MAX SSD Cache size on Synology

MAX SSD Cache size with MD218

I have Synology ds1618+ with 6 10TB drive in RAID 5. I have 32GM memory.

Now I’m thinking of buying MD218 to add SSD Cache. There are two slots in MD218. What is the maximum size NVMe SSD I can add in each m.2 slot? Is there a maximum limit on SSD cache size? I believe I can add two 1TB SSD for cache but not sure if there is a limit on SSD cache size. I do have enough RAM for 2TB cache. I could not find this information on Synology website.

DS1618+ Intel Atom C3538 Quad Core 4 Yes Denverton DDR4 Non-ECC SO-DIMM 4GB

Max 

Read and Write cache -2x 1TB cache would require  416MB RAM   Read-only   832MB of RAM

Read and Write cache -2x 2TB cache would require  832MB RAM   Read-only  1.7GB of RAM

Read and Write cache -2x 4TB cache would require  1.7GB  RAM    Read-only   3.3GB of RAM

None would need more then 4TB od cache, but you can have more if you add additional RAM stick,

Here are the calculations from SYnology:

Memory requirements depend on the CPU of your Synology NAS. To see which CPU is used in your Synology NAS, please refer to this article.

For models with Alpine CPUs:

Every 1 GB of SSD cache requires approximately 416 KB of system memory (expandable memory can be counted), and the maximum cache size is 930 GB. Therefore, if the read-only cache configuration is 2 x 128 GB SSD, the total cache size will be 256 GB and require at least 104 MB of memory, if the read-write cache configuration is 2 x 128 GB SSD, the total cache size will be 128 GB and require at least 52 MB of memory. If memory is insufficient, SSD cache creation will be restricted or the size of SSD cache will be limited.

For all other models:

SSD cache requires a certain amount of system memory depending on the cache size, so mounting larger SSD cache might require upgrading the memory of the Synology NAS. To keep the system stable and fast, only 1/4 of the pre-installed system memory can be used for SSD cache.

Every 1 GB of SSD cache requires approximately 416 KB of system memory (expandable memory can be counted).
Note: This does not apply to read-write SSD caches created on DSM 5.1 or earlier versions. That is, every 1 GB of SSD cache created before DSM 5.2 will still require 4 MB of system memory even when used in the DSM 5.2 environment.

Therefore, if the read-only cache configuration is 2 x 128 GB SSD, the total cache size will be 256 GB and require at least 104 MB of memory; if the read-write cache configuration is 2 x 128 GB SSD, the total cache size will be 128 GB and require at least 52 MB of memory. If memory is insufficient, SSD cache creation will be restricted or the size of SSD cache will be limited.

Alpine models:

DS1817 Annapurna Labs Alpine AL-314 Quad Core 4 Yes Alpine DDR3L 4GB
DS1517 Annapurna Labs Alpine AL-314 Quad Core 4 Yes Alpine DDR3L 2GB
DS416 Annapurna Labs Alpine AL-212 Dual Core 2 Yes Alpine4k DDR3 1 GB
DS2015xs Annapurna Labs Alpine AL-514 Quad Core 4 Yes Alpine DDR3 4 GB
DS1515 Annapurna Labs Alpine AL-314 Quad Core 4 Yes Alpine DDR3 2 GB
DS715 Annapurna Labs Alpine AL-314 Quad Core 4 Yes Alpine DDR3 2 GB
DS215+ Annapurna Labs Alpine AL-212 Dual Core 2 Yes Alpine4k DDR3 1 GB

MD218 compatible models:

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