http://www.mysqlperformanceblog.com/2009/10/16/finding-your-mysql-high-availability-solution-%e2%80%93-the-questions/

How to determine which MySQL High-Availability solution is best?

What is really tricky with MySQL is the number of possible HA solutions. From the simplest the most complex let’s list the most common ones:

- MySQL replication with manual failover
- Master-Master with MMM manager
- Heartbeat/SAN
- Heartbeat/DRBD
- NDB Cluster

These technologies are by far, not a one size fits all and many deployments use combination of solutions. I will not cover ScaleDB and Continuent because I know almost nothing of these solutions. There are many more questions you need to ask yourself before being able to pick the right one. Below, I listed the most common questions, I might have missed some.

1. What level of HA do you need?

Since all the technologies do not offer the same level of availability, this is a first important sorting factor. Here are estimates of the level of availability offered by the various solutions.

 Level of availability
Simple replication98 to 99.9+%
Master-Master with MMM manager99%
Heartbeat/SAN (depends on SAN)99.5% to 99.9%
Heartbeat/DRBD99.9%
NDB Cluster99.999%

From the table, if your requirements are for 99.99%, you are restricted to NDB Cluster while if it is only 99% you have more options. I recall that the level of availability is hard to estimate and subject to debate. These are the usually accepted level of availability for these technologies.

2. Can you afford to lose data?

Obviously, if you are concerned about loss of data, you are most likely using the InnoDB storage engine, since MyISAM is not transactional and do not sync data to disk. Similarly, MySQL replication is an asynchronous process and although it is fairly fast at transferring data between the master and the slaves, there is a window of time where data loss is possible.

If you can afford to lose some data, you can consider “MySQL replication” and “Master-Master with MMM manager” otherwise, you can only consider the other three solutions.

 Data 100% safe
MySQL replicationno
Master-Master with MMM managerno
Heartbeat/SAN (depends on SAN)yes
Heartbeat/DRBD yes
NDB Clusteryes

3. Does your application use MyISAM only features?

There are some features like Full text indexes and GIS indexes that are supported only by MyISAM. The HA solutions that work well with MyISAM are “MySQL replication” and “Master-Master with MMM manager”. Depending on the application, the MyISAM Full text indexes might be replaced by another search engine like Sphinx in order to remove the restriction. There is no HA solution other than the ones based on replication that handles GIS indexes.

 HA solutions
Need MyISAM Full text or GIS indexes “MySQL replication” and “Master-Master with MMM manager”
Don’t use any special MyISAM featureAll
Can change MyISAM Full text to SphinxAll

4. What is the write load?

The HA solutions we present are not equal in term of their write capacity. Due to the way replication is implemented, only one thread on the slave can handle the write operations. If the replication master is multi-cores servers and is heavily writing using multiple threads, the slaves will likely not be able to keep up. Replication is not the only technology that put a strain on the write capacity, DRBD, a shared storage emulator for Linux, also reduce by about 30% (very dependent on hardware) the write capacity of a database server. In term of write capacity here are you choices.

 Write capacity
MySQL replicationFair
Master-Master with MMM managerFair
Heartbeat/SAN (depends on SAN)Excellent
Heartbeat/DRBDGood
NDB ClusterExcellent

5. For what level of growth are you planning?

Since NDB Cluster is an integrated sharding environment, if you are planning for a growth that will need sharding (splitting the database over multiple servers), then you might need to take a serious at that solution. If not, then, apart from the write capacity, all the solutions are approximately equal.

6. How qualified is your staff or support company?

There is a quite direct relationship between the level of availability and the complexity of the solution. In order to reach the promised level of availability, the staff maintaining the HA setup, either internal or external, must have the required level of expertise. The required expertise level is summarized in the table below.

 Expertise level
MySQL replicationTypical, average MySQL DBA + some Sysadmin skills
Master-Master with MMM managerGood, average MySQL DBA + good Sysadmin skills
Heartbeat/SAN (depends on SAN)High, Good MySQL DBA + strong Sysadmin skills
Heartbeat/DRBDHigh, Good MySQL DBA + strong Sysadmin skills
NDB ClusterVery high, Specific NDB knowledge, strom MySQL skills and strong Sysadmin skills

7. How deep are your pocket?

The last aspect that needs to be considered is the budget, complexity is expensive. We will consider two types of setup. The first one is a basic proof of concept of the technology with the hardware tested, the data imported and basic testing and documentation. A proof of concept setup is a good way to get used to a technology and experiment with it in a test environment. The other type of setup we will consider is a full production setup that includes extensive testing, fire drills, full documentation, monitoring, alerting, backups, migration to production and post migration monitoring. Of course, it is the safest way to migrate an HA solution to production. All the times here are estimates based on field experience, the values presented here are fairly typical and contains some buffers for unexpected problems. Although an HA solution can be built remotely through a KVM over IP and adequate remote power management, an on site intervention with physical access to the servers is the preferred way, especially for the most complex solutions.

 Proof of conceptMigration to Production
MySQL replication4 hours12 hours
Master-Master with MMM manager8 hours24 hours
Heartbeat/SAN (depends on SAN)32 hours120 hours
Heartbeat/DRBD40 hours120 hours
NDB Cluster40 hours120 hours+

Editor’s Note: We’ve gotten many questions about the time estimates mentioned here. The above estimates shouldn’t be used to compare against any specific situation. Time will vary greatly depending on your project. For example, “setting up replication” can be as simple as CHANGE MASTER TO, and can take as little as a few minutes in some circumstances. Yves’s estimate is for a project to create a replication slave for HA purposes, not for “setting up replication.” There is a big difference between an HA project and a DBA task. – Baron Schwartz

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