Chapter 03 - Capacity Planning

When I bought my first NAS I didn’t do any capacity planning at all. I simply bought a DS212J and put in two 2TB drives that I had available. It was the least expensive 2-bay Synology NAS and a way for me to get data protection and to see what Synology could do. Getting a low cost model and dipping your toe in the Synology pool is a great way to get familiar with DiskStation Manager (DSM) and to see what it can do.

When I outgrew the DS212J I was able to reuse those hard drives so it wasn’t a bad investment. I still have the DS212J and use it with older drives for testing and experimentation. A single drive Synology NAS would be even less expensive but it doesn’t have any RAID data protection so I wouldn’t recommend it.

A 2-bay model may be ideal for you. With today’s 4TB drives you can get 4TB of RAID protected, usable space. This is a lot of space. Unless you are doing video, have a lot of high-quality music, or take a lot of photos this is may be all you need. I didn’t outgrow that DS212J until I started using it to store video files. Typical data such as word processing documents, spreadsheets and presentations don’t use a lot of space. Unless you're a professional or prolific photographer, a two drive model can also handle your photo library.

Of course, there are exceptions to these rules so you should at least go through the file section of this chapter. But don’t worry about getting it right down to the last byte.

File Planning

One of the major factors determining the Synology model that you need is your file storage needs. Determining your current file needs is a fairly straight-forward process. Locate the files that you plan to move to your Synology NAS and figure out how much space they currently use.

The total will include all subdirectories so there’s no need to also check sub-directories. While you don’t want to widely overestimate you should always round up.

Current Files - Windows

Once you identify a directory that will be moved to your Synology NAS right-click on it and select Properties.

File size on Windows

Current Files - Mac

Once you identify a folder that will be moved to your Synology NAS right-click (or control-click) on the folder and select Get Info.

File size on a Mac

OS X Time Machine

While I love and use Time Machine on OS X it does have it’s quirks, especially when it’s backing up across a network. Despite Time Machine's quirks my Synology NAS is a reliable backup destination. (I wouldn’t make it my only backup as there a hiccups from time to time, but it’s as reliable as any other network destination, including Apple’s Time Capsule). I’ll cover setting up Time Machine in a future chapter.

When allocating space for Time Machine backups I’d recommend twice your computer’s hard drive size if you have a full hard disk or want backups that extend back a long time. You can reduce this if you’re more concerned with recent backups.

For my computers I allocate just slightly more than my hard disk size for Time Machine. But I typically keep my hard drives well under 50% full since I store most data on my NAS.


You’ll also need to plan for file growth. If you intend to save new photo and video files you should be generous in your estimates. Video and photo files are getting bigger all the time so if you use your current file sizes as the basis for growth you may underestimate your needs.

If you can’t come up with an accurate estimate you can approach this in other ways. All of the options listed below are for a Synology NAS using Synology Hybrid Raid (SHR) which is extremely flexible and easy to use. This is the default when installing DSM. Other RAID options impose addition restrictions which will complicate growth.

  1. Simply buy a Synology NAS with as many drive bays that you can afford. Then only add enough hard drives for your current needs. Add drives to the empty bays as your needs grow. Since hard drive prices generally decline (per GB at least) this has the added benefit of saving money over having unused space. Plus, the drive warranties will be spread out.
  2. Pick a model that supports expansion units. If you outgrow the NAS you can add an expansion unit and drives. This can be expensive since you’ll need to buy the expansion to even add one drive. But again, the expense is deferred until you actually need the space.

Due to the cost this would be a better choice when you expect significant future growth. You don’t want to buy an expansion unit and only ever put one drive in it. 1. If you’re using all your drive bays you can take advantage of ever increasing hard drive sizes. Upgrade your hard drives as larger drives become available.

Be aware that you’ll need two hard drives of the larger size before you gain any space. For example, if you have a NAS with four 2TB drives you’ll have 6TB of usable space. If you replace a single drive with a 4TB drive you’ll still have 6TB of usable space. If you add two 4TB drives you’ll gain 2TB of usable space (for a total of 8TB) and you’ll gain 2TB for each additional 2TB drive that you replace with a 4TB drive.

