The newest addition to my home lab is a Synology RS3413xs+ NAS. While installing it, I came across a couple of details that I didn't know before buying it. So for other people thinking of buying this unit, here's what I found out:
- If you add network interfaces in the available PCIe slot, they might be numbered _before_ the four onboard interfaces. They were in my case. So onboard 1-4 are eth2-5, and add-on interfaces 1-2 are eth0-1.
- the SSD cache feature only works with identical drives in the both cache slots. You can buy two 120GB SSDs, but you can't just add one 240GB SSD. Except if you configure it manually through the CLI, and want to work without Synology support.
- as explained in an earlier post, there's no multiple-VLAN-over-one-interface support in the GUI, but you can work around that in the CLI
- the DSM web interface counts VLAN-tagged packets twice in its "Total Network" graph. The per-interface/per-bond counters are correct however. PS that looks like the bug I solved three years ago in dstat 0.7.0!
- a Synology RAID group is used as an LVM volume group. Volumes and block-based iSCSI LUNs you create afterwards are implemented as LVM logical volumes. File-based iSCSI LUNs are just placed on formatted volumes like other files.
- the SSD cache can only be used for one LVM logical volume! Read on for a manual workaround.
- activating or deactivating the SSD cache for a volume means stopping all services temporarily.
- both SSDs are configured as a software RAID0 volume, with 64KB segments.
- the SSD partitions aren't aligned at all. Makes sense I guess. The regular disk partition for data is aligned at a 512MB boundary. PS the swap partition is aligned at a 128MB boundary, and the DSM root partition is aligned at 128KB.
- Synology implements its SSD cache feature using the "flashcache" driver in Linux (the one Facebook developed). Flashcache has three caching modes (writeback, writethrough, writearound) of which Synology currently uses writearound in DSM4.1. Just like writethrough this only accelerates read performance, as is clearly indicated in Synology documentation. If you insist on having write cache as well - with all the consequences that brings! - you could manually change this mode to writeback. Not supported ofcourse. See the flashcache doc for details on the three modes.
- if you absolutely need SSD cache for multiple volumes, another manual tweak is possible: dividing your SSDs into multiple partitions, making different md RAID0 devices from those, and activating those as flashcache for multiple volumes.
Get info from your own Synology device using:
# fdisk -u -l /dev/sdk; fdisk -u -l /dev/sdl
(sdk and sdl are the two SSDs in a 10-bay Synology, where sda..sdj are the 10 regular disks)
# cat /proc/mdstat
# dmsetup table cachedev_0
# dmsetup status cachedev_0
# vgdisplay -v vg1