Re: netapp-alike snapshots?

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It is beneficial to not have snapshots in-place. With a local
directory of snapshots, [ ... ]

Indeed and there is a fair description of some options for
subvolume nesting policies here which may be interesting to the
original poster:

  https://btrfs.wiki.kernel.org/index.php/SysadminGuide#Layout

It is unsurprising to me that there are tradeoffs involved in
every choice. I find the "Flat" layout particularly desirable.

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Netapp snapshots are invisible for tools doing opendir()/
readdir() One could simulate this with symlinks for the
snapshot directory: store the snapshot elsewhere (not inplace)
and create a symlink to it, in every directory.

More precisely in every subvolume root directory.

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My users want the snapshots locally in a .snapshot
subdirectory.

Btrfs snapshots can only be done for a whole subvolume. Subvolumes
and snapshots can be created by users, but too many snapshots (see
below) can cause trouble. For somewhat good reasons subvolumes
including snapshots cannot be deleted by users though unless mount
option 'user_subvol_rm_allowed' is used.

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Because Netapp do it this way - for at least 20 years and we
have a multi-PB Netapp storage environment. No chance to change
this.

Send patches :-).

Not only du works recursivly, but also find and with option
also ls, grep, etc.

Note also that subvolume root directory inodes are indeed root
directory inodes so they can be 'mount'ed and therefore the
transition from a subvolume into a contained subvolume can be
detected at the mountpoint.

So 'find' has the '-xdev' option and 'du' has the '-x' options and
so similarly nearly all other tools, so perhaps someone expects
that to happen :-).

And it would require a bind mount for EVERY directory. There can
be hundreds... thousends!

Assumptions that all Btrfs features such as snapshots are
infinitely scalable at no cost may be optimistic:

  https://btrfs.wiki.kernel.org/index.php/Gotchas#Having_many_subvolumes_can_be_very_slow