DS1621+ Cache hit rate very low

[Furbz85]

Hi Folks,

first of all: thanks in advance for people who know the answer to my question!

Alright, I have got problems with my 1tb WD RED cache stick in my DS1621+.

When I look at the hitrate in DSM it tells me it hits 0%, sometimes 30% max 50%.
Looking around on the internet most folks get a hitrate about 99/100%. Yesterday
I was at my dads and he has the 1618+ with cache getting a 100% hitrate.

What could be the problem? Am I doing something wrong?

Benchmarking and stats tell that the SSD is fine! It ran for about 200 days now in the NAS.

I use my nas for photo-editing via 10gbe (16gb ram on the nas).

Hope someone can help me.

Greetings Furbz

 

[speedmaster]

Maybe you are mostly utilising files over 1 Megabyte.
File size limit in SSD cache is 1 Megabyte. Files over that limit bypass SSD cache and are read from/ written directly to the volume (Harddrives).

 

[Furbz85]

Thank you so much for your reply @speedmaster.
I have monitored it this morning and I think that 1mb size is not the problem.

Since this morning I see the hit rate going up! (and down also).
BUT! I have a hypothesis:

I check the hit rate in the morning. The percentage is low and the "reusable data" as well.
Now, I've done some light work and the percentage still changes, but the "reusable data" grows.
So I've been thinking why the data in low in te morning and I think the answer is simple:

My NAS turns down at night to save the energy usage. But perhaps the fact that the nas turns on and off
is the reason the cache goes back to 0? Does the cache disappear at reboot? And is there a way to prevent this?!

 

[speedmaster]

Let you NAS run turned on, for a couple of days, and see if "cache hit rate" goes up?

Personal observation (not facts!)!!:
There seem to be consensus among users in this forum, that SSD cache makes the user interface feel snappier.
But hardly makes a difference for home users file operations.

I have not been able to find info about cache rebuild after reboot/hibernation.

 

[Furbz85]

I Thank you’re absoluty right!
I would not recommend Any one with my usecase to get cache!

I Will leave it on for a couple of days!
Thanks

 

[Furbz85]

My theory was right: read only cache gets cleared on NAS reboot or off/on.

Is there a safe way to bypass this? @Rusty I know you know a lot !

It saves so much energy by turning the NAS off @ night, but it makes the whole point of cache
really non useable.

Doest anyone know how to NOT clear the cache @ reboot?

Thank you so much

 

[Rusty]

Is there a safe way to bypass this?

Nor really, apart from setting the configuration of the cache to be read/write (2 drives). If you are using read only cache (even with 2 drives in stripe) you will still get a wipe on boot.

 

[Furbz85]

Thanks @Rusty. That means I have to buy another SSD .

Sync still running without reboot and hit rates are OK.
But from a new user point of view: I'm not really noticing a fundamental difference with or without cache
for my use case (Photo+Video editing).

 

[EAZ1964]

SSD prices are low at the moment, 4TB samsung 870 2.5” for 170$.
it may be worth setting up a few of these in SHR, will save energy and you do not need the cache.

 

[Furbz85]

That is indeed a good point You make.
The problem is that I need more space than the current ssd can provide.

But man! I’d love to have an all ssd box via 10gbe.

 

[EAZ1964]

Yes, then there is an issue...
Is there a possibility you setup two pools? One pool with eg 2 SSD containing: all packages, databases, Docker and frequently accessed data.
A second pool can hold 4 spinning HDD with data where a small lag upon access (because that is probably where we are talking about) is acceptable?

 

[Furbz85]

I've been thinking about that. As I understood 7.2 gives the option to use the NVME slots as a ssd pool.
But only with Synology's own ssd's. I can not find any users on the internet that ran some test to see
if it is actually worth it or not.
The Syno SSD's are way overpriced and in essence that is not such a big of a deal, but when the benefits are minor
than I'll skip that investment.

 

[Telos]

But only with Synology's own ssd's.

There are workarounds published on the net. Though they are not Synology supported, they have been robustly tested. Proceed at your own risk.

 

[Furbz85]

Thanks @Telos for your input.
I use the Syno for business. Dare not to take that risk.