Synology NAS – Cache Disks as Storage

Wow, this is actually my first real post to this blog.
I set this up as a place for me to brain dump information and talk about things that interest me in general.
But anyway! Enough of that. Let me dive right into it!

Synology NAS Machines with Cache Disks

Synology really, really, does not want you using your cache disks as storage volumes.

I can partially understand why, but it is also incredibly limiting in many ways for users.

I’ve dug around the web, found a number of posts on Reddit and other various blogs regarding how to turn your cache disks into actual storage volumes.
But the problem I had was that they all seemed to go off in multiple different directions, and often had multiple paths of going about accomplishing this.

None of the posts I saw really brought up anything related to what I was seeing on my system. So, here I am to write about it a bit.

A bit on my hardware setup:

I have a Synology DS1817+ running DSM 7.1 filled with eight 8TB Seagate IronWolf disks. I also have the Synology M2D18 Dual M.2 SSD adapter card installed, slotted with two Samsung 860 EVO 250GB SSDs.

Utilizing these SSDs as a cache for my array wasn’t the best solution. So I disabled the cache, but it left me with two unused cache devices.

Through the web ui, I was unable to do anything with them except initialize a caching disk.

Source Materials:

I’m going to link some of the source material I used in my exploration to attempt to get things working:

How I did it

Since these posts didn’t really cover the scenario I was looking at, I fumbled around a bit while getting things working.

There were some notable differences.

  1. My NAS did not have lsblk installed (lsblk is not required, but I had a use for it, wanted to see how my system was registering the disks and partitions)
  2. My Cache Disks were not mounted at /dev/nvme*
  3. Most all of the Synology commands used are poorly documented


  • Ensure the cache device is not in use and the drives are not initialized
  • Log into your NAS and elevate to root
  • Locate your cache disks
    • Mine were located at /dev/nvc*, while all other posts referenced /dev/nvme*
  • Run this command on each cache disk:
    • synopartition --part /dev/nvmc1 13
    • synopartition --part /dev/nvmc2 13
    • The other posts I linked use 12 at the end of the command; as I mentioned, the Synology commands are poorly documented publicly, I am not entirely sure the difference between 12 & 13.
  • Perform an fdisk -l /dev/nvc[12] on your disks to validate the partitions were created properly
  • I deviated from the other blog posts by creating two storage pools, with each disk on its own pool, effectively just mounted as plain disks. The other posts go into detail on RAIDing them together.
  • I ran these commands to create a “simple” volume:
    • mdadm --create /dev/md4 --level=1 --raid-devices=1 --force /dev/nvmc1p3
    • mdadm --create /dev/md5 --level=1 --raid-devices=1 --force /dev/nvmc2p3
    • Each disk becomes its own “single disk RAID 1 array” – if you want to phrase it like that
  • After performing the mdadm commands, I formatted the newly created volumes
    • mkfs.btrfs /dev/md4
    • mkfs.btrfs /dev/md5
  • Reboot the NAS
  • Once booted, Storage Manager should now show two new Storage Pools (In addition to your existing one, if you have one set up)
  • Go to the storage area of the window from overview
  • The new pools will show as “unassembled”
  • Click on the three dots to the right of the new volumes and select the option to assemble the volume
  • Give them a name, and, like magic, you have configured your SSD cache devices as storage volumes!

The reason why I am posting this, is because many of the things I found involved touching and modifying internal Synology system files, such as /etc/synoinfo.conf – which, I do not find to be the best solution, as it is likely to break on upgrade.

Here is a screenshot of my current Storage Manager page, pools 2 & 3 are the SSDs, pool 1 are my main 8 disks.

Not too much hackery was involved in getting this all set up, and it seems to be persistent and survive reboots.

Hopefully this can at least help somebody out if most of the major guides aren’t working for them, or if you don’t want to go the route of creating a RAID volume from the SSDs, or if you don’t want to mess with LVM to set the disks up.
It’s all fairly straight forward.

As a side note, during the process, I did happen to install entware/opkg on my system, because it offered tools such as iotop, iftop, strace, which SynoCommunity doesn’t seem to have, and most other Synology Package Centers seem to be dead or vastly undermaintained.

Hopefully I will start posting here more regularly.

Thank you for reading my post, feel free to leave feedback


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