- Improved write hit performance with small blocks
- Improved write caching performance
I visited a client running 10.2.4 to see if these could be detected in the XIV performance statistics. In this clients case, the upgrade occurred on Feb 14. First up I wanted to show that in the period I am examining there was no major variation in write IO. In other words, before and after the code load, I wanted to confirm the client performed the same level of IO.
Having confirmed that the write IOPS did not vary over the period in question, did the latency change? Here we have some good news. Firstly the latency for Write Hits improved (slightly). A write hit is a write into a 1MB partition that already has some data in cache. It is faster than a write miss because some of the address allocation work has already been done. Write hits and misses both hit cache as I explained here. You can see a change on Feb 14 (when the code was updated):
I then looked at the latency for write misses. Again the latency dropped. This suggests that cache operations in general are being handled faster.
I then started thinking.... are we getting more write cache hits? The answer was YES! This is curious because the client normally does not have much control over where they actual write data to... Clearly the XIV firmware is managing the write cache in a more efficient manner. This is good not only because write hits normally have lower latency than write misses, but also because a write hit can save us destaging a block of data to disk. This is because a write hit could involve over-writing data that had not yet been destaged to disk. So two writes to the same LBA would only result in one write to backend disk.
So in conclusion, the upgrade to 10.2.4 code resulted in a measurable improvement in write IO performance at a real world client. Nice!