L2ARC cache on QuTS Hero vs Read cache on normal QuTS: Performance, Sequential and Random
There has been a lot of questions about cache and it's performance now that we have the new Hero OS that uses a different kind of cache. So after asking for some clarifications, I thought I would answer some questions about our cache performance.
First of all, it is worth noting, that cache performance can't be faster than the combined performance of the SSDs used for cache. Many of the new NVMe SSDs can claim very high numbers around 3000MB/s. But when talking about caching for sequential, most SSDs slow down when under a heavy load. So, when caching for sequential, the SSD likely won't maintain the 3000MB/s that is advertised. How much slower the SSD gets under a heavy sequential load may depend on if it is an SLC, MLC, TLC or QLC SSD. SLC is the fastest, QLC is the slowest and performance degrades the most under a sequential load.
Personally, I would not recommend QLC for cache, not even Random I/O. I expect most TLC to be helpful for random I/O.
As for cache set to "All I/O" to try to increase the sequential read performance, my answer differs for QuTS and QuTS Hero.
Our L2ARC on QuTS Hero is slower sequentially than our read cache on normal QuTS.
For normal QuTS, I would be cautious even about even some of the faster TLC SSDs. Because almost any TLC SSD should get slower under a heavy load. Some fast MLC drives in a RAID0 like Samsung Pro should be able to increase sequential read performance for normal QuTS.
But because L2ARC on QuTS Hero is not as fast sequentially as read cache on normal QuTS, I am not even recommending MLC for caching "All I/O". I would recommend set to random I/O instead. While I am not sure there aren't any SSDs in existence fast enough to make sequential reads faster, I am not aware of any specific drive we have found to be fast enough to use for All I/O on L2ARC. And I have been told that we don't recommend L2ARC for video editing because it is not normally expected to help with sequential performance. So at this time, I would not recommend L2ARC for "All I/O" because with most drives it would not help. And even if we found the very fast drive that helped, the average customer may not find the cost of a drive that fast to be worth the benefit.
QuTS Hero has good sequential performance from a RAID of HDDs. And an all SSD storage pool can be considered for those who want even faster throughput. One advantage of Hero is that inline compression can improve sequential throughput of a RAID of HDDs when dealing with data that is not precompressed already. So if you set L2ARC to Random only, you might still find that the sequential performance of the HDD RAID group is still quite good.
Zdroj: https://www.reddit.com/r/qnap/comments/ ... on_normal/