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SSD Cache - Multiple Pools with ONE SSD?


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It requires a separate SSD volume per pool.

Note the use of the word "volume" - you could use the same physical SSD for multiple pools by partitioning it into multiple volumes and adding each volume to a different pool, although you'd then have less SSD space available for each individual pool.

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Another option to consider may be using a product like PrimoCache which can use both your system RAM as level 1 cache and you SSD as level 2 cache for all your computer reads/writes - so, not just limited to DrivePool. DrivePool will use the SSD cache for writes only, but does not use it for caching reads. PrimoCache is a try before you buy software, and I think they offer a 30 day trial period. It worked good for me, but I chose just to use the DrivePool SSD plugin which is all I needed for my DrivePool home media server.

If you decided to go with something like PrimoCache, then you would not have to partition your SSD for multiple pools in DrivePool. In theory, you would have access to the entire SSD for every drive in your system and nothing wasted sitting in a partition that is not being used. Additionally, it is possible to overrun your DrivePool SSD cache if your data transfer is larger than your available space on the SSD. That has never happened to me, but evidently it is possible in DrivePool. With PrimoCache, on the other hand, it is more like a true buffer and you would not be able to overfill it. If you ever filled the PrimoCache buffer, the data would slow down/stop until more buffer space was available.

 

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10 hours ago, HTWingNut said:

Thanks. So either partition my SSD or go with something like PrimoCache. I think I will look into Primocache. That looks pretty slick. I may want to bump up my RAM to 64GB then. :D

When I tried out PrimoCache years ago, I only had 4GB RAM and did not have a SSD in my system. You can get some really impressive transfer rates on reads/writes when using PrimoCache RAM buffer as compared to writing to a slower HDD. However, with only 4GB RAM, I quickly filled my RAM and everything slowed back down to the write speed of my 5200rpm drive at that time. So, essentially, I saved only maybe 30-45 seconds actual transfer time on large file transfers. The cost of the program was not worth it to me, at that time, to save less than 1 minute of real life time.

My current system has 8GB RAM and a 224GB SSD, so using both RAM and SSD cache, I think PrimoCache would be a more viable option than years ago. Howerver, for me, the DrivePool SSD plugin does everything I needed it to do for my home media server at present. When I have a large transfer of files, it goes directly into the SSD cache and then later writes it out to the archive HDDs. Perfect for me. I am not too concerned about buffering reads. Additionally, I set my DrivePool SSD cache not to flush until it hits 100GB of stored data. That way, I am able to use that 100GB SSD cache for both reads and writes for temp working folders. Not as fast as using RAM, but still much faster than using slower HDDs.

In your case, if you plan on having multiple DrivePools nested, PrimoCache may be a real option to consider since you can take advantage of both system RAM and SSD cache to set up buffers for your DrivePools. One thing I don't like about the idea of partitioning the SSD is that you might end up wearing out sectors of the SSD prematurely because you have limited the area (partition) where the data is to be stored. So that area on the SSD might end up getting worn out faster than if you had the entire SSD available and the SSD software does its magic in balancing out the sectors where it reads/writes the data so the drive can have a longer service life. If I have that right, then the $30 cost of PrimoCache might well be worth the cost just for the expected extended service life of your SSD, let alone the advantages you should get by using both your system RAM and the SSD as buffers.

At least with PrimoCache they let you try it before you have to buy it. If you plan on bumping up your system RAM to 64GB, I would suggest doing that before you start the PrimoCache trial period. Of course, you can immediately try out the DrivePool SSD plugin and see if that works for you. That was all I needed for my home media server. It's good to have options to consider.

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A note of caution: if you're using caching software to defer writes, then in the event of a crash or power failure you WILL lose any data that hasn't yet been written to the disk. It's not likely to be a problem, at least if you've got a UPS and/or a stable setup, but it's still something you should take into consideration.

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OK, I finally downloaded PrimoCache and started the trial with a spare 240GB SSD I had laying around. The SSD has good performance, much better than any hard drive at least.

In any case, it doesn't seem like it's making a difference. I included the volumes in the cache task that I'm copying files to. I set up 16GB of my 32GB RAM for L-1 cache. I set up my 240GB SSD using all its capacity as an L-2 cache. I set it to use 100% for write caching only.

