I've set mysql parameter innodb_flush_log_at_trx_commit=0. It means that mysql flushes transactions to HDD 1 time per second. Is it true that if mysql will fail with this flush (because of power off) i will lose my data from these transactions. Or mysql will save them in data file (ibdata1) after each transaction regardless of binlog flush?
Thanks.
The binary log contains “events” that describe database changes such as table creation operations or changes to table data. It also contains events for statements that potentially could have made changes (for example, a DELETE which matched no rows), unless row-based logging is used. The binary log also contains information about how long each statement took that updated data. The binary log has two important purposes:
For replication, the binary log on a primary replication server provides a record of the data changes to be sent to secondary servers. The primary server sends the events contained in its binary log to its secondaries, which execute those events to make the same data changes that were made on the primary.
Certain data recovery operations require the use of the binary log. After a backup has been restored, the events in the binary log that were recorded after the backup was made are re-executed. These events bring databases up to date from the point of the backup
The binary log is not used for statements such as SELECT or SHOW that do not modify data.
https://dev.mysql.com/doc/refman/8.0/en/binary-log.html
Here is the entry in the MySQL reference manual for innodb_flush_log_at_trx_commit. You can lose the last second of transactions with the value set to 0.
Note that the binlog is actually something different that is independent of innodb and is used for all storage engines. Here is the chapter on the binary log in the MySQL reference manual.
Related
One of our users is using Cloud SQL(MySQL).
They turn on general logs flag and log_output is file.
They need these general logs because of some special circumstances.
MySQL generates about 8TB of general logs and these logs result a bigger use of the disk.
Here is the tricky part:
They want to remove these general logs file [1] to decrease the size of the disk.
However, this is their production database. They afraid this operation will impact their database's performance.
Since these log files are located in /var/log/mysql.log, the remove logs operation will execute on the OS level, right? -> This is the part we are not so sure.
If our user executes this truncateAPI, will this operation affect their database's performance?
Is there any best practice for this kind of situation?
P.S: Our user doesn't want to turn off general logs flag. They will try to truncate these logs once for a while. But for now, they need to truncate the huge amount of logs that they accumulated in the past few momths.
[1] https://cloud.google.com/sql/docs/mysql/admin-api/v1beta4/instances/truncateLog
I understand that you have turned on general logs flag and log_output is FILE and you want to remove these general logs files to decrease the size of the disk.
According to to the official documentation link:
To make your general or slow query logs available, enable the
corresponding flag and set the log_output flag to FILE. This makes the
log output available using the Logs Viewer in the Google Cloud
Platform Console. Note that Stackdriver logging charges apply.
If log_output is set to NONE, you will not be able to access the logs.
If you set log_output to TABLE, the log output is placed in a table in
your database. If this table becomes large, it can affect instance
restart time or cause the instance to lose its SLA coverage; for this
reason, the TABLE option is not recommended. If needed, you can
truncate your log tables by using the API. For more information, see
the instances.truncateLog reference page.
Instances: truncateLog truncate MySQL general and slow query log tables.
If I understood correctly, you can not "truncate the huge amount of logs that they accumulated in the past few months" because you did not set log_output to TABLE, therefore there are no tables to be truncated.
Regarding database performance: TRUNCATE TABLE Statement
On a system with a large InnoDB buffer pool and
innodb_adaptive_hash_index enabled, TRUNCATE TABLE operations may
cause a temporary drop in system performance due to an LRU scan that
occurs when removing an InnoDB table's adaptive hash index entries.
The problem was addressed for DROP TABLE in MySQL 5.5.23 (Bug
13704145, Bug #64284) but remains a known issue for TRUNCATE TABLE (Bug #68184).
Here you can check MySQL Server Log Maintenance.
Removing MySQL general_log FILES should not impact the performance of the database.
I have a Mysql master-slave(s) replication with MyISAM tables. All updates are done on the master and selects are done on either the master or slaves.
It appears that we might need to manually lock a few tables when we do certain updates. While this write lock is on the tables, no selects can happen on the locked table. But what about on the slaves? Does the lock propagate out?
Say I have table_A and table_B. I initiate a lock on table_A and table_B on the master and start performing the update. At this time no other connection can read table_A and table_B off the master? But what if at this time another connection tries to read the tables off of a slave, can they do so?
Everything that MySQL replicates can be found in the binary logs.
You can run the following command to see the details.
show global variables like 'log_bin%';
log_bin_basename will tell you the path to your binary logs with base file name.
and run
show binary logs
to find the binary files that are currently present on your server.
You can check the actual commands that are written to the file by using mysqlbinlog command together with the file name or by running show binlog events ... from the MySQL CLI.
Also, check what binlog_format are you using.
Basically - the lock of the tables is not directly propagated to slaves, but at the time, whey will execute the performed updates they will perform a lock of the updated table if needed.
As far as I know write locks do not propagate into the binlog, You can verify that by doing quick test and looking at the binlog. If you want to avoid issues on the master aswell and for some reason can not migrate to InnoDB consider integrating something like GET_LOCK() into your application instead of completely locking a table. MyISAM is quite iffy when it comes to concurrency.
Is there any methods to retrieve deleted records from a mysql database?
No.
Deleted records are gone (or munged so badly you can't recover them). If you have autocommit turned on, the system commits each statement as you complete it (if you have auto commit turned off, then do a rollback NOW - phew, you're saved -- but you are running with autocommit, aren't you?).
One other approach is to reply the activity that created the missing records - can you do that? You can either re-run whatever programs did the updates, or replay them from a binary log (if you still have the binary log). That may not be possible, of course.
So you need to recover the data from somewhere - either a backup of your db (made using mysqldump) or of your file system (the data files of MyISAM tables are all simply structured and on the disk - recovering InnoDB tables are complicated by the shared use of ibdata files).
There is a possible way to retrieve deleted records (depending upon your situation). Please check here:
https://stackoverflow.com/a/72303235/2546381
I want to get mysqldump after certain checkpoint e.g. if i take the mysqldump now then next time when i will take the dump it should give me only the commands which executed between this time interval. is there anyway to get this using mysqldump.
One more thing how to show the commands delete, update in the mysqldump files.
Thanks
I dont think this is possible from a MySQLdump, however that feature exists as part of MySQL core - its called Binlogging or binary logging.
The binary log contains “events” that describe database changes such as table creation operations or changes to table data. It also contains events for statements that potentially could have made changes (for example, a DELETE which matched no rows). The binary log also contains information about how long each statement took that updated data
Check this out http://dev.mysql.com/doc/refman/5.0/en/binary-log.html
Word of warning, binlogs can slow down the performance of your server.
There are times when a table / database is dropped unintentionally.
I have to check the date-time of the start position from the binary when the backup was taken.
I do also have to check the date-time of the position where the "drop" statement is found. I do run the mysqlbinlog statement with those parameters.
I can not use start-position and stop-position parameters because the binaries are spread across different files. Is there any better way to handle such human mistakes?
every time you take a backup, you should be using FLUSH TABLES WITH READ LOCK to force all of the tables in to a consistent state, followed by FLUSH LOGS to close the current binary log. then, when you apply the backup, all you have to do is replay one binary log.