VMware vSAN – A closer look [Data Availabilty] – Define Tomorrow
An important consideration when choosing which method of protection is required is the additional overhead generated by erasure coding. This is commonly referred to as I/O amplification. During normal operations there is no amplification of read I/O however there is I/O amplification of write I/O as the parity data needs to be updated every time new data is written. The process can be described as follows
- read the part of the fragment that needs to be modified
- read the relevant parts of the old parity data to recalculate their values
- combine the old values with the new data to calculate the new parity
- write the new data
- write the new parity
For RAID5 this results in 2 reads and 2 writes on the storage. For RAID6 this results in 3 reads and 3 writes on the storage. This means there is an increase of network traffic between nodes during write operations compared to mirroring. Additionally, if a node were to fail I/O amplification may also occur during read operations. Don’t let this put you off. As flash devices can provide a substantial number of IOPS I/O amplification can be less of a concern when weighed up against the capacity savings when compared to mirroring. There is no one size fits all approach and with vSAN you are free to pick and choose which workloads require high performance (mirroring) and which require capacity efficiency (erasure coding). Remember this is all driven by storage policies that you control and can change on the fly.
Read the entire article here, VMware vSAN – A closer look [Data Availabilty] — Define Tomorrow™
Via the fine folks at ComputerWorld Group.