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Improve server code to read files as part of a base backup.

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Don't use fread(), since that doesn't necessarily set errno. We could
use read() instead, but it's even better to use pg_pread(), which
allows us to avoid some extra calls to seek to the desired location in
the file.

Also, advertise a wait event while reading from a file, as we do for
most other places where we're reading data from files.

Patch by me, reviewed by Hamid Akhtar.

Discussion: http://postgr.es/m/CA+TgmobBw-3573vMosGj06r72ajHsYeKtksT_oTxH8XvTL7DxA@mail.gmail.com
This commit is contained in:
Robert Haas 2020-06-17 11:39:17 -04:00
parent 453e0e3f0e
commit 2fd2effc50
4 changed files with 86 additions and 67 deletions
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4
doc/src/sgml/monitoring.sgml
View File

@ -1193,6 +1193,10 @@ postgres 27093 0.0 0.0 30096 2752 ? Ss 11:34 0:00 postgres: ser
</thead>
<tbody>
<row>
<entry><literal>BaseBackupRead</literal></entry>
<entry>Waiting for base backup to read from a file.</entry>
</row>
<row>
<entry><literal>BufFileRead</literal></entry>
<entry>Waiting for a read from a buffered file.</entry>

3
src/backend/postmaster/pgstat.c
View File

@ -3931,6 +3931,9 @@ pgstat_get_wait_io(WaitEventIO w)
switch (w)
{
case WAIT_EVENT_BASEBACKUP_READ:
event_name = "BaseBackupRead";
break;
case WAIT_EVENT_BUFFILE_READ:
event_name = "BufFileRead";
break;

