Using LVM for MySQL Backup and Replication SetupPeter Zaitsev
If someone asks me about MySQL Backup advice my first question would be if they have LVM installed or have some systems with similar features set for other operation systems. Veritas File System can do it for Solaris. Most SAN systems would work as well.
What is really needed is ability to create atomic snapshot of the volume, which can be later mounted same as original file system
Why snapshot based MySQL backups are great ?
There are number of reasons:
Almost Hot backup In most cases you can perform this type of backup while your application is running. No need to shut down server, make it read only or anything like it.
Support for all local disk based storage engines It works with MyISAM and Innodb and BDB, It also should work with Solid, PrimeXT and Falcon storage engines.
Fast Backup You simply do file copy in the binary form so it is hard to beat in speed.
Low Overhead It is simply file copy so overhead to the server is minimal.
Easy to Integrate Do you want to compress backup ? Backup it to tape, FTP or any network backup software – it is easy as you just need to copy files.
Fast Recovery Recovery time is as fast as putting data back and standard MySQL crash recovery, and it can be reduced even further. More on this later.
Free No extra commercial tools as Innodb Hot Backup are required to perform backup.
Are there any downsides ?
Need to have snapshot campatibility – this is obvious one.
May need root access In some organizations DBA and System Administrator are different people from different departmnents which might not like to trade access rights between each other.
Hard to predict downtime I mentioned this solution is often hot backup, but bad thing it is hard to estimate when it is hot and when it is not – FLUSH TABLES WITH READ LOCK may take quite a while to complete on systems with long queries.
Problems with data on multiple volumes If you have logs on separate devices or just your database spanning across multiple volumes you will be in trouble as you will not get consistent snapshot across all the database. Some systems may be able to do atomic snapshot of many volumes.
Lets speak a bit about how LVM and snapshotting in general works. Really there are different implementations but the sake of them is to provide you with volume which consistently matches state of the volume at the time storage is created. In LVM it is implementeed as copy on write. Special storage area allocated on device where old version of changed pages are stored. You can think about it as about simplified form of versioning like in Innodb if it is closer to you. In other cases snapshot may be implemented by tripple-mirroring. Ie you have RAID1 volume but there are 3 copies of data rather than 2. So you can move one devices out of mirror and use it as snapshot while still having your data safe and secure.
There are two types of snapshots – some of them are read-only while others can be read-write. read-only snapshots may sound good enough as you’re only going to read data anyway, but in reality read-write snapshots have number of benefits. First no extra handling is needed for journaling file sytems – you can simply do journal recovery on snapshot. With read-only snapshot you need to make sure filesystem synchronizes device before snapshot is taken so no journal replay is needed.
The other benefit of read-write snapshot is you can actually start MySQL Server on it and perform recovery, check tables or do whatever else you might need to do to ensure your backup is consistent. Backing up database which was already corrupted is very nasty problem you want to avoid.
Let’s now see what exactly you need to do to perform backup of MySQL Database (or create slave) using LVM2 on Linux.
1) Connect to MySQL and run FLUSH TABLES WITH READ LOCK
Note – this command may take a while to complete if you have long running queries. The catch here is FLUSH TABLES WITH READ LOCK actually waits for all statements to complete, even selects. So be careful if you have any long running queries. If you’re using only Innodb tables and do not need to synchronize binary log position with backup you can skip this step.
2) While holding connection open run: lvcreate -L16G -s -n dbbackup /dev/Main/Data – This will create snapshot named dbbackup for Logical Volume Main/Data . You should specify enough of undo space to hold modifications during backup process – I’ve specified 16GB in this case. If your undo size is not large enough snapshot will get invalidated and backup will be aborted.
Sometimes you might run into the errors on this step, The most common one I’ve resently seen is: snapshot: Required device-mapper target(s) not detected in your kernel – This means snapshot module is not loaded in your kernel by default and you need to load it, which is done by running modprobe dm-snapshot
3) Now you have created logical volume and can unlock the tables, but before that you should probably record binary log position which is done by running SHOW MASTER STATUS – This is binary log position you’ll need to point your MySQL Slaves created from this snapshot.
4) Snapshot created, now you want to let MySQL Server to continue, which is done by running UNLOCK TABLES or simply closing connection.
5) Mount backup Filesystem: mount /dev/Main/dbbackup /mnt/backup
6) Copy data to backup. Normally you can skip slow query logs and error log while taking backup. You also can skip most of binary logs – however if some of your slaves are far behind you might want to keep some of last binary logs just in case, or you can assume in case of recovery from the backup you will need to restore slaves as well and skip binary logs in your backup process.
7) Unmount filesystem umount /mnt/backup
8) Remove snapshot: lvremove -f /dev/Main/dbbackup
If you want to create slave based on such snapshot you need to perform couple of more simple steps
9) Extract/Copy database to the slave database directory.
10) Start MySQL Server. Wait for it to perform recovery.
11) Use CHANGE MASTER TO to point slave to saved binary log position:
change master to master_host="master", master_user="user", master_password="password", master_log_file="host-bin.000335", master_log_pos=401934686;
12) Run SLAVE START to restart replication.
With slightly modified process you can clone slaves from the slaves without stopping them – you just need to use SHOW SLAVE STATUS instead of SHOW MASTER STATUS to find out appropriate binary log position. Be careful however – cloning slave from the slave also clones inconsistences in data which slave could have accomulated – especially if you use slave_skip_errors or sql_slave_skip_counter. Cloning master you’re starting from consistent copy.
If you’re interested in ready script you can try mylvmbackup by Lenz Grimmer