GEFS on OpenBSD: A Early Preview | Hacker News Reader

GEFS on OpenBSD: A Early Preview

marc.info

> While snapshot consistency is useful to keep data consistent, disks often fail over time. In order to detect corruption, block pointers contain a hash of the data that they point at. If corrupted data is returned by the underlying storage medium, this is detected via block hashes. And if a programmer error causes the file system to write garbage to disk, this can often be caught early. The corruption is reported, and the damaged data may then be recovered from backups, RAID restoration, or some other means.

Okay! It's got CoW, snapshots, and data checksums. Therefore, it's good enough to compete with ZFS while being way smaller and permissively licensed. Now I just want it ported to Linux and the other BSDs:)

> It's got CoW, snapshots, and data checksums. Therefore, it's good enough to compete with ZFS while being way smaller and permissively licensed.

Does it have a built-in RAID layer? Because if it doesn't, then it can't compete with ZFS in many use cases. For example, what does "data may then be recovered from […] RAID restoration" mean?

With ZFS, if you have a (e.g.) mirrored/RAID-1 configuration, and you fetch some data from one drive and the checksum is wrong, ZFS can check the other drive, and if that checksum is good it can (a) pass the good data up, and (b) use the good data to fix the bad data. Most mirroring systems can't do that both-drives checking: ZFS is self-healing.

(This isn't to say that GEFS won't be useful in many other situations.)

Author here: correct, it's currently on the user to deal with corrupted data. I'm not opposed to adding a RAID layer, but I also want to explore what adding trim-style feedback to a generic RAID layer may look like, so that different file systems could share the work.

We'll see where things go.

> […] but I also want to explore what adding trim-style feedback to a generic RAID layer may look like […]

TRIM commands sent to the block layer would probably help with SSD/NVMe wear leveling, as well as thin-provisioning in virtualized environments.

And the lack of RAID is not a critique: if you're mostly interested in the file system layer that's fine, and a useful area to solve problems in. My comment was more towards the comparison to ZFS and its "rampant layering violation" [1] and some of the trade-offs that happen when you focus on more or fewer parts of the stack.

[1] https://web.archive.org/web/20070602005153/http://blogs.sun....

Unrelated: and since you're surfing the comments, with regards to your superblocks [1], you may wish to look into some of the lessons learned by ZFS; specifically see perhaps Allan Jude's "Large Label and Blockpointer-V2" from the 2025 OpenZFS Dev Summit: https://www.youtube.com/watch?v=3tqOBv8BmXI

[1] https://orib.dev/gefs.pdf § 9.1

I don't mean a literal trim command to the block device, but hints that get passed to the RAID layer to mark how blocks should be spread among the backing devices. I haven't put much thought into the details, but I suspect that there might not be so much information needed, so the RAID layer could be both generic and smart. It may not work out.

I'll probably be picking Allan's brain on a few things in the coming few months; amusingly, he mentioned doesn't use ZFS for ZFS development so that he can unload and reload the module, and he's interested in something that won't corrupt when he panics the kernel.

> I don't mean a literal trim command to the block device, but hints that get passed to the RAID layer to mark how blocks should be spread among the backing devices.

At this point in software/file system history, having some kind of TRIM functionality I think is pretty important: either built in, or a separate utility (like fstrim(8) for ext4, etc).

Are you planning to (eventually?) do a RAID (or at least mirror) layer, or will this strictly stick do file systems?

Hi author :) Must say that 8k+ lines is a little bit annoying and uneasy to handle... But also bribgs a lot of hope and happiness :)

To the point:

- llm'ed or not, dasn't matter, just quality matter

- raid ? I know nothing about that but looks like pure hardware raid (if that exists), no "raid layers" at all and fs raid aware are three different animals. But adding something about raids will complicate things before "production ready" stability. Which, of course, we want ASAP ;)

- 8k+ seems easy to manage and bend so maybe key-value only partitions ?

