The screen that trips up most new Synology owners is not the one where you install DSM. It is the tiny drop-down halfway through creating your first volume, where the box quietly asks you to choose between Btrfs and ext4.
Most people click the default and move on. That default is Btrfs on almost every model that supports it, and honestly, for the majority of home users that is the right call. But not always.
I have set up close to forty Synology units for friends, family, and a couple of small offices, and this one setting causes more second-guessing than RAID type does. So let me walk you through what actually changes depending on which one you pick, using real behavior rather than spec-sheet bullet points.
What the two file systems actually do
A file system is just the bookkeeping layer that decides how your files are laid out on the disk and how the drive tracks what lives where. Both options here are mature and reliable, so you are not choosing between safe and risky.
Ext4 is the older, simpler one. It has been the default on Linux servers for well over a decade, it is fast, and it barely asks anything of your hardware. If you have ever run a Linux box, you have used it. There is a solid plain-English rundown on the ext4 Wikipedia page if you want the deep background.
Btrfs (say "butter FS" or "better FS", people argue about it) is newer and does a lot more. It adds snapshots, checksums that catch silent data corruption, and space-efficient copies. Synology has spent years hardening it, and it is the file system behind features like Snapshot Replication and the self-healing you see advertised.
The feature that sells Btrfs: snapshots
This is the real reason to care. A snapshot is a frozen, read-only picture of your shared folder at a moment in time, and Btrfs takes them almost instantly because it only records what changes afterward.
Say your kid drags the family photos folder into the trash on Tuesday, and nobody notices until Friday. On ext4, if that folder is gone, it is gone, and you are into backup-restore territory. On Btrfs with hourly snapshots, you right-click the folder, pick a snapshot from Tuesday morning, and restore it in about ten seconds.
I have used this to recover from ransomware on a client machine that encrypted a mapped drive. The NAS itself was untouched because the snapshots are read-only, so we rolled the whole share back to the previous hour and lost nothing.
Setting them up takes about two minutes in Snapshot Replication. You pick a folder, choose a schedule (hourly plus daily is the usual pairing), and set a retention rule so old snapshots clean themselves up automatically. On ext4 none of this exists, because the file system has no way to freeze a point-in-time copy in the first place.
Self-healing, and why it needs the right setup
Btrfs stores a checksum for every block of data. On a scrub, or when you read a file, DSM compares the block against its checksum and can spot bit rot, the slow silent corruption that ext4 would hand you without a word.
Here is the catch a lot of articles skip. Self-healing only actually repairs the bad block if you are running a redundant array like SHR with two or more drives, or RAID 1, 5, 6, or 10. With a single drive, Btrfs will detect the corruption and tell you, but it has no good copy to rebuild from.
So on a one-bay or single-drive setup, you get the warning but not the fix. Still useful, but do not expect miracles.
DSM runs a data scrub in the background on a schedule you set, usually monthly, and that is when most bit rot gets caught and quietly repaired on a healthy array. I have seen a scrub on a client's DS920+ flag and fix two corrupted blocks on a three-year-old drive that SMART reported as perfectly healthy. That is the kind of silent problem ext4 would never have told anyone about until a photo failed to open.
The cost: RAM and a little speed
Btrfs asks more of your NAS. It uses more memory and adds a bit of overhead, which is exactly why Synology does not offer it on the smallest, cheapest models. Units with 512 MB of RAM, like some older J-series boxes, are ext4 only.
For everyday file serving, streaming Plex, or Time Machine backups, you will not feel the speed difference. Where it shows up is heavy random writes and databases. If you run a busy database container or a Surveillance Station setup with a dozen cameras writing constantly, ext4 can be noticeably lighter.
| Factor | Btrfs | Ext4 |
|---|---|---|
| Snapshots | Yes, near-instant | No |
| Detects bit rot | Yes (checksums) | No |
| Self-heals corruption | Yes, with redundant RAID | No |
| RAM needed | More (2 GB+ recommended) | Less (runs on 512 MB) |
| Raw write speed | Slightly slower under heavy load | Slightly faster |
| Best for | Photos, documents, family data | Low-RAM units, heavy DB writes |
So which should you actually pick
For the vast majority of home users, pick Btrfs. The snapshot recovery alone has saved more of my clients' data than any other single feature, and the modern DS220+, DS224+, DS923+, and similar boxes have plenty of RAM to run it without breaking a sweat.
Choose ext4 in three situations. First, your model only offers ext4 anyway, so the decision is made. Second, you have an ultra low-RAM unit and want every drop of performance for something write-heavy. Third, you are running a single drive purely as a fast dumping ground with backups handled elsewhere, and you genuinely do not want snapshots.
A quick note on the conversion trap
Because there is no live conversion, people sometimes assume they can add a Btrfs volume next to an ext4 one and slide data across. You can, but only if you have free bays or a second unit. On a fully populated two-bay NAS, there is no room to shuffle.
If you are reading this before buying, that is the ideal moment. Get the file system right on day one and you never have to think about it again. Synology's own volume creation help page confirms the choice is locked at creation.
My honest default
If someone hands me a Synology and asks which to choose without any special requirements, I pick Btrfs every time and set up hourly snapshots with a 30-day retention on their photos and documents folders.
It costs a bit of RAM you already paid for, and it buys you an undo button plus corruption detection you will hopefully never need. That trade is easy. The only time I reach for ext4 is when the hardware forces my hand or the workload is punishingly write-heavy, and for a normal home NAS, neither of those is you.
Frequently asked questions
Is Btrfs slower than ext4 on Synology?
For everyday tasks like file sharing, Plex streaming, and Time Machine backups, you will not notice a difference. Ext4 pulls ahead only under heavy random writes, such as a busy database container or a large Surveillance Station install with many cameras writing at once.
Can I change from ext4 to Btrfs without losing data?
No. There is no in-place conversion on DSM. You have to back up your data, delete the volume, recreate it as Btrfs, and copy everything back, which needs either free drive bays or a second device to hold the data temporarily.
Do I need snapshots if I already have a backup?
They serve different purposes. A backup protects you if the whole NAS dies, while a snapshot is a near-instant undo for accidental deletions or ransomware, restoring in seconds instead of hours. Ideally you run both, since snapshots live on the same volume as your data.
Why does my Synology only offer ext4?
Btrfs needs more RAM, so Synology restricts it on the lowest-end models, typically those with 512 MB of memory like some older J-series units. If Btrfs is greyed out or missing, your model does not support it and ext4 is your only option.
Does Btrfs self-healing work on a single drive?
Partly. Btrfs will detect corruption using its checksums and warn you, but with only one drive it has no redundant copy to repair the bad block from. You need a redundant array like SHR with two or more drives, or RAID 1, 5, 6, or 10, for the automatic repair to actually work.
How many snapshots can Btrfs keep?
Synology supports up to 65,000 snapshots per shared folder, with 1,024 you can actively browse at once. For a home user taking hourly snapshots with a 30-day retention, that ceiling is far beyond anything you will realistically reach.
