Bill Scales - All Flash Storage - Why RAID Matters [Guest Post]
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Today we are lucky to have the first in a series of guest posts from Dr Bill Scales. Bill is one of the lead product architects in the storage team in Hursley. Bills depth and breadth of knowledge of storage systems is second to none, which means there is always a queue of people waiting to pick his brains in the office! Enjoy.
All Flash Storage - Why RAID Matters - by Dr Bill Scales
The high speed of Flash storage has changed where the performance bottlenecks are in storage systems. In the magnetic world users had to make difficult trade offs between performance, usable capacity and reliability by choosing whether to use mirroring, single parity or dual parity RAID. With all Flash storage systems the choice is simple - use dual parity RAID-6!
In these systems the choice of which type of RAID to use has a big impact on performance. RAID-1 or mirroring will provide the best performance while providing redundancy against device failures, but because all your data is stored twice means you need twice as much raw capacity as the amount of data you want to store. RAID-5 stripes data across a number of devices and has a single parity to provide redundancy, the ratio of data to parity can be varied and with ratios like 10:1 can provide a much more economic reduction in raw capacity while still providing protection. However while a write update to RAID-1 requires two write I/Os to mirror the data an update to a RAID-5 array requires two reads and two writes to update the data and the parity. RAID-6 provides even more redundancy by having dual parity and hence being able to protect against two simultaneous device failures, but the penalty is that write updates require three reads and three writes because both parities have to be updated.
Note that it is only write I/Os that require more device I/Os - the cost of a read is the same for all types of RAID, but for a mixed I/O workload this still means that using RAID-6 will reduce performance by about 25% compared to using RAID-5. So with these systems users end up balancing the requirement for high performance with the economics of providing enough storage capacity and often end up using a mixture of different RAID types and tiering their data by putting the more performance critical data on the more expensive RAID-1 storage.
So in summary if you're using all flash then use RAID-6! In my next post I'll talk more about advances in Distributed RAID.