- obviously wishlist is easy to construct (key-value storage, encryption, power off resistance, mail reading ;) ) so maybe some plugin system or at least guidelines for peoples before we will have repos of patches DWM-style ? :)

- that 5 second is clearly stated but a bit strange limit and obviously peoples will want to do something with that

- man give us producion readiness before you will be puched by some corporation, swamped with work and hidden from daylight, pls ? Pleas do not fall into Compiz trap...

- thank you :)

I've always wondered about similar designs: Doesn't calculating a hash of every block, on every read and every write, create lots of overhead? Why isn't that a problem?

Some systems have dedicated crypto co-processors for confidentiality (encryption) - e.g., I think drives with FDE, and I think Apple Silicon SoCs might have them. Can those be repurposed for hash calculation? What about systems that lack them?

Both ZFS and modern btrfs support a large set of checksums.

Both implement sha256, which does impose a heavy speed penalty.

ZFS allows you to adjust the checksum on the fly, using something faster (Fletcher) if desired.

In btrfs, a global checksum is set at filesystem creation; xxhash is the best modern option.

There is a website: https://xxhash.com

Deduplication adds concerns for a strong hash free of collisions.

Fletcher has been the default for quite some time.

Specifically "fletcher4":

* https://openzfs.github.io/openzfs-docs/man/master/7/zfsprops...

* https://openzfs.github.io/openzfs-docs/Basic%20Concepts/Data...

* https://en.wikipedia.org/wiki/Fletcher%27s_checksum

* https://people.freebsd.org/~asomers/fletcher.pdf

* https://www.intel.com/content/www/us/en/developer/articles/t...

Originally documented in the paper "An Arithmetic Checksum for Serial Transmissions" (referenced also in RFC 1146, in the context of TCP).

Yes, it adds some overhead, but it's fine IME. Granted, it helps that compression can significantly speed up performance. (I was very confused the first time I saw ZFS reading data faster than its drives were physically capable of, because it turned out the CPU could decompress faster than the drives could read)

Programs like filesystems typically have their own schedulers that aggregate writes, it would be an extreme performance hit if they wrote each checksum individually, as a separate I/O operation. They are certainly bundled with some other data that needs to be written.

And if you are concerned about the compute rather than storage, then writing to a block device is still slow enough so that computing a checksum isn't important performance-wise.

I don't think it will compete with ZFS or BTRFS (e.g. I don't think ppl will use GEFS over ZFS or BTRFS for a storage server), but it's a modern, much needed FFS replacement.

Who said anything about storage servers? I'm using zfs on laptops and desktops right now because I want data checksums and a filesystem that doesn't have a history of breaking horribly (I dropped btrfs after the second time it hosed my rootfs). Given the license issue with zfs - and in particular, the technical fallout like needing dkms - I'd be very pleased to replace it.

> I dropped btrfs after the second time it hosed my rootfs

btrfs fans use the "you're using it wrong" excuse a lot.

I recall a failure mode that activated when you fill the FS to 100% and their response was "you should never fill a filesystem to capacity"

They also fucked up their parity RAID repair so that the next disk failure on a different disk will destroy the data, argued that the pathological B-tree performance reported by a user couldn't be real, because they didn't understood the tree depth would only be logarithmic to the base of the fanout if the entries are the same size. insert shocked pikachu face

That failure mode is still there although it's been mitigated a bit. I hit it a few months ago but was able to recover after a few hours of doing various stuff.

Yeah that's not how filesystems are supposed to work.

You should be able to dd /dev/zero to a file and not worry about the entire filesystem shitting the bed.

It's really a shame, though. btrfs is permanently unfinished, and zfs has the wrong license. End result is people stick with ext4 or xfs most of the time anyway.

Otoh, good luck bringing a CoW filesystem back from 100%. Delete a file? Sure, let me just make a copy of all the metadata that was pointing at it using... the zero blocks I have left.

Tradeoffs are a bitch, bitch.

Which is why ZFS reserves "slop space" to make sure that doesn't happen, instead of defaulting to making it easy for users to corner themselves like that.

I have been hearing noise recently that btrfs is risky and unstable but (knocks on wood) i've been running it for years now with zero issues. What am I missing?

