jimsalterjrs/sanoid

Maximum space used by snaps, feature request

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#337 opened on Jan 29, 2019

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enhancementhelp wanted

Description

It would be useful have an (optional) limit on the space used by snapshots. This is of benefit because quotas in ZFS include the snapshot data space (potentially leading to snapshots 'stealing' available space from a filesystem).

This would permit reserving space for the filesystem of a dataset.

For instance, if one wants 50GB available and 20GB for snapshots, then the following could be set:

zfs set quota=70G pool/dataset

potential change to sanoid config:

[pool/dataset]
	use_template = hardlimit
	recursive = yes
	process_children_only = yes

[template_hardlimit]
        frequently = 0
        hourly = 36
        daily = 30
        monthly = 3
        yearly = 0
        autosnap = yes
        autoprune = yes
        spacelimit = 20G
        spaceprune = oldest

then there would always be 50GB available to the filesystem spaceprune could have optional algorithms for smart pruning, e.g. oldest|largest|prompt, which could be:

  • oldest: deletes oldest snaps first to get space back under quota (if the next snap fails, it deletes more)
  • largest: deletes largest snap first
  • prompt: prompts user (by email) when snaps exceed the spacelimit

I haven't thought through the algorithm too much but the space occupied by snaps is pretty easy to access:

zfs list -ro space pool/dataset
NAME  AVAIL   USED  USEDSNAP  USEDDS  USEDREFRESERV  USEDCHILD
 pool/dataset  70.0G  46.3M     23.5M   22.8M             0B         0B

Contributor guide

Tech stack
None
Domain
infrastructuredata
Issue type
feature
DifficultyEstimated implementation difficulty for a new contributor, from 1 for very small changes to 5 for expert-level work.
3
Estimated timeA rough time range for an experienced contributor to investigate, implement, test, and prepare a pull request.
1-2 days
Activity statusHow available the issue appears right now: fresh, active, stale, blocked, or waiting on maintainer input.
fresh
ClarityHow clearly the issue explains the expected change, acceptance criteria, and next step.
mostly clear
Prerequisites
ZFS basicssanoid configuration knowledge
Newbie friendlinessA 1-100 score estimating how approachable this issue is for first-time contributors.
30
Research direction
The issue proposes adding a spacelimit option to sanoid's configuration templates, similar to the example shown. The implementation would need to integrate with ZFS's quota system and add a pre snapshot check to ensure total snapshot usage stays under the limit. The user should examine sanoid's existing prune logic (likely in the Perl scripts) and consider how to call 'zfs list ro space' to get usage. The discussion in the issue mentions possible pruning algorithms like oldest or largest, which would need to be implemented as part of the feature.