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Hello everybody. Maybe I'm missing something, here, but I can't figure this out.

My system:

- DSM version  DSM 6.2.3-25426
- Loader version and model: JUN'S LOADER v1.04b - DS918+
- Using custom extra.lzma: IG-88's extra and extra2
- Installation type: BAREMETAL - ASRock Fatal1ty Z170 Gaming-ITX/ac, Intel Pentium G4400 SkyLake, 2x4 GB DDR4 RAM, Raid 5 for testing with:
    - SSD Intel 120 GB
    - SSD Crucial 120 GB
    - HDD Toshiba 300 GB
    - HDD Hitachi 240 GB
- Cache:
    - READ and WRITE 2x 1 TB SATA SSD Crucial

Here's the deal.
Since Synology declares here:

https://blog.synology.com/xmas-wishlist-optimizing-your-nas-with-ssd-cache

"Apart from SSD endurance, you should also consider the memory requirements of SSD caching. As an SSD cache requires a certain amount of system memory depending on the cache size, you may have to upgrade your memory if you want to mount a larger SSD cache. To maintain system stability, only 1/4 of the pre-installed system memory is allocated for SSD caching.

Since a 1GB SSD takes up approximately 416KB system memory (including expandable memory), a 2 X 128GB SSD read-only cache (total 256GB) requires at least 104MB memory, while a 2 X 128GB SSD read-write cache (total 128GB) consumes 52MB memory. It should be noted that a lack of memory will, therefore, limit the SSD cache size."

I can't figure out why when I create my cache raid 1 with the two 1TB SSDs it allows me to gain 321 GB instead of 900/1000 GB. 1000 GB cache would require 416 MB of my RAM, which is far less than 2 GB (a quarter of my 8 GB).
Here's a screenshot.
Thanks in advance for any help.
G.

939370098_Schermata2020-11-22alle18_31_46.png.27e7215d8dd68fe89d02f2e60fe4b0f1.png

 

Edited by giacomoleopardo
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2 hours ago, giacomoleopardo said:

I can't figure out why when I create my cache raid 1 with the two 1TB SSDs it allows me to gain 321 GB instead of 900/1000 GB. 1000 GB cache would require 416 MB of my RAM, which is far less than 2 GB (a quarter of my 8 GB).

 

go into the dsm resource monitor and check the memory section, there should be information about memory consumption

Edited by IG-88
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the RAM looks ok

 

5 minutes ago, giacomoleopardo said:

Yes, but not more than 321 GB

 

maybe that relates to your RAID5 array, its a 4 x 120GB, after the overhead it might be the size of 3 x 120GB (one less because of redundancy)

as its only for testing you could make the 4 disks a jbod array and see how the cache suggestion turns out (should be way more if i'm rigt)

 

 

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Just now, IG-88 said:

 

 

the RAM looks ok

 

 

maybe that relates to your RAID5 array, its a 4 x 120GB, after the overhead it might be the size of 3 x 120GB (one less because of redundancy)

as its only for testing you could make the 4 disks a jbod array and see how the cache suggestion turns out (should be way more if i'm rigt)

 

 

Good idea! Thanks for the advice. I'll try asap and report back here.

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28 minutes ago, IG-88 said:

 

 

the RAM looks ok

 

 

maybe that relates to your RAID5 array, its a 4 x 120GB, after the overhead it might be the size of 3 x 120GB (one less because of redundancy)

as its only for testing you could make the 4 disks a jbod array and see how the cache suggestion turns out (should be way more if i'm rigt)

 

 

Yep, I guess you're right.

Here are the cache creation wizard's results, upon a 700 GB JBOD

 

1476713307_Schermata2020-11-22alle21_57_49.thumb.png.8eb539526f10f28dc019043133206311.png

170644778_Schermata2020-11-22alle21_58_27.png.f899b7ae297a9d899b685f065f58fed1.png

 

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