mysql binlog replay / max_allowed_packet

mysql binlog replay / max_allowed_packet

  • Written by
    Walter Doekes
  • Published on

Trying to replay MySQL binlogs? And running into max_allowed_packet errors? Fear not. Likely this is not corruption, but packets that really are that big.

In 2021 I wrote about mariabackup and selective table restore. That blog entry shows how one might restore a mariabackup-saved snapshot of a MariaDB (MySQL) database. Specifically, it focuses on recovering only one or two tables selectively and it details optional GPG decryption and decompression.

Here's a recap of the mandatory steps for full recovery and additionally how to replay binlogs to a specific point in time.

Recap snapshot restore

Assume we're starting with a directory with the snapshot of the DB, already decompressed. It might look like this:

$ ls -F1 /path/to/snapshot

Move the log to /var/log/mysql, the rest to /var/lib/mysql and give the files the correct ownership (mysql:mysql).

# mv /path/to/snapshot/ib_logfile* /var/log/mysql/
# mv /path/to/snapshot/* /var/lib/mysql/
# chown -R mysql:mysql /var/lib/mysql/ /var/log/mysql/

From there, become the mysql user and “prepare” the data.

$ cd /var/lib/mysql
$ mariabackup --prepare --use-memory=20G --target-dir=.

This might take some time, depending on how large the snapshot is.

Then, start the database daemon — optionally with --skip-grant-tables if you don't have credentials on hand.

NOTE: If you're restoring something live, make sure clients cannot start connecting before you're all done with the restore.

Replaying binlogs

Time to replay the binlogs.

$ ls -F1 /path/to/binlogs

Check in xtrabackup_binlog_info where we should start.

$ cat /var/lib/mysql/xtrabackup_binlog_info
mariadb-bin.079219  93481903  0-1-2297759094

Dump the binlogs to stdout and feed that to mysql. You want --binary-mode in case there are binary blobs in them. (ASCII '\0' appeared in the statement, but this is not allowed unless option --binary-mode is enabled and mysql is run in non-interactive mode.)

$ mysqlbinlog --start-position=93481903 \
    /path/to/binlogs/mariadb-bin.{079219,079220,079221,079222} | \
    mysql --binary-mode

If all goes well, this replays all the log statements from the binlog, starting from the last position seen in the snapshot.

... unless you run into this:

ERROR 2006 (HY000) at line 21267838: MySQL server has gone away

... and see this in the error.log:

[Warning] Aborted connection 9 to db:
  'unconnected' user: 'root' host: 'localhost'
  (Got a packet bigger than 'max_allowed_packet' bytes)

Darn. An error, and we don't even know which binlog caused it.

And also: why would packets be bigger than they were when they were originally ran? If the MariaDB configuration is unchanged, then surely replaying should work equally well as the original playing. Right?



The binary log (binlog) records changes in the database either as statements or as changed rows. Recording statements can be cheaper in terms of space but riskier in the face of simultaneous updates. Recording changed rows is cheaper to process (because indexed) and double checks data integrity across nodes (if there's an update to a row you didn't have, you can abort early and tell the user to fix the corruption).

If we look at a binlog by passing it to mysqlbinlog, an UPDATE might look like this:

UPDATE aboutconfig_item SET datatype_id = 2 WHERE datatype_id = 1

That's about 65 bytes of packet size. And for binlog-format=STATEMENT style binary logging, that is about all the space it takes.

But that simple query can affect many rows, and when they are logged as binlog-format=ROW (and sometimes MIXED) they are instead recorded as follows:

# at 4112460
#240505 14:51:46 server id 1  end_log_pos 4112460 CRC32 0x91652288      Annotate_rows:
#Q> UPDATE aboutconfig_item SET datatype_id = 2 WHERE datatype_id = 1
#240505 14:51:46 server id 1  end_log_pos 4112537 CRC32 0x55707891      Table_map: `example_db`.`aboutconfig_item` mapped to number 799
# at 4112537
# at 4120607
# at 4128557
#240505 14:51:46 server id 1  end_log_pos 4120607 CRC32 0xcd652823      Update_rows: table id 799
#240505 14:51:46 server id 1  end_log_pos 4128557 CRC32 0xebf94293      Update_rows: table id 799
#240505 14:51:46 server id 1  end_log_pos 4133783 CRC32 0x0275ce04      Update_rows: table id 799 flags: STMT_END_F

# Number of rows: 54
# at 4112299
#240505 14:51:43 server id 1  end_log_pos 4112330 CRC32 0xc9363528      Xid = 271936

The #Q> is purely a comment. The BINLOG statement holds the data of all affected rows. And its “packet size” is significantly larger than 65 bytes.

