x0, rd_toastoid = 0,
pgstat_info = 0x182a030}2-vacrelstats
存在索引,pages总数为124,扫描pages为124,原存活tuple为9501,新tuples为9401,已删除tuples为100,已删除的tuples的ItemPointer存储在dead_tuples数组中(大小为num_dead_tuples)
(gdb) p *vacrelstats
$15 = {hasindex = true, old_rel_pages = 124, rel_pages = 124, scanned_pages = 124, pinskipped_pages = 0,
frozenskipped_pages = 0, tupcount_pages = 124, old_live_tuples = 9501, new_rel_tuples = 9401, new_live_tuples = 9401,
new_dead_tuples = 0, pages_removed = 0, tuples_deleted = 100, nonempty_pages = 124, num_dead_tuples = 100,
max_dead_tuples = 36084, dead_tuples = 0x1884820, num_index_scans = 0, latestRemovedXid = 397073,
lock_waiter_detected = false}
(gdb)1.初始化变量
(gdb) n
1474 pg_rusage_init(&ru0);
(gdb)
1475 npages = 0;
(gdb)
1477 tupindex = 0;
(gdb) p ru0
$16 = {tv = {tv_sec = 1548743482, tv_usec = 626506}, ru = {ru_utime = {tv_sec = 0, tv_usec = 40060}, ru_stime = {
tv_sec = 0, tv_usec = 114769}, {ru_maxrss = 8900, __ru_maxrss_word = 8900}, {ru_ixrss = 0, __ru_ixrss_word = 0}, {
ru_idrss = 0, __ru_idrss_word = 0}, {ru_isrss = 0, __ru_isrss_word = 0}, {ru_minflt = 5455, __ru_minflt_word = 5455},
{ru_majflt = 0, __ru_majflt_word = 0}, {ru_nswap = 0, __ru_nswap_word = 0}, {ru_inblock = 2616,
__ru_inblock_word = 2616}, {ru_oublock = 376, __ru_oublock_word = 376}, {ru_msgsnd = 0, __ru_msgsnd_word = 0}, {
ru_msgrcv = 0, __ru_msgrcv_word = 0}, {ru_nsignals = 0, __ru_nsignals_word = 0}, {ru_nvcsw = 814,
__ru_nvcsw_word = 814}, {ru_nivcsw = 2, __ru_nivcsw_word = 2}}}2.遍历vacrelstats->num_dead_tuples行指针数组(ItemPointer)
(gdb) n 1478 while (tupindex < vacrelstats->num_dead_tuples) (gdb)
2.1获取块号/读取块到缓冲区中
1485 vacuum_delay_point(); (gdb) 1487 tblk = ItemPointerGetBlockNumber(&vacrelstats->dead_tuples[tupindex]); (gdb) 1488 buf = ReadBufferExtended(onerel, MAIN_FORKNUM, tblk, RBM_NORMAL, (gdb) (gdb) p tblk $17 = 29 (gdb) p buf $18 = 175
2.2加锁,如不成功,则处理下一个元组
1490 if (!ConditionalLockBufferForCleanup(buf)) (gdb)
2.3调用lazy_vacuum_page释放空间,整理碎片
1496 tupindex = lazy_vacuum_page(onerel, tblk, buf, tupindex, vacrelstats, (gdb) p tupindex $1 = 0 (gdb) n 1500 page = BufferGetPage(buf); (gdb) p tupindex $2 = 2 (gdb)
2.4获取page,获取该page的空闲空间