Failures in storage software are very rare. Which makes it very hard to test... (you need to run it a lot, for a very long time if you hope to find errors by chance).

Also, some failure modes are worse than others. The failures known as DI (data integrity) are the worst. Even though they aren't expected to happen to everyone at a certain frequency (because, again, mature storage software is comparatively very reliable), even a single DI error that happened to any user sets up a major alarm.

In the storage industry, the running joke is that after first DI in your product you lose funding, after the second DI you loose the product.

And it did happen to Btrfs quite a bit... I've seen it with my own eyes when a system didn't come back after power failure. (But I'm in the business of testing software storage products, so, it's less surprising that it happened to me).

So... it's perfectly plausible that you have never seen Btrfs fail, and it's been more error prone than eg. EXT4. The error rate is low enough so that if you don't actively try to cause the error you will never experience one. But, over a large group of diverse use patterns, the rate is still worse than expected.

> Failures in storage software are very rare.

Famous last words. Most failures are not reported, because people move on.

It still has an issue with getting stuck in a bad state if you let the free space drop too low (e.g. under 5% / 2GB). It might still be recoverable but it takes a lot of fiddling and work to do that.

btrfs is almost 20 years old now, so lots of people only used it back when it was newer and far buggier.

In my experience, btrfs is actually more reliable than other filesystems due to its checksumming abilities, but when it does fail, it's much harder to fix than with other filesystems (which will often try to continue on even when stuff is broken).

> Who said anything about storage servers?

I did.

> I'm using zfs [...]

ZFS is primarily used on single-storage appliances.

It’ll replace brtfs if it gets ported. Almost anything is better than that pile of garbage.

Running it for more than two years now, without any hassles. On single SSDs though. But still, waddya even talkin about?

> Running it for more than two years now, without any hassles. On single SSDs though. But still, waddya even talkin about?

Running ZFS since it was originally released on Solaris (June 2006) with zero problems (then FreeBSD, then Linux), including on RAID-5/6 (RAID-Z1/2) configurations, which btrfs still does not recommend using all these years later:

* https://btrfs.readthedocs.io/en/latest/btrfs-man5.html#raid5...

What have the btrfs developer(s) been doing for the last decade(+) that the code is still of debatable dependability?

yup, this.

When I last took a serious look at btrfs its RAID 5 and RAID 6 modes could lose a whole lot of data if a single drive crashed or if the system lost power during a write. That's not supposed to happen on a RAID system with a journal.

From what I understand single copy setups, RAID 1, and RAID 10 never had the same problem. It's left a bad taste for a lot of people though.

Very early ext4 versions also had some data loss or data corruption, but I never hear people dismiss it completely for that brief spot of its history. Maybe part of that is because it was around 17 years ago and it's been a reliable default OS for many years now.

The recommendation is still to store metadata on a raid 1 or 1/0 partition and the data on a raid 5/6. Apparently there's still some issues with raid 5/6

That's good information to have. Thanks.

let's just say I'd put my bitcoin wallet with eleventy million btc on a zfs pool of a single disk before I did that on brtfs.

There was a recent presentation on this at EuroBSDCon for those interested.

https://events.eurobsdcon.org/2026/talk/NVMSCJ/

https://exquisite.tube/w/3QQimMdswWJxrsPaJtak2u

All respect to the author on their work, but for me personally - I'm be more interested in Hammer2 (from Dragonfly BSD) to come to OpenBSD.

https://github.com/kusumi/openbsd_hammer2

I've been following (and helping test) gefs on 9front for a while now. 9front's nightly builder has been running off of it for quite a while. Ori's done a fantastic job.

Oh my gosh, while I may have heard of 9front in passing I had a little look today and it's super cool! also shithub what an amazing alternative forge, it feels like one of those days when I'm part of the 10,000!

For reference, the shithub that plan9 uses is not shithub.sh (to any of my knowledge)

no they are different, our shithub is shithub.us to be specific.

Ahh thanks for getting my shit straight!