I did a large file transfer (80 GB, 25 files), and zero activity on the SSD, and performance was same as just a regular hard drive. My hard drive was tapped out at 100% but no sign of any cache activity. Any suggestions? Thanks.

EDIT: It seems that you NEED to enable "Defer-Write" otherwise it acts as a read only cache. I guess I just assumed that it would act as a cache from RAM to SSD to HDD in real-time. But I guess you need to enable "Defer-Write" to have it act as an actual write cache. Weird. I thought "Defer-write" was just to add a delay to write to hard drives, but apparently not.

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19 hours ago, HTWingNut said:

I did a large file transfer (80 GB, 25 files), and zero activity on the SSD, and performance was same as just a regular hard drive. My hard drive was tapped out at 100% but no sign of any cache activity. Any suggestions? Thanks.

I think what you are seeing is that if you transfer 80 GB from one source drive, PrimoCache is sending it to the RAM L1 buffer and immediately (no Defer-Write) out to your second target drive. If your source drive and your target drive have essentially the same speed on your system, then you would never get any real advantage to using PrimoCache because it cannot speed up your drives. Where I see an advantage to using PrimoCache is in a scenario where the source drive is internal to the system, and very fast, maybe even a system SSD, and you are transferring files to much slower archive HDDs maybe attached via USB. Then you would see the advantage to the PrimoCache buffer as it accepted data as fast as your source HDD can send it and then it buffers out that data to the slower speed of the external archive drive. In that case, you would see the buffer continue to fill up RAM Level 1 and maybe spill over to your SSD Level 2 cache.

When I was trying out PrimoCache years ago, I only had 4 GB RAM and no SSD. If I transferred data from my faster internal HDD to a much slower archive HDD via USB 2.0, you can bet that the RAM buffer filled up very fast. But the speed difference between the internal source and USB 2.0 target drives was huge. The internal drive might have been 6X faster than the HDD attached via USB 2.0. In my case, any transfer larger than my PrimoCache RAM buffer would quickly fill up the RAM Level 1 cache and then immediately slow down to the write speed of the external USB 2.0 HDD. Which is why I stated that I only saved maybe less than a minute of real-life time using PrimoCache because my ~2 GB set aside PrimoCache RAM buffer quickly filled up and then had to wait for the slower archive drive to write. Again, I had no SSD to act as Level 2 cache. For me, at that time, paying for PrimoCache to save 1 minute of real life time on my system was not worth it.

If your read/write specs on your source drive and target drives in DrivePool is the same speed, then you will not see any advantage to using PrimoCache as a go between buffer. Yes, you can set a defer-write mode, but that may not actually improve your real life transfer times if the source and target have the same speed on your system.

Ditto with using the DrivePool SSD plugin as a front end write cache. If I transfer files over my home network, I get maybe 30 MB/s transfer speed. My server DrivePool archive USB 3.0 HDDs get up to 100 MB/s. So, obviously, I get no advantage in that case using a front end SSD. However, if I transfer files from my fast server drive(s) at up to 480 MB/s to my slower DrivePool archive USB 3.0 HDDs at 100 MB/s, then I see the SSD cache as a real advantage. In my case, I also set my DrivePool SSD cache to "Defer-write" until it hits 100 GB in the buffer. In that way, I can also use my SSD as a read/write cache for working files in other programs. That works good enough for me on my system so I did not buy PrimoCache. Remember, the SSD will save your "defer-write" data in case of a power loss, whereas RAM will lose all that data in a power outage.

If you are only hitting your RAM Level 1 cache while using PrimoCache, then you probably don't need it either. Windows 10 does a pretty good job itself using RAM as a buffer for file transfers. Where I think PrimoCache might be of more value to you, is that you mentioned that you needed multiple pools and that requires either multiple SSDs or will need to partition you SSD into more volumes. As I said, creating patitons on the SSD might increase the chances of wearing out your SSD early as you would be using less SSD drive space for repeated read/writes. And obviously, the cost of PrimoCache is probably much less than buying multiple SSDs if you went that route.

In any case, you get to try out PrimoCache for 30 days to see if it is worth it for you. Compare that performance against using DrivePool's SSD optimizer setup, or even no SSD at all. Please post your findings if/when you get a chance.

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