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I have been using union mounts, such as unionfs on FreeBSD or mergerfs on Linux, and now I would need to do the same on illumos (SmartOS).

Reading illumos or SmartOS man pages is not helpful. Same with searching them via external search engine.

If possible, how can one create a union mount (bind two or more directories to one mountpoint) on illumos / solaris?

If not possible, is there anything close to same effect?

Why

In my humble opinion, union-like filesystems are used in relatively complex scenarios. Describing some here would be out of scope for this question. I would like to know if this is possible a-priori in any case, as I would not want an answer solving only a subset of all unionfs possible use-cases.

As an example: I need to pick and choose from the subdirectories of read-only zfs datasets, mounted DVD and readonly USB drives, overlaid with read-write datasets. The resulting mount point needs to be available as a cifs share. It would need to be done programmatically after boot, depending on various system factors.

For the interested, see referenced articles with some use cases. There are many such sysadmin scenarios.


[1] https://whattheserver.com/what-are-unionfs-mounts-and-how-to-use-them/

[2] https://medium.com/@knoldus/unionfs-a-file-system-of-a-container-2136cd11a779

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    Old SunOS4 had the Translucent File Service which do something like Linux unionFS but it was removed in SunOS 5.7 ie Solaris 7. So how to do something like it. What happens if multiple zfs volumes have the same mountpoint as a property ? Jul 11, 2020 at 16:10
  • How have you used the union mounts ? Why ? Usage case ? Hint: clones and snapshots of zfs volumes (and SunOS zones) replaces unionfs. Jul 11, 2020 at 16:16
  • Ie the volumes have the same mountpoint assigned in theirs mountpoint property. Jul 11, 2020 at 16:17
  • Clones of a zfs volume can be readonly. One user = one clone of that volume. Jul 11, 2020 at 18:52
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    TFS is probably the oldest similar implementation, as it is from January 1986. TFS was needed by NSE, the first distributed version control system. Since IBM and HP did not like to implement TFS in their OS, Sun developed TeamWare for working on CDE. Given that the TFS sources contain hints that it was a development payed by the US military, I guess this was the final reason for not porting it to Solaris even though this had been a minor effort. Anyway, attendees on the Sun User Group meeting in 1992 have been sad for that decision.
    – schily
    Jul 20, 2020 at 11:44

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