If you add verbosity (-v) to mysqlbinlog you'll also see:

### UPDATE `example_db`.`aboutconfig_item`
###   @1='2011-03-04 14:36:15'
###   @2='2011-03-04 14:36:15'
###   @3=2
###   @4='autolog.path'
###   @5='/srv/data/example/logs'
### SET
###   @1='2011-03-04 14:36:15'
###   @2='2011-03-04 14:36:15'
###   @3=1
###   @4='autolog.path'
###   @5='/srv/data/example/logs'

This means you get one of those — fairly efficiently stored, but still — for every one of those 54 affected rows. You can see how that might add up if you're doing a coarse update on a big table.

A statement (query) might be large, like an UPDATE with a very long list of WHERE id IN (1, 2, 3, 4, .... 99999). But that is nothing compared to the size of affected data for some queries. This toy example of 54 rows already produces a BINLOG statement of about 30 kilobytes.


When you're using Galera wsrep replication — like we do — then that replication also uses a form of row based replication. The changes get recorded in the binlog as those BINLOG statements we saw above. And, as we've just seen, those statements can be quite large. And when feeding those statements to the mysql client and the mysqld server they are bound by their respective max_allowed_packet settings.

During regular operation, wsrep itself is not limited by that setting. However, when you're replaying binlogs, the setting suddenly becomes important.

After running into the Got a packet bigger than 'max_allowed_packet' bytes error, we whipped up a little script to run on the binlogs that appeared to be problematic:

#!/usr/bin/env python3
# Take mysqlbinlog input. Show longest lines (queries) and longest blocks of
# data (rows to update). Set max_allowed_packet to a higher value.
import sys
from collections import namedtuple

Stats = namedtuple('Stats', 'line blocksize linelength')

def numsep(value):
    "1234567 -> '1_234_567'"
    value = str(value)[-1::-1]
    return '_'.join(value[i:i+3] for i in range(0, len(value), 3))[-1::-1]

def count(fp):
    blockstart = linelength = blocksize = -1

    for lineno, line in enumerate(fp, start=1):
        if line.startswith(b'# at '):
            if blocksize != -1:
                yield Stats(
                    line=blockstart, blocksize=blocksize,
            blockstart = lineno
            blocksize = 0
        blocksize += len(line)
        linelength = max(linelength, len(line))

    if blocksize != -1:
        yield Stats(
            line=blockstart, blocksize=blocksize, linelength=linelength)

def main(fp):
    maxblocksize, maxlinelength = -1, -1
    maxblocksizepos, maxlinelengthpos = -1, -1
    for stats in count(fp):
        if stats.blocksize > maxblocksize:
            maxblocksize = stats.blocksize
            maxblocksizepos = stats.line
        if stats.linelength > maxlinelength:
            maxlinelength = stats.linelength
            maxlinelengthpos = stats.line
    print('largest block at line', maxblocksizepos, ':', numsep(maxblocksize))
    print('longest line at line', maxlinelengthpos, ':', numsep(maxlinelength))

if __name__ == '__main__':
    if sys.stdin.isatty():
        print('please feed me mysqlbinlog output', file=sys.stderr)

Running this on the aforementioned files got us this:

$ mysqlbinlog /path/to/binlogs/mariadb-bin.079219 | ./
largest block at line 13099650 : 5_655_846
longest line at line 13099646 : 4_186_803
$ mysqlbinlog /path/to/binlogs/mariadb-bin.079220 | ./
largest block at line 10869443 : 130_846_886
longest line at line 17253384 : 4_708_572

Some of these statements might be very long — up to 5 megabytes — but the row updates can apparently be huge — up to 130 megabytes.

In this case, we determined that slightly over 130MiB was the max_allowed_packet we needed, significantly more than the value we had configured. And indeed, increasing it to 256MiB for good measure made the replay succeed.

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