Application Planning

These days disk space is measured in terabytes (TB) so the phrase “a small amount of disk space” is relative to that. Because of this the amount of disk space used by any Synology application is insignificant. I typically allocate 1TB of space to applications, overhead and as a cushion. If you are using smaller solid state drives (SSD) then you’ll need to be more concerned about space and will need to spend more time researching the disk usage of the applications you’ll be using.

General speaking, Synology’s own applications don’t require significant disk space unless you are adding files to them. For example, Video Station and Music Station also create backend databases but these are significantly smaller than the video or music files themselves.

Photo station does create thumbnails for your photos which makes it one of the applications with larger backend space needs. But even this doesn’t move the needle on disk usage. For example, my library of over 45,000 photos uses 154GB of disk for the photos themselves while the thumbnails don’t use enough space to move the space used number to the next gigabyte.

If you plan on using a third party application, such as Zarafa or DokuWiki, you should research the disk usage requirements of that application. Typically, applications will use the same amount of disk space on Synology as they would on other platforms.

For sample, WordPress usage is about the same on Synology as it is on a Linux server. Plus, disk usage is relatively small unless you’re adding a lot of photos, videos or file downloads. The WordPress files and backend database itself are small.


As I previously mentioned, I typically allow 1TB of disk for overhead and to have a cushion for unanticipated growth. This is for a new Synology setup where there’s no history to base growth on. This is generous estimate for overhead so it’s less on Synology NAS’s that have less room overall. It seems as if files grow to take up any space made available. That’s not literally true, but as you use your Synology NAS you’ll find it’s capable of doing more and more. Also, people (including me, although I’ve gotten better over time) tend to underestimate future usage. So the 1TB includes a cushion for bad estimates.

This doesn’t mean on day 1 that I have only 1TB free. This is 1TB after my estimates for growth. If you’ve got less than 4TB of usable space in your Synology NAS you should be sure to have at least 25% free on day 1 and consider adding disk, or removing files, if you get below 20% free.

Network Capacity

If you plan to have a lot pf people accessing the NAS at the same time, or a few people heavily accessing the NAS at the same time you can improve performance by using multiple network connections.

At one time this took additional, specialized network hardware that could support link aggregation. Link aggregation is still an option, but now you can use any network hardware to benefit from load balancing as long as your NAS has two or more network connections. Load Balancing was added with the recently released DSM 5.2.

Neither load balancing or link aggregation will improve things if all you have is one computer connecting to the NAS. Today I would pick load balancing over link aggregation since it’s easier and less expensive. I benefit from link aggregation every evening when all my computers do their backup to my NAS and one of my NASs backs up to the other. All this traffic from different devices can benefit from link aggregation and load balancing.

If you stream video to multiple devices, such to iPads for the entire family, there will also be a benefit from link aggregation or load balancing.

The Synology models with two or more network interfaces are the more expensive models. The two-bay plus model, currently the DS214+, is the least expensive model with multiple NICs.

If you have a model with multiple NICs there’s no reason not to use load balancing since the only cost is a network cable. That cable is often included with the Synology NAS.

Memory & CPU

Memory and CPU are the hardest to estimate. Except for some of the higher end models the built-in memory is all you get. The CPU is always picked for you and it is typically what determines the cost of the model compared to it’s peers.

If you’re going to be running a lot of apps, or have a lot of people access apps at the same time you’ll need more memory and CPU than you would for a NAS just doing file storage. In this case I’d recommend you look at the plus versions of the NAS models (for example, the DS214+ instead of the DS214). These have faster CPUs and more built-in memory.

If you get a NAS that has expandable memory I recommend adding as much memory as it can hold right from the beginning. The only time I don’t recommend this is when the NAS will be used purely as a backup destination. Memory is currently inexpensive and once you get a NAS you’ll find more and more to use it for.

Unless you have a good handle on what the NAS will use for and don’t plan to grow beyond that I typically always recommend getting the plus model if it’s within the budget. This is because they have the most memory and faster CPU related to their peers. If you outgrow the memory or CPU the only upgrade option is a new NAS.