SSD caching could decrease performance – part 2

In the second part of the “SSD caching could decrease performance” I will cover IO read and write ratio and IO size affects to SSD. Part 1 is accessible here.

Read IO and write IO ratio

Most real workloads are mixed IO workloads – both disk reads and writes. Read and Write ratio is split between disk reads and disk writes. Many cases enterprise SSD disks have equally good read and write performance. But lately MLC and especially TLC drives have made their way into enterprise market and with some of them read and write performance is not equal. In addition SSD disks may become slower over time due to small amount of free blocks. To mitigate the free block issue SSD vendors are installing extra capacity inside the disks – example Intel DC S3700 SSD has about 32% extra capacity.

SSD disks usually handle reads better than writes. Before selecting your SSD disk vendor and model I recommend to do some research. If possible purchase some different models that would suite your needs and test them in real life scenarios.

IO size

When it comes to performance IO size matters. Large IO could potentially overload a small number of SSD disks and with it affect the performance. I would avoid caching workloads with IO size above 128KB. In my personal experience I have seen a situation with a database where SSD caching was hindering performance due to database multi-block reads.

My recommendations for a more successful SSD caching project

  • Determine VMs that would benefit from SSD caching – VMs doing certain amount or more IO.
  • Analyze the IO workloads – no point of doing read caching when server is only writing. IO size.
  • Check your hardware – controller speeds and capabilities. No point to connect fast SSD to a crappy controller.
  • Find a suitable SSD disks for caching. Price vs performance.
  • Talk with storage admins – might be that SSD in array would make more sense than SSD in server.
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