SSD Optimizer question

[CptKirk1]

Just moved from WHS2011 to a new Win10 setup.

I have 4x 6TB drives in the pool (H:, I;, J:, K:), 1x 200GB SSD as OS drive (C:), 1x 1TB SSD (partitioned as 2x 500GB drives F:, G:).  I have duplication on.

The first image shows that I have added both SSD F: and G: to the pool.

I installed the SSD Optimizer installed.  But when I look as Balancing... it shows them as:

SSD1 (F:\), SSD2 (G:)

and I have it checked as an SSD (image 2)

 

I know from the documentation that if I have duplication enabled that I need to have two SSD's for caching.

 

Does that actually mean two physical SSD's?

Or can it be one physical SSD with two logical drives created on it?

 

[Jonibhoni]

Hi CptKirk1!

Quote

Does that actually mean two physical SSD's?

Yep, that's exactly the case.

The whole reason of file protection in DrivePool is to protect about failure of a disk, with emphasis on "disk". So it's "smart" enough to not treat two volumes on the same disk as different disks:

Quote

Files placed in a duplicated folder exist on at least 2 physical hard drives, so if one of the drives fail the other copy will still be safe.

https://stablebit.com/Support/DrivePool/2.X/Manual?Section=File Protection

 

There was some recent post where someone used a single SSD as cache for otherwise duplicated files by deactivating real-time duplication, but I have no experience with that; and it also means your files are not protected at all until (delayed) duplication:
https://community.covecube.com/index.php?/topic/6890-ssd-not-being-used-for-new-files/#comment-37104

 

[CptKirk1]

I was afraid of that.

 

How large of an SSD is actually needed for the cache?  2x 120GB ok?

Is there a recommended size mentioned anywhere?

Do not want to go overkill if not needed.

 

The 1TB SSD is actually in an M.2 slot.

I've got one SATA port left open.  I could get a 120GB SSD for about $20 on Amazon.  Then partition the 1TB M.2 as 120GB, then use the remaining 880GB for something else.

[Jonibhoni]

So, first a small hint regarding the 120 GB partition on the 1 TB M.2:
From meer technical side you wouldn't have to create a separate partition. DrivePool doesn't mind if you assign it a fresh and clean or old and used partition. It will "live" in just some hidden, ordinary folders in the root of the volume, without harming existing data in any way.

Regarding the optimal SSD size, well...

You have to understand that SSD Optimizer isn't a transparent cache like PrimoCache is (some users here on the forum are using that one). Instead it's kind of tiered storage with automatic rebalancing from time to time.

Comparing flushing:
PrimoCache: A fixed time delay (or immediately, if you run out of cache) after you wrote a file, the content of the cache will be written to the HDDs.
SSD Optimizer: Placing files on the pool will put them onto the SSD, which will reduce the "balance ratio" of your whole pool. The "balance ratio"  is a measure of how much your pool is balanced. Once the balance ratio of the pool falls below a certain threshold, which is configured as part of your general balancing settings, an automatic balancing pass is triggered for the whole pool. The files from the SSD will be moved as part of the rebalancing like any other file placement rules or balancing plugins. Means if you set your balancing settings to not balance more often than every 12 hours, your SSD will not get flushed at all before the next scheduled time. This can lead to a known problem:

Comparing what happens when running out of cache:
PrimoCache: If you run out of PrimoCache, your writes will be slower because PrimoCache is flushing your cache while you write. That's it. Kind of transparent to the user.
SSD Optimizer: If you run low on SSD space with the SSD optimizer plugin, DrivePool may write new files that are small enough directly to your archive drives. But if the free space on your SSD is below a certain (configurable) threshold while your file to be written is larger than the remaing space on the SSD, you will get an error message telling you your disks are full. Your transfer will abort.

You can read those topics if you want to know more:
https://community.covecube.com/index.php?/topic/1881-ssd-optimizer-problem/
https://community.covecube.com/index.php?/topic/6060-using-ssd-cache-and-keeping-some-files-on-ssd
https://community.covecube.com/index.php?/topic/6277-2x-250gb-nvme-drives-ssd-optimization-or-primocache/
https://community.covecube.com/index.php?/topic/5776-ssd-optimizer-as-cache
https://community.covecube.com/index.php?/topic/6627-q-how-to-copy-if-a-file-is-too-large-for-the-ssd-optimiser/

So, regarding size, well.. it depends.

If you want to realize something like a cache that will never bother you, you will need SSDs which are somewhat larger than the combined size of all the files you transfer during your desired rebalancing interval (like 12 hours, or 2, or whatever). You can choose that interval yourself, but the lower that interval is, which means the more often your pool is rebalancing, the more files will be moved around in general. And if your 120 GB SSD is 100 GB full with to-be-balanced data and you try to copy a 21 GB file onto your pool, transfer may fail.

Correct me if I am wrong, anyone, because actually I'm not using SSD optimizer myself.