本篇内容介绍了“PostgreSQL中Old Master节点分析”的有关知识，在实际案例的操作过程中，不少人都会遇到这样的困境，接下来就让小编带领大家学习一下如何处理这些情况吧！希望大家仔细阅读，能够学有所成！
基于streaming replication搭建的PostgreSQL HA环境,如出现网络访问/硬件故障等原因导致Standby节点升级为Master节点,但Old Master节点数据库并未损坏,在排除故障后Old Master节点可以通过pg_rewind工具而不需要通过备份的方式成为New Master节点的Standby节点.
在执行命令pg_rewind时,到底做了什么?
在PostgreSQL HA环境中,Standby节点升级为Master节点后,时间线会切换为新的时间线,比如从1变为2.而Old Master节点的时间线仍然为原来的时间线,比如仍为1,那么使用pg_rewind工具,Old Master节点如何从New Master节点读取相关的数据成为新的Standby节点?
简单来说,有以下几步:
1.确定New Master和Old Master数据一致性的Checkpoint位置.在该位置上,New Master和Old Master数据完全一致.这可以通过读取新Old Master节点时间线历史文件可以获得,该文件位于$PGDATA/pg_wal/目录下,文件名称为XX.history
2.Old Master节点根据上一步获取的Checkpoint读取本机日志文件WAL Record,获取在此Checkpoint之后出现变化的Block,并以链表的方式存储Block编号等信息
3.根据第2步获取的Block信息从New Master节点拷贝相应的Block,替换Old Master节点相应的Block
4.拷贝New Master节点上除数据文件外的所有其他文件,包括配置文件等(如果拷贝数据文件,与备份方式搭建区别不大)
5.Old Master启动数据库,应用从Checkpoint开始后的WAL Record.
在执行主备切换后,New Master节点的时间线切换为n + 1,通过pg_rewind可使Old Master在分叉点开始与New Master同步,成为New Standby节点.
XLogRecPtr
64bit的WAL Record寻址空间地址.
/* * Pointer to a location in the XLOG. These pointers are 64 bits wide, * because we don't want them ever to overflow. * 指向XLOG中的位置. * 这些指针大小为64bit,以确保指针不会溢出. */ typedef uint64 XLogRecPtr;
TimeLineID
时间线ID
typedef uint32 TimeLineID;
pg_rewind的源码较为简单,详细请参考注释.
int
main(int argc, char **argv)
{
static struct option long_options[] = {
{"help", no_argument, NULL, '?'},
{"target-pgdata", required_argument, NULL, 'D'},
{"source-pgdata", required_argument, NULL, 1},
{"source-server", required_argument, NULL, 2},
{"version", no_argument, NULL, 'V'},
{"dry-run", no_argument, NULL, 'n'},
{"no-sync", no_argument, NULL, 'N'},
{"progress", no_argument, NULL, 'P'},
{"debug", no_argument, NULL, 3},
{NULL, 0, NULL, 0}
};//命令选项
int option_index;//选项编号
int c;//字符ASCII码
XLogRecPtr divergerec;//分支点
int lastcommontliIndex;
XLogRecPtr chkptrec;//checkpoint Record位置
TimeLineID chkpttli;//时间线
XLogRecPtr chkptredo;checkpoint REDO位置
size_t size;
char *buffer;//缓冲区
bool rewind_needed;//是否需要rewind
XLogRecPtr endrec;//结束点
TimeLineID endtli;//结束时间线
ControlFileData ControlFile_new;//新的控制文件
set_pglocale_pgservice(argv[0], PG_TEXTDOMAIN("pg_rewind"));
progname = get_progname(argv[0]);
/* Process command-line arguments */
//处理命令行参数
if (argc > 1)
{
if (strcmp(argv[1], "--help") == 0 || strcmp(argv[1], "-?") == 0)
{
usage(progname);
exit(0);
}
if (strcmp(argv[1], "--version") == 0 || strcmp(argv[1], "-V") == 0)
{
puts("pg_rewind (PostgreSQL) " PG_VERSION);
exit(0);
}
}
while ((c = getopt_long(argc, argv, "D:nNP", long_options, &option_index)) != -1)
{
switch (c)
{
case '?':
fprintf(stderr, _("Try \"%s --help\" for more information.\n"), progname);
exit(1);
case 'P':
showprogress = true;
break;
case 'n':
dry_run = true;
break;
case 'N':
do_sync = false;
break;
case 3:
debug = true;
break;
case 'D': /* -D or --target-pgdata */
datadir_target = pg_strdup(optarg);
break;
case 1: /* --source-pgdata */
datadir_source = pg_strdup(optarg);
break;
case 2: /* --source-server */
connstr_source = pg_strdup(optarg);
break;
}
}
if (datadir_source == NULL && connstr_source == NULL)
{
fprintf(stderr, _("%s: no source specified (--source-pgdata or --source-server)\n"), progname);
fprintf(stderr, _("Try \"%s --help\" for more information.\n"), progname);
exit(1);
}
if (datadir_source != NULL && connstr_source != NULL)
{
fprintf(stderr, _("%s: only one of --source-pgdata or --source-server can be specified\n"), progname);
fprintf(stderr, _("Try \"%s --help\" for more information.\n"), progname);
exit(1);
}
if (datadir_target == NULL)
{
fprintf(stderr, _("%s: no target data directory specified (--target-pgdata)\n"), progname);
fprintf(stderr, _("Try \"%s --help\" for more information.\n"), progname);
exit(1);
}
if (optind < argc)
{
fprintf(stderr, _("%s: too many command-line arguments (first is \"%s\")\n"),
progname, argv[optind]);
fprintf(stderr, _("Try \"%s --help\" for more information.\n"), progname);
exit(1);
}
/*
* Don't allow pg_rewind to be run as root, to avoid overwriting the
* ownership of files in the data directory. We need only check for root
* -- any other user won't have sufficient permissions to modify files in
* the data directory.
