[Samba] Time Machine incompatibility with ZFS and 4.23 was: No luck with Time Machine and 4.23.5

[Stefan Bethke]

On 8. Feb 2026, at 11:29, Stefan Bethke via samba <samba at lists.samba.org> wrote:
> 
> On 30. Jan 2026, at 09:48, Stefan Bethke via samba <samba at lists.samba.org> wrote:
>> 
>> I have a few Macs backup up to my Samba server, and this has worked without a hitch for many years.
>> 
>> While trying to restart a backup on a machine where Time Machine was unhappy, I decided to wipe the share and start over fresh. Since then, Time Machine gets stuck after about 3.8 GB, never progressing any further. Another Mac is happily backing up to the same server.
>> 
>> The server is Debian Trixie with the backport packages (2:4.23.5+dfsg-1~bpo13+1), the Mac is running 26.2.
>> 
> Lots of informatin trimmed, see the archive for all details: https://lists.samba.org/archive/samba/2026-January/252570.html
> 
> I've just tested creating a sparseimage manually to see if having it extend is the main culprit, but it seems Time Machine is doing something special. I cannot reproduce the error with a plain sparseimage.
> 
> What I've done:
> 1. Mount the volume locally through Finder.
> 2. cd /Volumes/Time\ Machine\ Backups
> 3. hdiutil create -size 1000g -fs apfs -volname test test.sparsebundle
> 4. hdiutil attach test.sparsebundle
> 5. cd /Volumes/test
> 6. dd if=/dev/random of=x bs=1m count=10k status=progress
> 
> dd completes successfully, and a du on the volume shows that 10G are being used.
> 
> When Time Machine backs up, the size never gos above 4G. The local log is filled with:
> 
> attrVolumeWithMountPoint 'file:///Volumes/.timemachine/REDACTED/92D967CA-8376-413C-8E84-694D4706AA30/Time%20&Machine%Backup/' failed, error: Error Domain=NSPOSIXErrorDomain Code=13 "Permission denied"
> 
> This was from a completely empty volume, and Time Machine was set up fresh.
> 
> Is there some option that I need to set? From looking at the bug/patch https://bugzilla.samba.org/show_bug.cgi?id=15926, it seems to be that it should just work?

Huh, that's interesting. I figured I make sure there are no weird interactions with the underlying filesystem, so I set up a fresh VM with plain ext4, and lo and behold, it appears to be working just fine.

The problematic configuration is Samba running inside a Proxmox LXC container that I have mounted a ZFS dataset from the host into. I believe Proxmox/LXC are using bind mounts for that.

I then set up a fresh CT and tried to backup to the root disk (which is a directly mounted ZFS dataset). Here, I'm getting the same issue, so it's not the bind mount.

Next, I added ZFS to my fresh VM and tried backing up to a fresh pool. This is exhibiting the same problem, so it seems the culprit is ZFS.

To see if it is a general problem with ZFS, I next tried with FreeBSD 15. It appears the same problem occurs, the backup gets stuck at around 4GB.

Unfortunately I don't think I have the capabilities to debug this further, but I'm happy to create a Bugzilla bug for it.

On the Mac side, the log entry that seems to correlate with this situation is

attrVolumeWithMountPoint 'file:///Volumes/.timemachine/tmtest/505BFF6B-38CB-467F-896C-84FCDD3325C9/Backup%20test/' failed, error: Error Domain=NSPOSIXErrorDomain Code=13 "Permission denied"


Stefan

-- 
Stefan Bethke <stb at lassitu.de>   Fon +49 175 3288861

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