When we set out to build our own network storage server, it's very common to be disappointed to see that the storage space listed on the drive box doesn't match what's displayed on the screen. It's not that we've been ripped off; it's simply that a number of technical and mathematical factors mean that the nominal capacity never exactly matches the actual space available for storing our files.
To avoid guesswork, it's crucial to understand that the usable storage space of a NAS isn't a fixed figure, but rather the result of a complex equation involving the chosen hardware, security configuration , and even how the operating system manages data. Without this understanding, we risk running out of space much sooner than expected.
Why is the advertised space not the actual space?
The first thing to understand is that there's a difference between advertised and actual storage capacity. Manufacturers typically measure disks using decimal, but operating systems use binary. That's why an 8 TB drive magically becomes about 7,3 TB usable as soon as you plug it in. On top of that, the NAS operating system itself and installed applications consume some of the storage space to function properly.
The pillars that define your useful capacity
To calculate how much space you'll actually have, you need to look at three key points:
- The number of bays: These are the physical spaces where the drives fit. A single-bay NAS is very limited, while one with four, eight, or more bays gives you a... brutal margin for maneuver to grow in the future.
- The individual capacity of each disk: Obviously, installing 1 TB drives is not the same as installing 20 TB drives. You should always check that the NAS is compatible. support the maximum capacity of the record we want to buy.
- The selected RAID level: This is the key to everything. RAID is how the NAS organizes the disks to gain speed or, more commonly, security.
Understanding RAID and its impact on space
Depending on how you configure your disks, the available space will vary drastically. Here's an explanation of the most common configurations to avoid confusion:
- RAID 0: You take advantage of 100% of the space, but it's like playing Russian roulette; if one disk fails, you lose everything.
- RAID 1: It's the standard for two-bay NAS. The data is mirrored, so You only have 50% of the total capacity. If you install two 16 TB drives, you will have 16 TB of usable space.
- RAID 5: Very typical in 3 or 4 bay systems. The usable capacity is the sum of the drives minus one. It's a Ideal balance between safety and space.
- RAID 6: Similar to the previous one, but with two fewer disks. It allows two drives to fail simultaneously without the system crashing.
- RAID 10: Combine mirror and add, but you still lose half of the total space (50%).
A clear example: if you set up a server with 64 TB of physical storage but choose RAID 1, you'll end up with about 32 TB of usable space . It's a price we pay for the peace of mind that our data won't disappear if one disk fails.
Choose the number of bays according to the usage profile
You don't need to buy the biggest NAS on the market if you don't need it, but it's also not wise to fall short. Depending on your needs, there are more sensible options:
For freelancers or small businesses that only want to store documents and make basic backups, a two-bay model like the Synology DS223 is usually sufficient. RAID 1 is the norm, although the problem is that to expand storage space, you then have to replace the old drives with larger ones.
If you're looking for something versatile for a small or medium-sized business, a four-bay NAS is the sweet spot. It allows you to start with just a few drives and leave space for future expansion. RAID 5 shines here, and you can also experiment with hybrid storage , using mechanical drives for large amounts of data and SSDs for lightning-fast performance.
For professional environments with virtualization or video surveillance, we move up to 6 or 8 bays. These units allow for the creation of separate storage groups and offer significantly greater fault tolerance thanks to RAID 6. In these cases, it is very common to opt for the rack format to integrate them into technical cabinets with better cooling.
Practical tips and shopping checklist
Before you buy, ask yourself these questions to avoid making a mistake. Don't just look at your current storage capacity, but also at how much your storage will grow annually . If you're currently using 6 TB but generating 2 TB per year, a two-bay NAS will quickly become too small.
Keep in mind that if you plan to install security cameras, virtual machines, or use SSDs for caching, each of these functions may occupy a drive bay or consume space on the main volume. Also, always remember that RAID is not a backup. For peace of mind, apply the 3-2-1 rule : three copies of your data, on two different storage media, and one of them off-site (for example, in the cloud with services like Backblaze B2).
To properly size your system, always check disk compatibility, whether the system allows for future expansion, and calculate the actual usable capacity by subtracting the space used by the RAID and the operating system. Ultimately, choosing a system with room for growth is the best investment to avoid costly and tedious migrations in the near future.
