What storage means on a server
A server usually has two kinds of drive. The system drive holds the operating system and your apps, and it should be an SSD, because apps start and respond faster. The data drives hold the big things, such as films, photos and backups. Those are often hard drives, because they cost far less per terabyte.
The second question is what happens when a drive dies. Every drive fails eventually, often with warning signs and sometimes without. You can arrange two or more drives so the server keeps going when one fails. That arrangement is called RAID: several drives working as one, with spare copies or repair information spread across them. The spare information is called redundancy, or parity when it’s a calculated repair code rather than a full copy.
None of this is a backup. RAID keeps you running through a dead drive. A backup brings back a deleted file, or everything after a theft, a fire or ransomware. If you can only afford one, choose the backup; the backups page explains how.
SSD or hard drive
| SSD | Hard drive | |
|---|---|---|
| Speed | Much faster, especially with lots of small files | Fine for films and backups |
| Noise | Silent | A hum, and clicks when busy |
| Cost per terabyte | Higher | Much lower |
| Best for | The system, apps, databases and photo thumbnails | Films, music, photo originals and backups |
SSDs come in two kinds. SATA SSDs are either the 2.5-inch laptop-drive shape or a small M.2 stick. NVMe SSDs are M.2 sticks that run several times faster. For a home server either is plenty for the system drive, but a slot only takes the kind it was made for, so check your machine’s specifications before you buy.
Which layout to choose
| Layout | Drives | Usable space | Drives you can lose | Good for |
|---|---|---|---|---|
| One drive | 1 | All of it | None | Starting small, with a backup |
| Mirror (RAID 1) | 2 | Half | One | Most first servers |
| RAIDZ1 or RAID 5 | 3 or more | All but one drive | One | More space from more drives |
| RAIDZ2 or RAID 6 | 4 or more | All but two drives | Two | Many large drives |
| Unraid or SnapRAID | 2 or more, any sizes | All but the parity drives | One per parity drive | Media libraries on mismatched drives |
The more drives you have, and the larger they are, the longer a rebuild takes and the more it matters to survive a second failure during it. That’s why big arrays use two parity drives.
What it needs
- Matching drives for a mirror or RAIDZ. Mixed sizes work, but every drive is treated as the size of the smallest.
- Somewhere to plug them in. A desktop or NAS has SATA ports and bays. A mini PC has room for SSDs only, so its hard drives go in USB enclosures or a NAS.
- CMR hard drives. The Skip list below explains why SMR drives cause trouble.
- Memory to spare for ZFS, which uses free memory as a cache. TrueNAS asks for 8 GB.
- A backup somewhere else, on another drive or online.
USB is fine for a single drive or a backup copy. Avoid building a mirror or RAID out of USB drives: a brief USB disconnection looks to the system like a failed drive.
How a mirror fits in
- Everything written twice. Both drives hold an identical copy at all times.
- Keeps working when one fails. The server warns you. Replace the dead drive and the mirror copies itself onto the new one.
- The simplest rebuild. Copying one drive onto another is quick compared with rebuilding from parity, so you spend less time unprotected.
- Faster reads. Data can be read from both drives at once.
- Checksums with ZFS or Btrfs. Each piece of data is stored with a fingerprint. If one copy has silently gone bad, it’s found and repaired from the other.
- Room to grow. Swap both drives for bigger ones, one at a time, and the mirror can then grow into the extra space.
- The cost. Half the space you paid for, and it doesn’t replace a backup.
The easy way to set it up
Two systems build a mirror from their web pages, with no terminal.
ZimaOS. Its free plan handles up to 4 disks.
- Fit both drives and start ZimaOS.
- Open Settings, then Storage, and choose Combine.
- Pick RAID 1, select the two drives, and confirm. Anything already on them is erased.
- Wait while it prepares the drives. Bigger drives take longer.
TrueNAS. It always uses ZFS, so you get checksums too.
- Install TrueNAS on an SSD of its own, 20 GB or more, separate from the data drives.
- In its web page, open Storage and choose Create Pool.
- Name the pool, choose Mirror as the layout, and add the two drives.
- TrueNAS schedules a regular scrub, a full check of every file, for you.
Unraid and Proxmox can also build mirrors from their web pages. On plain Ubuntu it takes a few terminal commands, which the OpenZFS documentation covers.
Questions people ask
Can I use two different-sized drives?
In a mirror, yes, but you only get the smaller drive’s space. Unraid and SnapRAID handle mixed sizes properly, as long as the parity drive is at least as big as the largest data drive.
Do I need RAID at all?
No. For a film library you could rebuild, one drive plus a backup of the irreplaceable parts is fine. A mirror earns its place when the data matters, like family photos, or when you’d hate the server to be down while you restore.
How will I know a drive is failing?
Drives keep health readings called SMART. The tools in the list below read them, and TrueNAS shows drive problems in its alerts. Reallocated sectors, which are damaged spots the drive has swapped out, are the classic early sign. One or two is a reason to watch; a number that keeps rising is a reason to replace the drive.
Can I add more space later?
Yes, in different ways. A mirror grows when you swap in bigger drives, or when you add a second mirrored pair. ZFS’s RAIDZ can grow one drive at a time, ZimaOS’s RAID 5 can take extra drives, and Unraid adds a drive whenever you like.
Where should photos go?
Keep the originals on the protected hard drives and the photo app’s database and thumbnails on the SSD. The app feels fast, and the files that matter have a second copy.