Talk at EuroBSDCon 2026: https://exquisite.tube/w/3QQimMdswWJxrsPaJtak2u

From NYC*BUG† May 2023, "GEFS, A Good Enough File System" By Ori Bernstein:

* https://www.youtube.com/watch?v=juFndFy72gI

September 2026 EuroBSDCon presentation from Sunday:

* https://www.youtube.com/watch?v=yPoU4QEv_u8&t=49m43s

† BSD User Group

GEFS: A Good Enough File System https://orib.dev/gefs.pdf

I've always wondered why HAMMER2 from DragonFlyBSD hasn't gotten more attention from other OSes?

I think its the way their kernel architecture has diverged to this point in time.Seeing as it depends on structures/primitives in DRagonfly that dont exist in the other BSDs. Making a port a bit of a challenge.

Some people have 'hammered' on it, to varying degrees:

https://github.com/kusumi/hammer2

https://github.com/jdmanring/linux_hammer2

What a wonderful surprise! The nearest thing seem to be modern (v5) XFS + dm-integrity, I'll have to see a comparison once it's stable enough.

A thing ZFS suffers from is fragmentation (no way to defragment in-place nor preallocate so stuff like bittorrent doesn't play well with it), which it justifies with its CoW design, wonder if/how it mitigates the problem.

>modern (v5) XFS

Is still silly, for the same reasons Hans Reiser documented in an article about Reiser4's design.

e.g. its b-tree stores directories and small files in the same level of the tree, which makes directory operations bloated, while also hurting cache lines.

Fortunately, neither ZFS, Hammer2 or GEFS suffer from this.

You can, to some degree, do an online defrag with zfs rewrite -P now although it is not the primary intent. The displayed fragementation may not go down or may even increase, but all the files will go through the allocation policy. zfs fragmentation display is a little misleading, it measures free space fragmentation.

If we're looking at 'future' filesystems, is fragmentation really an issue in an SSD world? Not that there isn't a lot of spinning rust (and will be for quite a while), I don't think it's unreasonable to assume "most block storage is going to be SSD in the future" when allocating resources to priorities.

The day I can furnish my NAS in SSDs for roughly the same price as HDDs probably won't come before any current filesystem is obsoleted for some reason or another, methinks.

Probably true. My bad.

Fragmentation in ZFS wastes a lot of space. So it doesnt matter if it's SSD or not.

You're right...I didn't think that through.

>Error handling is largely commented out.

First we do the first 90%, and then we do the last 90%.

It's done on 9front. The mechanical work of translating it to OpenBSD is not yet done.

I have been running GEFS for a number of days and it hasnt crashed

248 ├gefs [ctl.1]

249 ├gefs [mutate.2]

250 ├gefs [sweep.3]

251 ├gefs [tasks.-1]

252 ├gefs [readio.4]

253 ├gefs [syncio.5]

254 ├gefs [srvio.-1]

255 ├gefs [stdio.-1]

up 13 days, 15:34:25

send it to production!!

Ori B. it's a great programmer, he fixed a small bug on the earlier GeFS on 9front versions in no time. It worked fine in my n270 based Atom netbook under 9front, so it will run perfectly well under OpenBSD in a near future.

It isn't as resource heavy as ZFS, and it will be more reliable than FFS, for sure.

Is there any chance of proving a filesystem is correct?

Is this one simple enough that it won’t have bugs??

Given the issues with well-known filesystems like ZFS and BetterFS, why shouldn’t I expect data-losing bugs in this one?

What well known data-losing bugs are there in zfs? It can be slow, and resource hungry, but afaik it's about as safe as they come (and I've been using it in prod since solaris 10)

There was a long-running data corruption issue with non-raw sends that was finally found and patched in 2025: https://github.com/openzfs/openzfs-docs/issues/494. But I agree, it's about as safe as they come. I trust it far more than any other file system, in large part due to all its built-in redundancy and the way it makes backups trivial (encrypted sends <3)...

I never dug into the failure, but I once had a ZFS get to a state where it would crash the kernel on mount, I was able to mount it with checks turned off an recover what was important, but it was a spooky experience. I live on the bleeding edge of file systems for my desktop, I was on reiser4 back when that was fresh, I daily drive bcachefs (it's been great!) Mostly I've been lucky, I don't usually keep an openbsd system around, but I love FreeBSD and I'll give OpenBSD a try with GEFS for sure!

