COW and volume management? [LWN.net]

COW and volume management?

Posted Jan 27, 2026 9:55 UTC (Tue) by marcH (subscriber, #57642)
In reply to: COW and volume management? by taladar
Parent article: Filesystem medley: EROFS, NTFS, and XFS

I tried it only briefly but the concepts and user interface looked better than everything else I knew at the time. Very impressive.

It has real, "big iron" users too and they seem generally pretty happy too.

I have not researched it extensively but I don't remember hearing about any serious issue with it.

> and the ZFS hype was another very off-putting thing about it.

How do you know it was hype? I don't think any company tried to promote it, or at least not recently...

> I am sure ZFS, ...

"sure" without ever trying it? What is this based on?

> ... being the first of this type of file system has many design flaws

Maybe you could find and list at least one flaw?

> ... that could be improved starting from scratch a generation later, much like any technology that tries something new and then gets stuck supporting existing behavior and design decisions as it gains users.

Again, not an expert but I try to stay informed and so far it seems like other filesystems are still playing catch up...

> wasted a lot of its potential on that whole licensing fight.

Agreed, a real pity. That could almost make you wish it's also bad technically :-)

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COW and volume management?

Posted Jan 28, 2026 8:33 UTC (Wed) by taladar (subscriber, #68407) [Link] (1 responses)

I did actually try it once for a little while in the early days when the hype was at its peak. From what i remember the ZFS design had some issues around requiring huge amounts of kernel memory which presented a big problem for alternate implementations even on license compatible systems like the BSDs and it quickly ran out of memory on systems with the 2GB of RAM that were still common at the time. Overall I was not very impressed, especially since 90% of its supposed advantages are meaningless on the small scale of just 2-4 disks or so that you might have in a typical home system.

COW and volume management?

Posted Feb 5, 2026 17:27 UTC (Thu) by georgh-cat (guest, #158897) [Link]

Is that memory issue something you experimented yourself?

I had a 750GB mirror running on OpenSolaris at home on 2008 (still using ZFS today and that pool was migrated once to 4TB disks), and I was afraid of the memory thing.

It was never an issue. Even when approaching full disk capacity, never had the memory usage be anywhere near high or even a potential problem. Then had the same server migrated to FreeBSD (OpenSolaris not being maintained anymore) and same experience.

If there ever were memory issues, I was somehow not affected or it was a myth or misunderstanding of how it worked.

COW and volume management?

Posted Feb 5, 2026 12:38 UTC (Thu) by anton (subscriber, #25547) [Link]

How do you know it was hype?

The kind of fearmongering used for selling ZFS was really off-putting. One supposedly-relevant scenario that ZFS defends against is when a HDD writes a block to the wrong disk sectors. This scenario has never happened in my experience, and the way HDDs work, it cannot happen in the way that the scenario suggests (by the read-write-head arm moving to the wrong track without any additional hardware failure).

That being said, we have used ZFS on many machines since about 2019 (mainly because it was not clear at the time whether RAID1 works well on btrfs), it works nicely. The only downside I see in it is that it is its own universe that ignores things like /etc/fstab and you need to learn the ZFS terminology even for terms that have a correspondence outside the ZFS universe. Another downside I read about is that ZFS supposedly does not utilize RAM as well as native Linux file systems which make better use of the buffer cache or somesuch. We certainly see little free RAM on our machines with ZFS. That's why I have decided to go for btrfs again for one machine recently.

COW and volume management?

Posted Feb 7, 2026 7:53 UTC (Sat) by cypherpunks2 (guest, #152408) [Link]

> Maybe you could find and list at least one flaw?

ARC is not well-integrated with the rest of the memory management system, so the system can OOM before ARC detects memory pressure and shrinks itself (so it's more than just an "ARC presents itself as used memory which confuses people" UI issue).

The cache for mapped files is unnecessarily duplicated in both ARC and the page cache.

There's still no support for defragmentation, which is an issue on high-speed IOPS-limited NVMe drives as CoW filesystems tend to experience heavy fragmentation.

COW and volume management?

Posted Apr 13, 2026 22:47 UTC (Mon) by dankamongmen (subscriber, #35141) [Link]

> Maybe you could find and list at least one flaw?

i've used ZoL on several machines for over a decade now. i've also written a large disk-management tool that has had to support all variety of filesystems. i've encountered zero problems with it from a user, despite generally running tip-of-git checkouts with most features enabled. can't say enough good about it from that perspective.

as an app writer and administrator, it has some annoying issues. it lives outside the standard filesystem management (fstab / systemd mount objects) with its own systemd services. i'm still unsure what exactly zed does, or what a ZEDLET is, or why it's written ZEDLET. you won't find the zpools in /sys/block, nor in /proc/diskstats, meaning you won't find them in say iostat, nor in blkid. compiling against libzfs is a shifting and rewardless PITA due to libspl clashing with all manner of userspace entities (random.h last i checked), and the OpenZFS developers have explicitly declared that one ought not compile against libzfs, essentially limiting sane userspace to the OpenZFS toolset.

whether or not one's bootup toolchain supports zfs this week is another question, one which i sidestep by never having zfs in my boot chain. as a result, i have an annoying mix of mdadm and zfs on these machines.

so it's great tech, but it's not without its annoyances. these annoyances seem furthermore not to be issues of bugs nor design flaws, but operational and philosophical decisions.