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Why Is 3-Way Synchronous Mirroring Better Than 2-Way?

Written by Roy Mikes on Thursday, 13 September 2012. Posted in General

2-way synchronous mirroring 

This configuration requires storage redundancy on the nodes. The use of RAID10* is recommended.
2-node HA ensures the synchronous mirroring of data between two storage nodes. Taking
into account that each storage node only has 50% usable capacity with RAID10, synchronous mirroring makes the further dividing of those 50% by half resulting in the underutilization of storage capacity - only 25% is used. * RAID5, RAID6 can also be used. However, it is not recommended because of the high probability of disk failure while rebuilding RAID5, and the low write performance of RAID6.

3-way synchronous mirroring 

This configuration does not require storage redundancy on nodes since 3-way synchronous mirroring already ensures the required level of data protection. Therefore, you can use either JBOD, or RAID0 for performance. Synchronous mirroring between 3 storage nodes results in 33% usable capacity and thus provides a higher level of utilization compared to 2-way synchronous mirroring.

Increased reliability  2-way synchronous mirroring provides 99.99% uptime. The outage of one storage node results in a single point of failure and immediately brings the system to a degraded performance mode. Cache is flushed and turned from write-back to write-through mode on the running node. A number of MPIO paths is reduced twofold because one node is down. Consequently, the storage performance falls by a factor of 2-4. Such performance degradation can be sometimes equal to downtime for performance-critical applications like databases, online sales, and bank services, etc. Obviously, a double fault cannot be tolerated by a 2-node cluster and inevitably results in downtime.

3-way synchronous mirroring provides 99.9999% uptime. No single point of failure occurs when one node of a 3-node storage cluster goes down. In such a situation, the storage performance falls by up to 33% because the system loses 1/3 of the MPIO paths. Cache policy is in no way affected. Performance-critical applications usually can continue running in an ordinary way. The 3-node HA configuration tolerates a double fault and retains the availability of service.

Six nines in my opinion is more a commercial figure than a real number. Of course you have a very high availability, but it depends on more factors. Six nines equals 31.5 seconds outage per year. When disaster strikes you are probably longer than 31 seconds from the air.

Higher performance  2-way synchronous mirroring. With Round Robin policy used, I/Os are processed up to two times faster comparing to a single-node configuration.

3-way synchronous mirroring Owing to the Round Robin policy, the I/Os throughput rises by a factor of 3 compared to singlenode storage. As a result, performance is increased by up to 50% compared to a 2-node configuration.

RAID10* vs RAID0 and JBOD. 2-way synchronous mirroring. This configuration strongly requires extra redundancy for data protection on the storage nodes themselves. This redundancy can be provided through the use of RAID. RAID10 is recommended as it ensures mirroring between the disk stripes and makes fast reads and writes. However, storage utilization is considerably low because the same data is mirrored and stored on two stripes of RAID.

* Use of RAID5, RAID6 is possible but not recommended because of the high probability of a disk failure while rebuilding RAID5, and the low write performance of RAID6.

With 3-way synchronous mirroring no extra redundancy is required on the node because of the existing redundancy. Therefore, RAID0 or JBOD can be used for performance. Both reads and writes are faster here as the system reads the data from all disks simultaneously.

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About the Author

Roy Mikes

Roy Mikes

Roy Mikes has developed a deep knowledge of virtualization, storage and Cloud in a broad perspective over the past 18 years, but also streamlines his focus recently more and more on AI and Blockchain. Because of that knowledge and focus, Roy works as an Advisory Partner Solution Development Lead & Evangelist at Dell EMC.

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