(gdb) n 1500 page = BufferGetPage(buf); (gdb) p tupindex $2 = 2 (gdb) n 1501 freespace = PageGetHeapFreeSpace(page); (gdb)
2.5释放缓冲,记录空闲空间
(gdb) 1503 UnlockReleaseBuffer(buf); (gdb) 1504 RecordPageWithFreeSpace(onerel, tblk, freespace); (gdb) 1505 npages++; (gdb)
lazy_vacuum_page
进入lazy_vacuum_page函数
1496 tupindex = lazy_vacuum_page(onerel, tblk, buf, tupindex, vacrelstats, (gdb) p tblk $3 = 30 (gdb) p buf $4 = 178 (gdb) p tupindex $5 = 2 (gdb) (gdb) step lazy_vacuum_page (onerel=0x7f4c70d95570, blkno=30, buffer=178, tupindex=2, vacrelstats=0x18676a8, vmbuffer=0x7fffaef4a19c) at vacuumlazy.c:1535 1535 Page page = BufferGetPage(buffer); (gdb)
输入参数:块号/缓冲区编号/tuple数组下标以及vacrelstats(统计信息+辅助存储信息,如废弃元组数组等)
(gdb) p vacrelstats->dead_tuples[0]
$6 = {ip_blkid = {bi_hi = 0, bi_lo = 29}, ip_posid = 168}1.初始化相关变量
(gdb) n
1537 int uncnt = 0;
(gdb)
1541 pgstat_progress_update_param(PROGRESS_VACUUM_HEAP_BLKS_VACUUMED, blkno);
(gdb)
1543 START_CRIT_SECTION();
(gdb)
1545 for (; tupindex < vacrelstats->num_dead_tuples; tupindex++)
(gdb) p page
$7 = (Page) 0x7f4c44f46380 "\001"
(gdb) p *page
$8 = 1 '\001'
(gdb) p *(PageHeader *)page
$9 = (PageHeader) 0x4ec2441800000001
(gdb) p *(PageHeader)page
$10 = {pd_lsn = {xlogid = 1, xrecoff = 1321354264}, pd_checksum = 0, pd_flags = 1, pd_lower = 1188, pd_upper = 7856,
pd_special = 8192, pd_pagesize_version = 8196, pd_prune_xid = 0, pd_linp = 0x7f4c44f46398}
(gdb)2.遍历废弃元组数组
2.1获取块号,如块号不一致,跳出循环
2.2获取偏移/行指针
2.3标记为未使用,记录偏移
(gdb) n
1551 tblk = ItemPointerGetBlockNumber(&vacrelstats->dead_tuples[tupindex]);
(gdb)
1552 if (tblk != blkno)
(gdb) p tblk
$11 = 30
(gdb) n
1554 toff = ItemPointerGetOffsetNumber(&vacrelstats->dead_tuples[tupindex]);
(gdb) p vacrelstats->dead_tuples[tupindex]
$12 = {ip_blkid = {bi_hi = 0, bi_lo = 30}, ip_posid = 162}
(gdb) n
1555 itemid = PageGetItemId(page, toff);
(gdb) p toff
$13 = 162
(gdb) n
1556 ItemIdSetUnused(itemid);
(gdb) p itemid
$14 = (ItemId) 0x7f4c44f4661c
(gdb) p *itemid
$15 = {lp_off = 0, lp_flags = 3, lp_len = 0}
(gdb) n
1557 unused[uncnt++] = toff;
(gdb)
1545 for (; tupindex < vacrelstats->num_dead_tuples; tupindex++)
(gdb)3.调用PageRepairFragmentation整理碎片
3.1判断和检查(严谨的编码!!!)
