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Backup MySQL in a Second with ZFS

Guest Author

MySQL backup soon becomes an important matter when the database is
used in production. The pain-point comes from the fact that while backuping the database is not available to respond
to client requests anymore. With mysqldump - the
standard tool for performing MySQL backups - and a large database the operation can go over many tenth of
minutes if not hours. If I am running my business on line this is simply not
acceptable.

The
classical approach to workaround this problem
is to take advantage of MySQL replication. I set up a master/slave
configuration where the slave acts as copy of the master. Then, when
needed, I run mysqldump on the
slave without any service interruption on the master.

But ZFS snapshosts bring a new straightforward approach that avoids the pain and the complexity of a master/slave replication.

Snapshots
are a key feature of ZFS that allows me to save a copy of any ZFS file
system in less than a second. Yesterday, Sept 27th I first created
a myDB
ZFS filesystem for my database, and since I decided
to backup my database everyday I also created a first snapshot. I am using InnoDB, so I made sure that both the data and the logs are located in the myDB filesystem. Today,
Sept 28th, I created a second snapshot. Here are the commands:

Yesterday, September 27th


# zfs create pool/myDB
# zfs snapshot pool/myDB@Sept27.2009


Today, September 28th


# zfs snapshot pool/myDB@Sept28.2009

Note
that as long as the dataset does not change, the snapshot does not use
any disk space, so since there was no change to the database when I
created the first snapshot zfs list reporst me a null USED space :


# zfs list -t snapshot
NAME                                     USED AVAIL REFER MOUNTPOINT
pool/myDB@Sept27.2009   0         -          667M   


The
commands are simple, but what about the performance? If I run the
commands just as is, the filesystem that I save takes 667M on disk. On my laptop I
have much more data in my home directory. Let's see how long it takes me to
snapshot this data.


# zfs list rpool/export/home/tmanfe
NAME                             USED  AVAIL  REFER  MOUNTPOINT
rpool/export/home/tmanfe   47.8G   22.5G  28.1G  /export/home/tmanfe

# time  zfs snapshot rpool/export/home/tmanfe@28.2009
real    0m0.370s
user   0m0.004s
sys     0m0.009s


I am able to snapshot 47.9GBytes of data in 0.37seconds.
Good enough. As long as I can stay bellow a second, this will certainly
go unnoticed by the users. By the way, should my dataset increase, the
snapshot time will stay the same.

Now, what do I need to do with MySQL?

For
my data to be properly and consistently saved, I need to make sure that
it gets written to disk. To do this, MySQL provides me with the command
FLUSH and its TABLES WITH READ LOCK option. About this option the documentation says: "Closes all open tables and locks all tables for all
databases with a read lock until you explicitly release the
lock by executing UNLOCK TABLES. This is a very convenient way to get backups
if you have a file system such as Veritas that can take
snapshots in time"
. Note that the documentation should be modified to add ZFS as another filesystem that supports snapshots. What FLUSH TABLES WITH READ LOCK really does is:

  1. Blocks any table update. No new update is accepted
  2. Wait for tables currently being updated to be closed/flushed
  3. Blocks any commit

Since I am using InnoDB I also need to do a couple of things:

  • First, InnoDB does not guaranty that when flushing tables they are actually written to the filesystem. To address this issue I need to snapshot both the tables and the MySQL binary log files, and make sure I save the MASTER STATUS before unlocking tables. The binary log contains all the changes that have been committed to the database, and the master status tells me which modifications in the log files have been applied to the tables and which are still pending. So even if tables are not in a consistent state on the filesystem, as long as I have the binary log I am able to make tables consistent by replaying all the changes from the log that have not yet been applied to the tables. To sum up, for InnoDB,  a straightforward solution consists in having both the log files and the tables on the same filesystem and save the MASTER STATUS while tables are locked by running SHOW MASTER STATUS.
  • I also need to turn autocommit off before running the FLUSH command otherwise every MySQL statement - such as FLUSH TABLES WITH READ LOCK
    - is wrapped into a transaction and at the end of each transaction all
    the table locks are released. In other words, my attempt to lock tables
    would be self-canceling, would have no effect.

No such thing is
required with MyISAM.

Then, I create the ZFS snapshot and I release the tables with the command UNLOCK TABLES. I am done with MySQL.

I need to run and synchronize commands at both the shell level and in MySQL. More specifically, after executing the FLUSH in MySQL, I must not close the connection, otherwise the locks will be freed. An easy way to meet these requirements is to use a Python script:


dbh = MySQLdb.connect(host='localhost', user='mysqdl-admin', passwd=mysql_pass)

dbh.query('FLUSH TABLES WITH READ LOCK;')

os.system('zfs snapshot pool/myDB@Sept29.2009 )

dbh.query('UNLOCK TABLES;')

dbh.close()



The
snapshot I created is located on the same disks and server than my
original data. To put a real backup system in place I need to transfer
the snapshot to another backup server or storage system. Once again ZFS
provides me with all I need. The zfs send command enables me to send a snapshot somewhere else. The only prerequisite is that ZFS must be available on my  backup server in order to take advantage of the snapshot that it receives.

Here is the exact syntax:


# zfs send -Ri pool/myDB@Sept27.2009 pool/myDB@Sept28.2009 | ssh backupHost “zfs receive -Fd pool”


With the -i option, only the difference between my two snapshots is sent to a system named backupHost.
This option enables me to do incremental backups. Ahead of incremental
backups I need to send a first complete snapshot to the backup host. I
did it yesterday, when creating my very first  snapshot:


# zfs send -R pool/myDB@Sept27.2009 | ssh backupHost “zfs receive -Fd pool”


Note that the stream generated by zfs send replicates the filesystem, not the snapshot.

The stream is then funneled into an ssh command that connects to the backup host. Once connected, the zfs receive
command is executed that takes the content of the stream - i.e. the
filesytem - and copies it into a zfs pool that is local to the backup
host.

I am done.

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