143
src/backend/replication/basebackup.c
View File

@ -81,6 +81,8 @@ static int compareWalFileNames(const ListCell *a, const ListCell *b);
static void throttle(size_t increment);
static void update_basebackup_progress(int64 delta);
static bool is_checksummed_file(const char *fullpath, const char *filename);
static int basebackup_read_file(int fd, char *buf, size_t nbytes, off_t offset,
const char *filename, bool partial_read_ok);
/* Was the backup currently in-progress initiated in recovery mode? */
static bool backup_started_in_recovery = false;
@ -98,18 +100,6 @@ static char *statrelpath = NULL;
*/
#define THROTTLING_FREQUENCY 8
/*
* Checks whether we encountered any error in fread(). fread() doesn't give
* any clue what has happened, so we check with ferror(). Also, neither
* fread() nor ferror() set errno, so we just throw a generic error.
*/
#define CHECK_FREAD_ERROR(fp, filename) \
do { \
if (ferror(fp)) \
ereport(ERROR, \
(errmsg("could not read from file \"%s\"", filename))); \
} while (0)
/* The actual number of bytes, transfer of which may cause sleep. */
static uint64 throttling_sample;
@ -600,7 +590,7 @@ perform_base_backup(basebackup_options *opt)
foreach(lc, walFileList)
{
char *walFileName = (char *) lfirst(lc);
FILE *fp;
int fd;
char buf[TAR_SEND_SIZE];
size_t cnt;
pgoff_t len = 0;
@ -608,8 +598,8 @@ perform_base_backup(basebackup_options *opt)
snprintf(pathbuf, MAXPGPATH, XLOGDIR "/%s", walFileName);
XLogFromFileName(walFileName, &tli, &segno, wal_segment_size);
fp = AllocateFile(pathbuf, "rb");
if (fp == NULL)
fd = OpenTransientFile(pathbuf, O_RDONLY | PG_BINARY);
if (fd < 0)
{
int save_errno = errno;
@ -626,7 +616,7 @@ perform_base_backup(basebackup_options *opt)
errmsg("could not open file \"%s\": %m", pathbuf)));
}
if (fstat(fileno(fp), &statbuf) != 0)
if (fstat(fd, &statbuf) != 0)
ereport(ERROR,
(errcode_for_file_access(),
errmsg("could not stat file \"%s\": %m",
@ -642,9 +632,10 @@ perform_base_backup(basebackup_options *opt)
/* send the WAL file itself */
_tarWriteHeader(pathbuf, NULL, &statbuf, false);
while ((cnt = fread(buf, 1,
Min(sizeof(buf), wal_segment_size - len),
fp)) > 0)
while ((cnt = basebackup_read_file(fd, buf,
Min(sizeof(buf),
wal_segment_size - len),
len, pathbuf, true)) > 0)
{
CheckXLogRemoved(segno, tli);
/* Send the chunk as a CopyData message */
@ -660,8 +651,6 @@ perform_base_backup(basebackup_options *opt)
break;
}
CHECK_FREAD_ERROR(fp, pathbuf);
if (len != wal_segment_size)
{
CheckXLogRemoved(segno, tli);
@ -676,7 +665,7 @@ perform_base_backup(basebackup_options *opt)
*/
Assert(wal_segment_size % TAR_BLOCK_SIZE == 0);
FreeFile(fp);
CloseTransientFile(fd);
/*
* Mark file as archived, otherwise files can get archived again
@ -1575,7 +1564,7 @@ sendFile(const char *readfilename, const char *tarfilename,
struct stat *statbuf, bool missing_ok, Oid dboid,
backup_manifest_info *manifest, const char *spcoid)
{
FILE *fp;
int fd;
BlockNumber blkno = 0;
bool block_retry = false;
char buf[TAR_SEND_SIZE];
@ -1594,8 +1583,8 @@ sendFile(const char *readfilename, const char *tarfilename,
pg_checksum_init(&checksum_ctx, manifest->checksum_type);
fp = AllocateFile(readfilename, "rb");
if (fp == NULL)
fd = OpenTransientFile(readfilename, O_RDONLY | PG_BINARY);
if (fd < 0)
{
if (errno == ENOENT && missing_ok)
return false;
@ -1637,8 +1626,27 @@ sendFile(const char *readfilename, const char *tarfilename,
}
}
while ((cnt = fread(buf, 1, Min(sizeof(buf), statbuf->st_size - len), fp)) > 0)
/*
* Loop until we read the amount of data the caller told us to expect. The
* file could be longer, if it was extended while we were sending it, but
* for a base backup we can ignore such extended data. It will be restored
* from WAL.
*/
while (len < statbuf->st_size)
{
/* Try to read some more data. */
cnt = basebackup_read_file(fd, buf,
Min(sizeof(buf), statbuf->st_size - len),
len, readfilename, true);
/*
* If we hit end-of-file, a concurrent truncation must have occurred.
* That's not an error condition, because WAL replay will fix things
* up.
*/
if (cnt == 0)
break;
/*
* The checksums are verified at block level, so we iterate over the
* buffer in chunks of BLCKSZ, after making sure that
@ -1689,16 +1697,15 @@ sendFile(const char *readfilename, const char *tarfilename,
*/
if (block_retry == false)
{
/* Reread the failed block */
if (fseek(fp, -(cnt - BLCKSZ * i), SEEK_CUR) == -1)
{
ereport(ERROR,
(errcode_for_file_access(),
errmsg("could not fseek in file \"%s\": %m",
readfilename)));
}
int reread_cnt;
if (fread(buf + BLCKSZ * i, 1, BLCKSZ, fp) != BLCKSZ)
/* Reread the failed block */
reread_cnt =
basebackup_read_file(fd, buf + BLCKSZ * i,
BLCKSZ, len + BLCKSZ * i,
readfilename,
false);
if (reread_cnt == 0)
{
/*
* If we hit end-of-file, a concurrent
@ -1708,24 +1715,8 @@ sendFile(const char *readfilename, const char *tarfilename,
* code that handles that case. (We must fix
* up cnt first, though.)
*/
if (feof(fp))
{
cnt = BLCKSZ * i;
break;
}
ereport(ERROR,
(errcode_for_file_access(),
errmsg("could not reread block %d of file \"%s\": %m",
blkno, readfilename)));
}
if (fseek(fp, cnt - BLCKSZ * i - BLCKSZ, SEEK_CUR) == -1)
{
ereport(ERROR,
(errcode_for_file_access(),
errmsg("could not fseek in file \"%s\": %m",
readfilename)));
cnt = BLCKSZ * i;
break;
}
/* Set flag so we know a retry was attempted */
@ -1768,20 +1759,8 @@ sendFile(const char *readfilename, const char *tarfilename,
len += cnt;
throttle(cnt);
if (feof(fp) || len >= statbuf->st_size)
{
/*
* Reached end of file. The file could be longer, if it was
* extended while we were sending it, but for a base backup we can
* ignore such extended data. It will be restored from WAL.
*/
break;
}
}
CHECK_FREAD_ERROR(fp, readfilename);
/* If the file was truncated while we were sending it, pad it with zeros */
if (len < statbuf->st_size)
{
@ -1810,7 +1789,7 @@ sendFile(const char *readfilename, const char *tarfilename,
update_basebackup_progress(pad);
}
FreeFile(fp);
CloseTransientFile(fd);
if (checksum_failures > 1)
{
@ -1996,3 +1975,35 @@ update_basebackup_progress(int64 delta)
pgstat_progress_update_multi_param(nparam, index, val);
}
/*
* Read some data from a file, setting a wait event and reporting any error
* encountered.
*
* If partial_read_ok is false, also report an error if the number of bytes
* read is not equal to the number of bytes requested.
*
* Returns the number of bytes read.
*/
static int
basebackup_read_file(int fd, char *buf, size_t nbytes, off_t offset,
const char *filename, bool partial_read_ok)
{
int rc;
pgstat_report_wait_start(WAIT_EVENT_BASEBACKUP_READ);
rc = pg_pread(fd, buf, nbytes, offset);
pgstat_report_wait_end();
if (rc < 0)
ereport(ERROR,
(errcode_for_file_access(),
errmsg("could not read file \"%s\": %m", filename)));
if (!partial_read_ok && rc > 0 && rc != nbytes)
ereport(ERROR,
(errcode_for_file_access(),
errmsg("could not read file \"%s\": read %d of %zu",
filename, rc, nbytes)));
return rc;
}

3
src/include/pgstat.h
View File

@ -913,7 +913,8 @@ typedef enum
*/
typedef enum
{
WAIT_EVENT_BUFFILE_READ = PG_WAIT_IO,
WAIT_EVENT_BASEBACKUP_READ = PG_WAIT_IO,
WAIT_EVENT_BUFFILE_READ,
WAIT_EVENT_BUFFILE_WRITE,
WAIT_EVENT_CONTROL_FILE_READ,
WAIT_EVENT_CONTROL_FILE_SYNC,