这篇文章主要介绍“PostgreSQL在响应客户端发出备份命令pg_basebackup时做了什么”，在日常操作中，相信很多人在PostgreSQL在响应客户端发出备份命令pg_basebackup时做了什么问题上存在疑惑，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在响应客户端发出备份命令pg_basebackup时做了什么”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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一、数据结构
basebackup_options
pg_basebackup的选项,在数据库服务器解析为该数据结构.
typedef struct
{
//备份的标签
const char *label;
//是否显示进度
bool progress;
//是否执行快速fast checkpoint?
bool fastcheckpoint;
//nowait?
bool nowait;
//是否包含wal data
bool includewal;
//
uint32 maxrate;
//是否包含表空间映射文件?
bool sendtblspcmapfile;
} basebackup_options;二、源码解读
数据库服务器接收到请求,postmaster启动新的postgres进程响应此请求,此进程被视为walsender,标记am_walsender设置为T,在PostgresMain函数中,将执行以下逻辑:
...
for (;;)//主循环
{
...
switch (firstchar)
{
case 'Q': /* simple query */
{
if (am_walsender)
{
//如为WAL sender,执行exec_replication_command
if (!exec_replication_command(query_string))
exec_simple_query(query_string);
...
...调用exec_replication_command函数,执行相关命令.该函数会调用SendBaseBackup函数执行具体的实现逻辑,其中重点的实现函数是sendFileWithContent/sendDir.
1.sendFileWithContent函数用于发送backup_label等文件到客户端
pq_putmessage发送消息,’d’的消息类型表示CopyData.
static void
sendFileWithContent(const char *filename, const char *content)
{
struct stat statbuf;
int pad,
len;
len = strlen(content);
/*
* Construct a stat struct for the backup_label file we're injecting in
* the tar.
*/
/* Windows doesn't have the concept of uid and gid */
#ifdef WIN32
statbuf.st_uid = 0;
statbuf.st_gid = 0;
#else
statbuf.st_uid = geteuid();
statbuf.st_gid = getegid();
#endif
statbuf.st_mtime = time(NULL);
statbuf.st_mode = pg_file_create_mode;
statbuf.st_size = len;
_tarWriteHeader(filename, NULL, &statbuf, false);
/* Send the contents as a CopyData message */
pq_putmessage('d', content, len);
/* Pad to 512 byte boundary, per tar format requirements */
pad = ((len + 511) & ~511) - len;
if (pad > 0)
{
char buf[512];
MemSet(buf, 0, pad);
pq_putmessage('d', buf, pad);
}
}2.sendDir遍历文件目录,调用sendFile发送到客户端
递归遍历数据库目录,调用sendFile发送文件
...
if (!sizeonly)
sent = sendFile(pathbuf, pathbuf + basepathlen + 1, &statbuf,
true, isDbDir ? pg_atoi(lastDir + 1, sizeof(Oid), 0) : InvalidOid);
if (sent || sizeonly)
{
/* Add size, rounded up to 512byte block */
size += ((statbuf.st_size + 511) & ~511);
size += 512; /* Size of the header of the file */
}
...sendFile发送相应的文件内容到客户端
...
while ((cnt = fread(buf, 1, Min(sizeof(buf), statbuf->st_size - len), fp)) > 0)
{
...
/* Send the chunk as a CopyData message */
if (pq_putmessage('d', buf, cnt))
ereport(ERROR,
(errmsg("base backup could not send data, aborting backup")));
...
}三、跟踪分析
客户端启动pg_basebackup
[xdb@localhost ~]$ pg_basebackup -h 192.168.26.25 -U replicator -p 5432 -D /data/backup -P -Xs -R
Password:
跟踪postmaster,设置跟踪子进程
(gdb) set follow-fork-mode child
(gdb) b PostgresMain
客户端输入密码后,进入断点,在执行BASE_BACKUP命令前,首先会执行SHOW data_directory_mode/SHOW wal_segment_size/IDENTIFY_SYSTEM三个命令,然后再执行BASE_BACKUP命令
...
(gdb) p input_message
$2 = {data = 0x20a1d78 "SHOW data_directory_mode", len = 25, maxlen = 1024, cursor = 0}
...
