Which DSM version shall I use? This is an oft-repeated question, complicated by the fact that different loaders support different DSM versions AND hardware platforms.
In simple terms, DS3615/DS3617 has the widest support for hardware and packages, and DS916/DS918 has support for newer, low-cost CPU's, transcoding and NVMe cache (DS918 only).
But the real questions are, what hardware do you have? What hardware do you want to buy in order to support what you want to do? And how does a DSM loader and platform support this? In order to answer, it's useful to understand what hardware is natively supported by DSM. Each DSM version is different as Synology does not need to support many types of hardware, as they build up a specific DSM for each hardware platform they sell. Fortunately, the base Linux kernel has much broader support for hardware than they intend.
Most of us guessed at the hardware requirements and made (hopefully) intelligent selections on DSM versus hardware. Some build systems and then are distressed when the hardware isn't fully supported by the DSM platform they choose. Unfortunately, Synology hardware knowledge is often imperfect, as the main boards are custom designed. Refer to the example below for DS3615 (I hope it will be completed more precisely in the future).
DS3615xs
Reference Hardware:
CPU:
Intel Core i3-4130 (Haswell)
Chipset:
Intel C20x?
4x1Gbe Ethernet:
???
Secondary SATA:
???
10Gbe OEM:
E10G18 (Aquantia AQC107)
E10G17 (Mellanox Connect X-2/3/4)
E10G15 (Tehuti)
With even this information, we can make some good guesses on what hardware might be supported. We certainly can act on the 10Gbe cards since those are well known. Wouldn't it be great if we knew ALL the different drivers that are natively supported? Unfortunately this is a fairly difficult process if you are not a Linux guru (and a bit laborious even if you are).
There are "user-reported" hardware compatibility threads out there, but many don't understand that those reports are both DSM version and platform specific. Furthermore, with the way hardware manufacturers reissue hardware with the same name but new PCI device number (such as the Intel PHYs on desktop motherboards), often not enough information is reported to confirm whether a specific piece of hardware is suitable for use.
If you aren't sure if your hardware is supported, this post and the complementary DS918 driver guide aim to help you. Download the appropriate Excel spreadsheet to see key driver support in the DS3615 6.1.7 or 6.2.1 Synology custom kernel, and via loadable modules supplied with DSM. Hopefully it will help you select the best DSM platform for your purposes, and possibly inform your hardware purchases.
Common drivers missing from this platform, or newer driver versions that support the latest silicon may be available by installing additional compiled modules.
2021-Sep update: Now that folks are testing RedPill loader, they are finding that drivers working in 6.2.3 and earlier are missing in 6.2.4 or 7.0. This is because many of the drivers are part of the Jun loader and not truly native to DSM. Yes, the title of this post says NATIVE but for the purposes of the analysis, Jun's injected drivers are considered along with the actual embedded native drivers.
DSM 6.1.7 DS3615 V1.04 2019-Feb-04.xls
DSM 6.2.1 DS3615 V1.0 2019-Feb-04.xls