How can I improve my read hit rate?

[esepc40991]

Here is my setup:

H/W
16GB DDR-3 1600
60GB Agility 3
2x160GB WD Caviar Blue in RAID-0

S/W
6GB RAM Drive (using primo, so it doesn't use this much ram all the time.)

SSD:
1GB RAM Cache
4kB cluster size
LRU Algorithm
Deferred Writing with 3s latency

RAID-O HDD
1GB RAM Cache
128kB Cluster size
LRU algorithm
Deferred Writing with 10s latency.



Normally, I see about a 1-2% read hit rate and a 50-110% deferred write rate on my HDD's, and I wasn't sure in what situations the read hit rate really shines. My HDD cache was the more important one to me, due to its wicked slow seek times compared to SSD and RAM. I wanted a buffer to help it deal with small files.



As I understand it, deferred writing is about like filling a balloon with water with a hole at the bottom. In inflates to a steady-state and then the output is a few seconds behind the input, but much more stable.

But in the case of reading, it seems like it's trying to "suck" data from a vacuum. The data has to be called in the first place, at the normal HDD rate, but it seems unlikely it would use the same data again unless it's the OS. (like my SSD)


So in what real-world situations could I really see the read cache being utilized? (Currently ~50MB free on the SSD and 500-600MB free on the RAID.)

Can I improve it much?

[dustyny]

You definitely have a good understanding of how FC works and where major your bottleneck is.

Hit rate is solely reliant on how frequently your application(s) reuse the data. Photoshop will see excellent hit rates because you are working on the same file which is being accessed over and over again, while a game might get a terrible hit rate because it's only using the files it reads one time. At the moment there is no way to precache data but the request has been made to Romex so perhaps we'll see something in a future version.

"So in what real-world situations could I really see the read cache being utilized?"
I get excellent hit rates using FC to cache my virtual machine hard disks (Hyper-V), content creators (video, photos) will probably also get high hit rates for everyone else it all comes down to what app your using and how the developers coded it (most shy away from slow HDD reads when possible).