Rules of Thumb for Determining Cache NVMe Drive Sizes?

[A_Bullish_Crab]

Does anyone have some good rules of thumb for how big the read/write NVMe SSDs should be for a home NAS with basic usage? (cache per volume size ratios?)

I have a 4 bay DS 423+... Was thinking of two Crucial 500 GB NVMes

Cheers,
Crab

 

[Jan Janowski]

Been fine here on 3x NAS

 

[fredbert]

What did the cache adviser in Storage Manager report?

 

[A_Bullish_Crab]

What did the cache adviser in Storage Manager report?

I've had the NAS for less than a week; so I'm not sure how accurate it will be.

 

[fredbert]

It's likely that you are using the NAS in what will become a non-typical way, meaning you're probably moving data on/off the NAS much more right now than what will become how you use the NAS. So I would wait to run the SSD cache manager because it monitors how much data is moving to/from the NAS, and it takes at least a week (I ran it and it was nearly two weeks).

 

[patel71]

Been fine here on 3x NAS

Hi Jan, when you say "3x NAS", what does that mean? (I think you might mean 3x the NVMe should equal the amount of capacity you should procure that you want to be able to cache?) I am trying to understand the best way for estimating NVMe storage need if you or anyone has thoughts...

For example, if you have a DS923+ with 32GB ECC RAM (2x16GB), and 4 HDD's with all 16TB drives RAID'd together into a RAID 5 (1 drive failure safety)... is there a recommended formula tied to RAM or HDD or HDD Volume or something else to consider that I might be missing? The size of the volume would make sense to me.... is there a limit to how much can be cached by DSM or some other hardware component to think about? If 4x16TB was in a RAID 5, the Synology RAID Calculator page said it would translate to:

37.3 GB - System Reserved

43.7 TB - Available Capacity

14.6 TB - Protection

Now if the available is 43.7 TB to make volumes out of.... what if it was just one monster 43.7 TB volume dumping ground for files? Is that ill-advised for some reason... or should there be a bunch of separate volumes created? Are you saying 3 x Cache should equal the volume that was created or the other way around? My thought process that I was thinking was the way to determine the NVMe capacity need might be by looking at what and how much you think you might be "caching" or needing to be accessible for "high-speed read/write"? So if I hypothetically setup a few VM's, Media Servers (Plex?/Jellyfin?/etc.?), and Docker containers, I would probably want them to be on the NVMe as a storage volume or point to a volume on an HDD RAID that has those frequently accessed files...right? Maybe someone has recommendations on how to best setup storage volumes and caches?

For example if I had a total of about 335GB of media server metadata and VMs:

Media Server A (Docker?) = ~240 GB of Metadata

Media Server B = ~15 GB of Metadata

VM A & VM B = ~80GB+

...does that mean 3x335GB=1005GB ... so let's say 1TB of NVMe is recommended for caching capacity for this? Does this mean I would want to possibly get something like 2x1TB NVMe's where I could theoretically be using 1 NVMe for caching the metadata and have room to spare, and then have the 2nd NVMe for dropping my VM's and/or Docker containers in as a high-speed Read/Write storage area? I would assume it would also be a good idea to periodically sync/replicate the contents of the 2nd high-speed Read/Write storage volume area NVMe into the HDDs as well in case the NVMe failed?

I appreciate if you or anyone with insight or thoughts on any of this can help poke holes or offer suggestions to me on any of this? I'm poking around in the forums for answers in parallel. Thank you in advance!

 

[Jan Janowski]

2x720+
One with 6GB Ram, 2x 7200 Ironwolf’s.
And 2x 256GB R/W M.2 Cache

Other With 10GB RAM, 2xSSD’s.
And 2x 128GB R/W M.2 Cache

And a 718+ With 6GB Ram, 2x 7200 Ironwolf’s.

Now, in all honesty, I have used These 3 NAS’s for years. In all that time, I’d expect I would see some difference in speed in these three systems, at one time or another.

I don’t: They all act identical.
Ram, Cache. No apparent difference!
SSD’s do appear a bit quicker than HDD’s do. But just a bit.

So, Yes They’ve been fine on 3x. NAS’s!

I don’t see an obvious improvement.

But I’ve got “Bragging rights” for owning these updates! That has to account for something. Not sure what….. But Something!

 

[patel71]

..., I have used These 3 NAS’s for years. In all that time, I’d expect I would see some difference in speed in these three systems, at one time or another.

I don’t: They all act identical.
Ram, Cache. No apparent difference!
SSD’s do appear a bit quicker than HDD’s do. But just a bit.

So, Yes They’ve been fine on 3x. NAS’s!

I don’t ...

Doh! Ok, I see what you are saying now I was definitely misunderstanding it, so thanks for helping me. Basically, you were only saying you have 3 different NAS models and agreeing with A_Bullish_Crab that 2x500GB modules he was thinking of would be enough in your original post. In your response to me in your post, I also now get that you said you were using M.2 Cache for 2 models (across 3 NAS devices) that worked for you and were using 256GBx2 and 128GBx2 for those. I misunderstood the original post and the "3x" part was not a factor to determine the size of cache storage needed or a way to estimate the M.2 SSD capacity one might want to purchase.

I guess maybe it's best to see what this "cache advisor" says for an answer to how much to buy and allocate for cache after some use....it all depends on what the storage is going to be used for anyway.

After all this, i'm thinking maybe it's best to have 1 SSD dedicated for performant SSD Caching (a place where frequently accessed small metadata can go to be read faster), and then a 2nd SSD dedicated for an SSD Read/Write storage location use case (a place for VM's, docker containers, maybe media servers transcodes, etc. other OS type stuff that might need that little extra fast read/write performance to go on)....faster than the HDD's anyway. Then, maybe schedule a backup of the read/write SSD slot to the HDD's. This way, if the read/write SSD failed there would be a backup basically and I can just replace the SSD module and copy things over. (The HDD's will be RAID'd together in a RAID 5 in case of 1 drive failure it would not be lost.) If the cache SSD died, I guess caching would stop, but nothing would be lost and I assume things will still operate without cache?..maybe there is no good answer to the capacity question because it depends on the use cases. Maybe there is a limit to what the specific system can cache in memory and/or SSD to consider...not sure. If anyone knows, please share!