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Questions on L2 Cache & "Prefetch Last Cache" Settings

Posted: Wed Jul 21, 2021 2:04 pm
by bushwacker611
Hi all,

I've been using PrimoCache for a year plus and it's awesome, but I have 2 questions I haven't been able to find answers for.

To begin, here's my basic setup:

RAM- 32GB DDR3 1600Mhz (Corsair Vengence)
Primary Disk- 500GB Samsung 860Pro SSD (512MB DDR4 onboard cache)
Secondary Disk/L2 Cache- 16GB SanDisk SDSA5GK-016G-1006. Transfer rate is a lot slower than primary (220MB/s read, 50MB/s write vs 560/530 for Samsung)
CPU- Intel Core I-7 4820k @ 4.1GHz (OCed from 3.7)

1. Does the "Prefetch Last Cache" feature only apply to the L1 cache? Also, where does it save it's data?
2. If this setting saves to my primary (faster) Samsung SSD, do I even need a L2 set up, given how much slower my secondary disk is?

Overall, my goals are to make use of that old 16GB SSD as a L2 (why not), but only if it's beneficial to performance.


Re: Questions on L2 Cache & "Prefetch Last Cache" Settings

Posted: Sun Jul 25, 2021 5:28 am
by InquiringMind
Welcome to the forums, Bushwacker611

"Prefetch Last Cache" applies to L1 only (since L2 cache is non-volatile). It does not save actual data, but the list of disk blocks previously cached - which are then copied onto L1 cache when you restart your system.

It looks like you are using a USB stick for L2 which is pointless. For L2 to provide any performance benefit, it needs to be using storage that is about an order of magnitude faster (so only an NVMe volume would make sense for caching SSDs, and then only slow ones). For your situation, an appropriately sized L1 cache only would seem a better setup.

Re: Questions on L2 Cache & "Prefetch Last Cache" Settings

Posted: Sun Jul 25, 2021 4:33 pm
by bushwacker611
Hi InquiringMind,

Thanks for the reply! L2 wasn't a USB stick, but it was a lower capacity/smaller SSD (both are SATA/PCIe 2.0). Looks like that old SSD would be more useful as an emergency backup drive, or I put Linux on there for the heck of it. Thanks for the clarification on the last cache prefetching. Hopefully the official documentation can be updated to make its purpose more clear.

Re: Questions on L2 Cache & "Prefetch Last Cache" Settings

Posted: Sun Jul 25, 2021 10:56 pm
by InquiringMind
Sorry about the mistake on the (not) USB stick, but 16GB is really small for an SSD - I had to rub my eyes when I checked the specifications. Setting it up as a recovery partition would seem a good idea.

I overlooked one part of your question - the prefetch data (list of blocks to load) is stored in a Prefetch folder in the PrimoCache installation folder. It will contain one file for each cache task using prefetch, with the name pf{cache task number}.pf1.