Chapter 02 – Features & Definitions

Chapter 02 – Features & Definitions

Before I get into the details of capacity planning and picking your Synology NAS I’ll cover what some of those features are and define some terms.

DiskStation Manager (DSM)

This is the software that runs on the Synology NAS. It’s often referred to as firmware and sometimes as the operating system.


When determining the capacity of your NAS you need to look at more than the traditional computer. So I use the term device to indicate that anything that can connect should be considered. This includes traditional computers along with phones, tablets, Roku, Chromecast and more.


Synology Hybrid RAID

RAID was originally short for Redundant Array of Inexpensive Disk but now it’s also considered an acronym for Redundant Array of Independent Disk. Wikipedia is a good place to start if you want to get into the details of RAID.

One key point to remember – for standard RAID implementation (standard meaning the ones discussed on the Wikipedia page and that follow industry standards) all the drives in the array must be the same size. It’s also generally recommended that they be of the same model and even firmware level. While implementations can vary, these RAID levels can require specific technical knowledge to manage them.

The cornerstone of DiskStation Manager (DSM) is Synology Hybrid RAID (SHR). This is proprietary to Synology so you won’t find it discussed on the Wikipedia RAID page. The benefit of SHR is it’s flexibility an ease of use. The drives do not need to be of the same size or model. If the drives are of different sizes then Synology will optimize them to provide the most redundant space. You can use the Synology RAID Calculator to determine usable space for different RAID levels and drive configurations.

Using SHR with two drives is similar to RAID 1 (mirroring) where a copy of a file is maintained on each physical drive. Your total usable space is 1/2 of the total combines drive sizes if they are the same size, or equal to the smallest drive if they are difference sizes.

Using SHR with three or more drives is similar to RAID 5 where enough data is kept on a third drive so that the data can be rebuilt if one drive fails. In this case the size of you largest drive is subtracted from the total to determine free space. Although, since drive sizes can vary there’s a lot of variation possible and you should use the RAID calculator to get the total.

Synology Hybrid RAID can also be setup to withstand two simultaneous drive failures. This lowers the usable space. In the RAID calculator select “SHR-2” to get these totals. At least 3 drives are required.

When you need more space you can simply replace one of the SHR drives with a larger drive. During this time the storage manager will report that the array is degraded while it rebuilds the drive. Performance will be impacted during the rebuild so you’ll want backups and you won’t want to do this frequently. But, it’s certainly easier than upgrading a standard RAID array.

Synology Hybrid RAID, while proprietary to Synology, does have comparable competitors. Drobo and Q-NAP both provide similar solutions which is proprietary to their hardware.

Other RAID Levels

Synology NASs also support standard RAID levels 0, 1, 5, 6, and 10 if the unit has enough drives to implement that RAID level. Some of these RAID levels do have advantages, such as better performance. Unless you need to optimize for every last bit of performance you’ll be better off picking SHR and you won’t notice a difference.

Synology does support expanding existing RAID levels by upgrading to larger disks or adding disks. For upgrades, you’ll need to upgrade all drives to get them to the same size before seeing a benefit. Support for this depends on the model so refer to the technical specifics to see which drives support the various RAID levels and upgrade options.

One of the reasons for using Synology, especially for the home and small business, is to simplify RAID. Even though other RAID levels don’t change much other than disk management, I based this guide on the assumption you will be using Synology Hybrid RAID.

Multiple Network Interface Cards

Multiple Network Interface Cards / Link Aggregation

For a technical explanation of link aggregation you can visit Wikipedia. For our purposes you can think of link aggregation of a way to increase the amount of data that the Synology NAS can send and receive on the network.

For link aggregation you’ll need to connect the Synology NAS to a network switch that supports link aggregation. Look for switches that mention support for 802.3ad or Link Aggregation Control Protocol (LACP). While there are many low-cost switches to choose from, most low-cost switches targeted toward home users don’t include link aggregation support. I currently use a TP-LINK TL-SG2216 16-Port Gigabit Smart Switch so I can confirm it works. Unfortunately I would consider the interface daunting for someone without experience setting up a network, and frustrating even for someone with experience. Search Amazon, or your favorite store’s website, for “802.3ad” and look for favorably reviewed switches. (Don’t trust search, verify the switch does support link aggregation.)