* 不需要以root用户运行pg_rewind,避免覆盖数据目录中的文件owner.
* 只需要检查root用户,其他用户没有足够的权限更新数据目录中的文件.
*/
#ifndef WIN32
if (geteuid() == 0)
{
//root用户
fprintf(stderr, _("cannot be executed by \"root\"\n"));
fprintf(stderr, _("You must run %s as the PostgreSQL superuser.\n"),
progname);
exit(1);
}
#endif
get_restricted_token(progname);
/* Set mask based on PGDATA permissions */
//根据PGDATA的权限设置权限mask
if (!GetDataDirectoryCreatePerm(datadir_target))
{
fprintf(stderr, _("%s: could not read permissions of directory \"%s\": %s\n"),
progname, datadir_target, strerror(errno));
exit(1);
}
umask(pg_mode_mask);
/* Connect to remote server */
//连接到远程服务器
if (connstr_source)
libpqConnect(connstr_source);
/*
* Ok, we have all the options and we're ready to start. Read in all the
* information we need from both clusters.
* 现在,我们有了相关的执行运行,准备开始运行.
* 从两个db clusters中读取所有需要的信息.
*/
//读取目标控制文件
buffer = slurpFile(datadir_target, "global/pg_control", &size);
digestControlFile(&ControlFile_target, buffer, size);
pg_free(buffer);
//读取源控制文件
buffer = fetchFile("global/pg_control", &size);
digestControlFile(&ControlFile_source, buffer, size);
pg_free(buffer);
sanityChecks();
/*
* If both clusters are already on the same timeline, there's nothing to
* do.
* 如果两个clusters已经是同一个时间线,没有什么好做的了,报错.
*/
if (ControlFile_target.checkPointCopy.ThisTimeLineID == ControlFile_source.checkPointCopy.ThisTimeLineID)
{
printf(_("source and target cluster are on the same timeline\n"));
rewind_needed = false;
}
else
{
//找到分叉点
findCommonAncestorTimeline(&divergerec, &lastcommontliIndex);
printf(_("servers diverged at WAL location %X/%X on timeline %u\n"),
(uint32) (divergerec >> 32), (uint32) divergerec,
targetHistory[lastcommontliIndex].tli);
/*
* Check for the possibility that the target is in fact a direct
* ancestor of the source. In that case, there is no divergent history
* in the target that needs rewinding.
* 检查目标是源的直接祖先的可能性.
* 在这种情况下,在需要调整的目标中就没有不同的历史.
*/
if (ControlFile_target.checkPoint >= divergerec)
{
//如果目标的checkpoint > 分叉点,则需要rewind
rewind_needed = true;
}
else
{
//目标的checkpoint <= 分叉点
XLogRecPtr chkptendrec;
/* Read the checkpoint record on the target to see where it ends. */
//读取目标的checkpoint记录,检查在哪结束?
chkptendrec = readOneRecord(datadir_target,
ControlFile_target.checkPoint,
targetNentries - 1);
/*
* If the histories diverged exactly at the end of the shutdown
* checkpoint record on the target, there are no WAL records in
* the target that don't belong in the source's history, and no
* rewind is needed.
* 如果正好在shutdown checkpoint Record处出现分叉,
* 那么在目标cluster中没有WAL Record属于源cluster历史,
* 不需要进行rewind操作,否则需要rewind.
