Expected benefits of read-only NVMe cache?

First, I’d like to say thank you to spacerex for your channel. I’ve learned so much about network-related topics there, it’s ridiculous!

Now, on the topic of ssd-caching. It is a little unclear to me which situations having a NVMe cache (specifically read-only cache) will be of benefit, so I need some advice.

Will a RO ssd cache help with things such as:

  • viewing frequently visited directories that contain many thumbnailed photos?
  • opening actual photos (~25MB, ie. above synology’s definition of “small file size” of 1MB)?
  • browsing in plex?

If any of these scenarios benefit from having a NVMe RO cache, would a 1TB drive be better that a 500GB one? (assume photo library of approx 4TB)

So for the most part 500 gigs should be enough for pretty much most SSD caches. It will end up with all the databases that are accessed often.

  1. ~slightly not too much
  2. Probably not at all (other than having the NAS not doing other things)
  3. Yes for sure!
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Only one thing to add: if you ever go for a read-write cache one day, you should use larger SSDs, because write speeds are slower for smaller capacities. (Of course, the choice also depends on your network speed: the fastest SSD won’t help if your network connections are slow.) Read speeds, however, are afaik not affected by capacity, so a 500GB drive should be OK, as @Will already wrote. It will just take a wee bit longer for the system to transfer frequently accessed files from your storage pool to the read cache, but I assume that’s barely noticeable, if at all.

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Thank you!

If I may ask a couple of follow up questions:

  • I’ve started a SSD cache advisor-job. Will the result of this analysis indicate anything about the potential benefit of installing a RO cache?
  • Is there any extra risk of installing a healthy five year old NVMe drive as a RO cache? (I’ve already purchased a new 1TB drive, but I’m considering putting it in my computer as a regular drive instead. The old NVMe (512GB) from my computer would then go into the NAS).

I don’t know what this SSD cache advisor job is. As for the second question, I’d say: yes, you can put the old SSD into the NAS, because even if it dies earlier, the data won’t be lost, because it’s just a cache using copies of files that are still on your main storage pool. However, I’d run a software first to look at the old SSD’s health. If (for example) you’re already way above the drive’s TBW, I’d probably not use it.

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On the other hand, new 500GB gen3 M.2 SSDs are really cheap now, just a couple of bucks, e.g. the TeamGroup MP34 TM8FP4512G0C101. Even a 1TB model wouldn’t be expensive.


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The “SSD Cache Advisor” is a job you can start from Storage Manager. It’s explained like this:

SSD Cache Advisor analyzes your current data usage pattern and recommends a suitable size for SSD read-write cache. The initial analysis takes at least 7 days and stops automatically after 30 days.

I’m not sure if this will indicate anything about a RO-cache though, but I guess not :slight_smile: