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SSD Level-2 Cache Size

Posted: Mon May 03, 2021 12:31 am
by Gamer2592
Hello everyone.

Just wondering if there is a guideline (RATIO) to select a SSD size for Level-2 caching vs the HDD size it will be caching ?

On my computer, I use a 240GB SSD to cache a 2TB HDD. Is my SDD too small? too big ? Or the correct size ?

I did not find this information in the documentation.

Thank you in advance for your responses.

PS. Being using PrimoCache for over 1 year and it works great.

Re: SSD Level-2 Cache Size

Posted: Mon May 03, 2021 5:20 am
by Jaga
This is really subjective for most people. I have a personal preference though: no less than 10% data coverage, and ideal would be 20% or higher. So if your HDD has 1TB of data you want to cache on it, your 240GB L2 (over-provisioned so it only has a 200-220 GB volume for the L2, right?) would be right around 20%.

When you go too far under 20%, your hitrate starts to suffer due to blocks being swapped in/out too much. Just my opinion on it, others may have their own. 10% would be my absolute minimum coverage rate, or the L2 SSD is really going to suffer write/re-write wear.

Re: SSD Level-2 Cache Size

Posted: Thu May 13, 2021 11:48 am
by Axel Mertes
If you have a rough estimate on how much data you touch per day, like newly written data, old read data, then you have an indication of how much you would need to run via SSD without necessarily touching the HDD at all. I tend to use twice the size of that and that keeps even a server cool...

You can actually measure the data touched by looking at the statistics in PrimoCache, which is excessively helpful. You can reset them at the beginning of your measurement period and check later how much data was written and how much was read. Add that up, times 2 and you should be running really smooth from my experience.