... (gdb) b vacuumlazy.c:1560 Breakpoint 2 at 0x6be604: file vacuumlazy.c, line 1560. (gdb) c Continuing. Breakpoint 2, lazy_vacuum_page (onerel=0x7f4c70d95570, blkno=30, buffer=178, tupindex=5, vacrelstats=0x18676a8, vmbuffer=0x7fffaef4a19c) at vacuumlazy.c:1560 1560 PageRepairFragmentation(page); (gdb) (gdb) step PageRepairFragmentation (page=0x7f4c44f46380 "\001") at bufpage.c:481 481 Offset pd_lower = ((PageHeader) page)->pd_lower; (gdb) n 482 Offset pd_upper = ((PageHeader) page)->pd_upper; (gdb) 483 Offset pd_special = ((PageHeader) page)->pd_special; (gdb) 500 if (pd_lower < SizeOfPageHeaderData || (gdb) p pd_lower $17 = 1188 (gdb) p pd_upper $18 = 7856 (gdb) p pd_special $19 = 8192 (gdb) n 501 pd_lower > pd_upper || (gdb) 502 pd_upper > pd_special || (gdb) 504 pd_special != MAXALIGN(pd_special)) (gdb) 503 pd_special > BLCKSZ ||
3.2获取偏移,初始化变量
(gdb)
513 nline = PageGetMaxOffsetNumber(page);
(gdb) n
514 itemidptr = itemidbase;
(gdb)
515 nunused = totallen = 0;
(gdb) p nline
$20 = 291
(gdb) p *itemidptr
$21 = {offsetindex = 162, itemoff = 8144, alignedlen = 48}
(gdb)3.3遍历行指针数组
3.3.1获取行指针lp
3.3.2如ItemId正在使用,记录到itemidbase数组中;否则标记ItemId未被使用
(gdb) 516 for (i = FirstOffsetNumber; i <= nline; i++) (gdb) n 519 if (ItemIdIsUsed(lp)) (gdb) 539 ItemIdSetUnused(lp); (gdb) 540 nunused++; (gdb) 516 for (i = FirstOffsetNumber; i <= nline; i++) (gdb)
跳出循环,继续执行
516 for (i = FirstOffsetNumber; i <= nline; i++) (gdb) b bufpage.c:544 Breakpoint 3 at 0x8b1d2d: file bufpage.c, line 544. (gdb) c Continuing. Breakpoint 3, PageRepairFragmentation (page=0x7f4c44f46380 "\001") at bufpage.c:544 544 nstorage = itemidptr - itemidbase; (gdb) (gdb) p nunused $22 = 284
3.4计算数组中存储的元素个数
A.如个数为0,重置page
B.否则调用compactify_tuples压缩页
(gdb) n 545 if (nstorage == 0) (gdb) p nstorage $23 = 7 (gdb) n 553 if (totallen > (Size) (pd_special - pd_lower)) (gdb) 559 compactify_tuples(itemidbase, nstorage, page); (gdb)
3.5为PageAddItem方法设置标记位
(gdb) 563 if (nunused > 0) (gdb) 564 PageSetHasFreeLinePointers(page); (gdb) 567 } (gdb)
4.标记buffer为dirty
(gdb) lazy_vacuum_page (onerel=0x7f4c70d95570, blkno=30, buffer=178, tupindex=5, vacrelstats=0x18676a8, vmbuffer=0x7fffaef4a19c) at vacuumlazy.c:1565 1565 MarkBufferDirty(buffer); (gdb) n
5.写入WAL Record
1568 if (RelationNeedsWAL(onerel)) (gdb) 1572 recptr = log_heap_clean(onerel, buffer, (gdb) 1576 PageSetLSN(page, recptr); (gdb) 1585 END_CRIT_SECTION();
6.如all-visible,则设置页面all-visible标记
(gdb) n 1593 if (heap_page_is_all_visible(onerel, buffer, &visibility_cutoff_xid, (gdb) 1595 PageSetAllVisible(page); (gdb)
7.如page为all-visible,设置vm
1602 if (PageIsAllVisible(page)) (gdb) 1604 uint8 vm_status = visibilitymap_get_status(onerel, blkno, vmbuffer); (gdb) 1605 uint8 flags = 0; (gdb) 1608 if ((vm_status & VISIBILITYMAP_ALL_VISIBLE) == 0) (gdb) 1609 flags |= VISIBILITYMAP_ALL_VISIBLE; (gdb) 1610 if ((vm_status & VISIBILITYMAP_ALL_FROZEN) == 0 && all_frozen) (gdb) 1613 Assert(BufferIsValid(*vmbuffer)); (gdb) 1614 if (flags != 0) (gdb) 1615 visibilitymap_set(onerel, blkno, buffer, InvalidXLogRecPtr, (gdb)
8.返回下一个page的起始数组编号
(gdb) 1619 return tupindex; (gdb) p tupindex $24 = 5 (gdb)
到此，关于“PostgreSQL的vacuum过程中lazy_vacuum_heap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，请继续关注创新互联网站，小编会继续努力为大家带来更多实用的文章！