*/
if (chkptendrec == divergerec)
rewind_needed = false;
else
rewind_needed = true;
}
}
if (!rewind_needed)
{
//不需要rewind,退出
printf(_("no rewind required\n"));
exit(0);
}
//找到目标cluster最后的checkpoint点
findLastCheckpoint(datadir_target, divergerec,
lastcommontliIndex,
&chkptrec, &chkpttli, &chkptredo);
printf(_("rewinding from last common checkpoint at %X/%X on timeline %u\n"),
(uint32) (chkptrec >> 32), (uint32) chkptrec,
chkpttli);
/*
* Build the filemap, by comparing the source and target data directories.
* 通过对比源和目标数据目录构建filemap
*/
//创建filemap
filemap_create();
pg_log(PG_PROGRESS, "reading source file list\n");
fetchSourceFileList();
pg_log(PG_PROGRESS, "reading target file list\n");
traverse_datadir(datadir_target, &process_target_file);
/*
* Read the target WAL from last checkpoint before the point of fork, to
* extract all the pages that were modified on the target cluster after
* the fork. We can stop reading after reaching the final shutdown record.
* XXX: If we supported rewinding a server that was not shut down cleanly,
* we would need to replay until the end of WAL here.
* 从在分叉点之前的最后一个checkpoint开始读取目标WAL Record,
* 提取目标cluster上在分叉后所有被修改的pages.
* 在到达最后一个shutdown record时停止读取.
* XXX: 如果我们支持非正常关闭的数据库rewind,需要在这里重放WAL Record到WAL的末尾.
*/
//构造filemap
pg_log(PG_PROGRESS, "reading WAL in target\n");
extractPageMap(datadir_target, chkptrec, lastcommontliIndex,
ControlFile_target.checkPoint);
filemap_finalize();
if (showprogress)
calculate_totals();
/* this is too verbose even for verbose mode */
//如为debug模式,则打印filemap
if (debug)
print_filemap();
/*
* Ok, we're ready to start copying things over.
* 现在可以开始拷贝了.
*/
if (showprogress)
{
pg_log(PG_PROGRESS, "need to copy %lu MB (total source directory size is %lu MB)\n",
(unsigned long) (filemap->fetch_size / (1024 * 1024)),
(unsigned long) (filemap->total_size / (1024 * 1024)));
fetch_size = filemap->fetch_size;
fetch_done = 0;
}
/*
* This is the point of no return. Once we start copying things, we have
* modified the target directory and there is no turning back!
* 到了这里,已无回头路可走了.
* 一旦开始拷贝,就必须更新目标路径,无法回头!
*/
//
executeFileMap();
progress_report(true);
//创建backup_label文件并更新控制文件
pg_log(PG_PROGRESS, "\ncreating backup label and updating control file\n");
createBackupLabel(chkptredo, chkpttli, chkptrec);
/*
* Update control file of target. Make it ready to perform archive
* recovery when restarting.
* 更新目标控制文件.在重启时可执行归档恢复.
*
* minRecoveryPoint is set to the current WAL insert location in the
* source server. Like in an online backup, it's important that we recover
* all the WAL that was generated while we copied the files over.
* minRecoveryPoint设置为目标服务器上当前WAL插入的位置.
* 与在线backup类似,在拷贝和覆盖文件时根据所有生成的WAL日志进行恢复是很重要的.
*/
//更新控制文件
memcpy(&ControlFile_new, &ControlFile_source, sizeof(ControlFileData));
if (connstr_source)
{
//获取源WAL插入的位置
endrec = libpqGetCurrentXlogInsertLocation();
//获取时间线
endtli = ControlFile_source.checkPointCopy.ThisTimeLineID;
}
else
{
endrec = ControlFile_source.checkPoint;
endtli = ControlFile_source.checkPointCopy.ThisTimeLineID;
}
//更新控制文件
ControlFile_new.minRecoveryPoint = endrec;
ControlFile_new.minRecoveryPointTLI = endtli;
ControlFile_new.state = DB_IN_ARCHIVE_RECOVERY;
update_controlfile(datadir_target, progname, &ControlFile_new, do_sync);
pg_log(PG_PROGRESS, "syncing target data directory\n");
//同步数据目录(除数据文件之外)
syncTargetDirectory();
printf(_("Done!\n"));
return 0;
}“PostgreSQL中Old Master节点分析”的内容就介绍到这里了，感谢大家的阅读。如果想了解更多行业相关的知识可以关注亿速云网站，小编将为大家输出更多高质量的实用文章！
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