Its native encryption has something of a poor history

I think the premise is basically yes, you should assume there will be data-losing bugs, but:

1. The filesystem should be reasonably good at detecting an error/corruption state and informing you, and

2. You should have backups of said data stored elsewhere, and backups should be tested (e.g. to verify that data can be read back)

I wonder what it would take to swap out the top layer for a FUSE layer? After all, ten thousand lines is comparatively small, but still a lot of work. On the other hand, having at least one modern file system with most of the bells and whistles that worked everywhere would benefit a lot of people.

> Error handling is largely commented out.

I guess we won't really know when the file system breaks or corrupts data

> The git repo is hidden on shithub

Heh :)

Is this classic 9front tomfoolery?

Indeed. If that URL is too crude then one may also use only9fans.com.

This is great timing considering I'm currently dealing with FFS corruption after my OpenBSD server lost power during a storm.

I've run OpenBSD continuously at home without backup power for something like eight years. They probably loose power about once or twice a quarter. I have yet to detect lost data that was important. Measures are taken. If a storm is coming through, I'll shutdown till it passes.

I've been lucky with release driver support. The little Lenovo ThinkCentre's being used seem to chug along without crashing on driver issues, at least with OpenBSD releases.

The standard OpenBSD partitioning scheme is also being used. Boot time fsck has never failed when the storage was properly attached and in a good state. Backups are performed using pax. So far, so good.

Are you sure it's not just a dodgy SSD that simply failed to persist data when losing power mid-write? I've had my share of sudden power cuts upon OpenBSD during the past 20+ years, and FFS has so far never gone corrupt on me.

I do not believe so?

Drive is a 2TB Intel 670p NVMe SSD (INTEL SSDPEKNU020TZ) with 9078 power on hours and 42TBW - so pretty spry, but not at the start of the bathtub curve either.

It was mounted as fast storage for a Bitcoin node.

Perhaps the only 'unique' thing is it is using a NVMe to PCIe adapter card (Synology M2D20) due to this being my "legacy" server that's still rocking a Broadwell chip.

That does sound pretty spry. But it doesn't rule out that it utilizes slow storage which is fronted by a (too) large RAM buffer making the device appear much faster than it really is. Most SSDs are like this.

FFS corrupted multiple times on me, with intel video driver freezing on OpenBSD. My short OpenBSD sidetrack ended after reliably corrupting itself the third time... every time video driver panicking, leaving a corrupted filesystem after reboot, when I had eg. ports install going on during panic.

Windows ran fine on the machine (Lenovo 200) before, and Linux ran fine after. FFS (and the intel video drivers) are the weakest part of OpenBSD in my experience, I liked many other aspects.

If writes are happening while the power cuts you will lose that data. Journaling doesn't help against this. FFS will need a consistency check if it doesn't "unmount cleanly", but this isn't the same as the file system having been corrupted and losing data. If something really got corrupted and it was something else besides what you were writing in the moment the machine died, it sounds like a lot of data (for whatever reason large amounts of data was still buffered; usually an SSD is the reason) just hadn't persisted yet. With the exception of the old unadvisable softdeps feature, FFS doesn't defer a lot of writing. And It's possible to tell FFS to be fully synchronous, with the caveat that it becomes even slower than the sluggish performer it already is.

I've seen file corruption with FFS on some of my OpenBSD servers, though them being VMs is likely a factor there.

Qcow2 will zero blocks on power loss, at least in some configurations.

What a nice surprise! It would be great if it finally gets integrated into OpenBSD tree. Corruption Detection and Crash Safety will be more than welcomed here.

I can't complain about FFS since it has been fully reliable for me. I have been directly powering off my obsd router for years and never had a problem... in the worst case if I use a very old and semi broken USB drive I have to access console and manually fsck -y some partitions, that's it.

I dig the GEFS acronym and concept too.

Finally . Very good work team !

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