(gdb) p input_message
$4 = {data = 0x20a1d78 "SHOW wal_segment_size", len = 22, maxlen = 1024, cursor = 0}
...
(gdb) p input_message
$5 = {data = 0x20a1d78 "IDENTIFY_SYSTEM", len = 16, maxlen = 1024, cursor = 0}
...
(gdb) p input_message
$7 = {data = 0x20a1d78 "BASE_BACKUP LABEL 'pg_basebackup base backup' PROGRESS NOWAIT ", len = 67, maxlen = 1024,
cursor = 0}
...跟踪SendBaseBackup
(gdb) b SendBaseBackup
...
4178 if (!exec_replication_command(query_string))
(gdb) step
exec_replication_command (cmd_string=0x20a1d78 "BASE_BACKUP LABEL 'pg_basebackup base backup' PROGRESS NOWAIT ")
at walsender.c:1438
1438 if (got_STOPPING)
1521 SendBaseBackup((BaseBackupCmd *) cmd_node);
...
进入SendBaseBackup
(gdb) c
Continuing.
[New process 1811]
[Thread debugging using libthread_db enabled]
Using host libthread_db library "/lib64/libthread_db.so.1".
[Switching to Thread 0x7f46389b58c0 (LWP 1811)]
Breakpoint 1, SendBaseBackup (cmd=0x2137da8) at basebackup.c:762
762 parse_basebackup_options(cmd->options, &opt);
(gdb) n
764 WalSndSetState(WALSNDSTATE_BACKUP);
(gdb) p opt
$1 = {label = 0x2137760 "pg_basebackup base backup", progress = true, fastcheckpoint = false, nowait = true,
includewal = false, maxrate = 0, sendtblspcmapfile = false}实际执行backup的是函数perform_base_backup
...
(gdb)
775 perform_base_backup(&opt);
(gdb) step
perform_base_backup (opt=0x7ffc96573180) at basebackup.c:232
执行sendXXX
320 if (ti->path == NULL)
(gdb)
325 sendFileWithContent(BACKUP_LABEL_FILE, labelfile->data);
(gdb) p *labelfile
$4 = {
data = 0x2138a50 "START WAL LOCATION: 0/66000028 (file 0000001", '0' , "66)\nCHECKPOINT LOCATION: 0/66000060\nBACKUP METHOD: streamed\nBACKUP FROM: master\nSTART TIME: 2019-03-26 17:05:45 CST\nLABEL: pg_basebackup base"...,
len = 227, maxlen = 1024, cursor = 0}
(gdb) n
331 if (tblspc_map_file && opt->sendtblspcmapfile)
(gdb)
337 sendDir(".", 1, false, tablespaces, true);
(gdb)
340 if (lstat(XLOG_CONTROL_FILE, &statbuf) != 0)
(gdb)
345 sendFile(XLOG_CONTROL_FILE, XLOG_CONTROL_FILE, &statbuf, false);
(gdb)
356 if (opt->includewal && ti->path == NULL)
(gdb)
361 pq_putemptymessage('c'); /* CopyDone */
(gdb)
309 foreach(lc, tablespaces)
(gdb)
364 endptr = do_pg_stop_backup(labelfile->data, !opt->nowait, &endtli);
(gdb)
366 PG_END_ENSURE_ERROR_CLEANUP(base_backup_cleanup, (Datum) 0);
(gdb)
369 if (opt->includewal)
(gdb)
605 SendXlogRecPtrResult(endptr, endtli);
(gdb)
607 if (total_checksum_failures)
(gdb)
623 }
(gdb)
SendBaseBackup (cmd=0x2137da8) at basebackup.c:776
776 }
(gdb)
exec_replication_command (cmd_string=0x20a1d78 "BASE_BACKUP LABEL 'pg_basebackup base backup' PROGRESS NOWAIT ")
at walsender.c:1522
1522 break;
(gdb) c
Continuing.
[Inferior 2 (process 1811) exited normally]
(gdb)到此，关于“PostgreSQL在响应客户端发出备份命令pg_basebackup时做了什么”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！
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