If all you have is one computer, or only use one computer at a time, link aggregation will not improve performance for you since you are limited by the PCs network connection. Even if the computer supports link aggregation, some operating systems will only support communication between any two computers (including the NAS)using only one of the NICs.

If you have a computer and also want to stream video to your TV, or have multiple family members or employees using Synology services then link aggregation on the NAS will provide better performance.

Multiple Network Interface Cards / Adaptive Load Balancing

Prior to DSM 5.2 link aggregation was the only way to increase the overall throughput into and out of the NAS. (OK, technically enterprise class network gear could be used to increase throughput if the NAS had multiple NICs but that requires expensive hardware and was independent of the NAS.) With DSM 5.2 Synology added load balancing to DSM.

With adaptive load balancing you can now use multiple NICs in the Synology NAS to increase the overall throughput without requiring any special hardware. You simply connect the network cables into the network switch, or your router if the network ports are built in. This can also be used with the otherwise obsolete hubs.

The network cables can also be connected into different switches to provide redundancy.

Like link aggregation, this has little benefit if you have only one device talking to the NAS but if multiple devices are connecting at the same time the NAS can handle additional traffic.

Multiple Network Interfaces / Additional Features

The dual network interface cards (NICs) can be used even without link aggregation or load balancing. Both NICs can be connected to a switch and each will be given an IP address. The dual NICs can also be configured to failover from one to the other if one NIC fails. Configuring for failover is a larger topic since the NIC in the Synology NAS is not the most likely point of failure.

Wake On LAN (WOL)

This allows you to remotely start your Synology NAS without having to push the power button. You can use the Synology DS Finder app to send the WOL command (sometimes called a magic packet) to start the Synology NAS. There is other software that can also send the WOL command. Not all Synology NASs support WOL and it must be turned on for those that do support it.

eSATA Port

This port is used to attach Synology expansion units. It can also be used to attach other external drives (if they can connect via eSATA). Some Synology units with a eSATA port do not fully support Synology expansion units. By this I mean that DSM software will see the expansion unit only as a external drive and not as drives it can manage internally, just like the built in drives. The DS415+ is an example of having an eSATA port that doesn’t support Synology expansion units. (The DS916+ does support the expansion units which is why the name changes to DS9xx even though it’s a DS415+ updated for the new model year.)

If you think you’ll outgrow your NAS you’ll have two choices – upgrade the physical drives to larger models or add more physical drives. If you think you’ll be adding drives you will want to get a model that supports expansion units.

SSD Caching

Some models support SSD Caching which is a technology that uses fast solid state drives to cache the most used files to improve performance.

Only one drive bay is needed for read-only caching while two drive bays are required for read/write caching. DSM includes a SSD Cache advisor so if you are unsure of what size drives you may need I recommend that you include the bay’s in your plan but don’t buy the drives until you’ve used the NAS for about a week and then see what the advisor recommends.

Synology recently introduced the M2D17 Dual M.2 SSD Adapter Card that moves SSD caching out of the drive bays and onto this card. This allows those drive bays to be used for storage. Support is currently limited to the DS1517+ and DS1817+ models.

SSD Caching also uses about 416KB of memory for each 1 GB of cache so you will need to size the memory accordingly.

Wireless Support

Many Synology NASs can be connected to your network use a wireless connection. Except for the DS213air, which has built-in wireless, you’ll need to buy a additional wireless dongle. Verify the NAS supports a wireless connection then be sure to buy a compatible wireless dongle.

Note that Wake On LAN (WOL) cannot be used to wake a wireless connected NAS. This is a limitation of the wireless network, not the Synology NAS.

Wrapping Up

This summarizes the hardware dependent features that differentiate the Synology models. You can look through the options and then visit the Synology website to see which models have the features you need.