-S automatically sets internal SSDs as DSM preferred

[wolffcatskyy]

Could you explain what this does? It sounds like it makes DSM prefer faster drives, which would speed up everything? Sounds incredible but I can't find anything about it!

[007revad]

Correct. It makes DSM read from the 2.5 inch SSDs instead of the slower HDDs. Note: This only applies to DSM's hidden system and swap partitions. It uses mdadm's write mostly option.

[wolffcatskyy]

2.5” only not nVME or raid volumes?I don’t currently have any 2.5”, does it make sense to add 2.5” just for this?Sent from my iPhoneOn Dec 29, 2025, at 3:37 AM, Dave Russell ***@***.***> wrote: Correct. It makes DSM read from the 2.5 inch SSDs instead of the slower HDDs. Note: This only applies to DSM's hidden system and swap partitions. It uses mdadm's write mostly option. —Reply to this email directly, view it on GitHub, or unsubscribe.You are receiving this because you authored the thread.Message ID: ***@***.***>

[007revad]

All 3.5 inch and 2.5 inch drives in the NAS contain a mirror of the DSM system partition and swap partition.

NVMe drives, and HDD/SSD in an expansion unit have the hidden system and swap partitions but they are empty and not used by DSM.

Personally if I was going to use this option I would install 1 small 2.5 inch SSD in bay 1. But the trade-off for a tiny DSM speed increase is 1 less bay for larger HDDs. I haven't compared the speed difference but I suspect it would be more noticeable in a NAS with slow RPM HDDs or one not using RAID.