DSM system partition missing on M.2 volume?

[TorbenSchreiter]

Hi there,

on a DS920+ with 4x HDD previously installed (as /volume1; ext4) I successfully installed 2x M.2 Samsung 990 EVO SSDs (SHR as /volume2; btrfs). The aim is to reduce spinning times of the HDDs (thus reduce power consumption) and add a bit of performance. As part of the effort I would like to recreate the HDD volume1 as btrfs as a last step.

I have finished migrating all packages and shared folders to the M.2s. Both Synology_HDD_db & Synology_app_mover worked nicely. I also moved the system Postgres following instructions here.

Then I started following the suggested procedure from here:

1. Migrate everything to vol2.
2. Shut down NAS.
3. Remove vol1 drives.
4. Boot NAS and mute the beep.
5. Start your packages and check they are okay.
6. Shut down NAS.
7. Install the drives you removed in step 3.
8. Boot NAS.
9. Delete vol1 storage pool.

However, after step 4, the NAS is not booting at all with the HDDs removed and only the M.2s inside. I had expected that the DSM system/OS partition would automatically mirror to all drives (volumes?). Originally, I had intended to next set the -S (write_mostly) switch for the script after confirming everything is working off the M.2s. But that does not seem to be the case.

What can I do to ensure the DSM system partition is also mirrored to the M.2s?

Thanks.

[007revad]

DSM does not mirror the system or swap partitions to M.2 drives (or drives in an expansion unit), so.

• You can't boot the NAS with just NVMe drives
• And write_mostly won't make DSM try to always read from the NVMe drives.

Technically there is a way to install DSM on NVMe drives... but it requires some seriously dangerous hacking.
See this reddit post and the reply by Alex_of_Chaos.

In your situation I'd just install a small 2.5 inch SATA SSD in the DS920+. But as you don't have a spare bay you'd have to:

1. Remove one of the HDDs (which will make the storage pool degraded).
2. Insert the small SSD drive and create a new storage pool (so DSM gets mirrored to it).

Then check your packages are all okay without the HDDs.

1. Shut down NAS.
2. Remove vol1 HDDs.
3. Boot NAS and mute the beep.
4. Start your packages and check they are okay.

Assuming step 4 was okay:

1. Shut down NAS.
2. Install the 3 HDDs you removed in the previous step 2.
3. Boot NAS.
4. Delete vol1 storage pool.
5. Create a new btrfs storage pool and volume 1.

The one downside of this is you now only have 3 HDDs for data instead of 4.

[TorbenSchreiter]

Understood about the limited capabilities of the M.2 slots. That the M.2s would not be bootable was not clear to me previously. Maybe worth a disclaimer or short Q&A entry in the fine print of README.md?

I shall admit, I have not fully understood though how write_mostly actually works. I only run 4TB HDDs and don’t really need more capacity anytime soon. Could I simply replace one of the 4x HDDs with a 4TB SATA SSD within a single large SHR2 volume across 3x HDD + 1x SSD and then flip the write_mostly switch? Or would it wake all four disks from sleep together each time?

As the DSM system partition should be on each drive (not each volume), would this direct most of the reading for the OS to that SATA SSD? Daily data + packages would stay on the M.2s, so no HDD activity from there either.

[007revad]

Setting write_mostly for the slower drives makes DSM try to avoid reading from those drives, so it reads, mostly, from the drive without write_mostly set. It only works for RAID 1, which the system and swap partitions are.

DSM 7.x will not let you use different drive types in the same storage pool. DSM 6 used to allow it, apparently. So you can't have an SSD in the same storage pool as HDDs.

Are you using SHR2? I would never use SHR2, or RAID 6, with less than 6 drives.