Snapshot Scheduling Module

This module implements scheduled snapshots for CephFS. It provides a user interface to add, query and remove snapshots schedules and retention policies, as well as a scheduler that takes the snapshots and prunes existing snapshots accordingly.

How to enable

The snap_schedule module is enabled with:

ceph mgr module enable snap_schedule

Usage

This module uses CephFS Snapshots, please consider this documentation as well.

This module’s subcommands live under the ceph fs snap-schedule namespace. Arguments can either be supplied as positional arguments or as keyword arguments. Once a keyword argument was encountered, all following arguments are assumed to be keyword arguments too.

Snapshot schedules are identified by path, their repeat interval and their start time. The repeat interval defines the time between two subsequent snapshots. It is specified by a number and a period multiplier, one of h(our), d(ay) and w(eek). E.g. a repeat interval of 12h specifies one snapshot every 12 hours. The start time is specified as a time string (more details about passing times below). By default the start time is last midnight. So when a snapshot schedule with repeat interval 1h is added at 13:50 with the default start time, the first snapshot will be taken at 14:00.

Retention specifications are identified by path and the retention spec itself. A retention spec consists of either a number and a time period separated by a space or concatenated pairs of <number><time period>. The semantics are that a spec will ensure <number> snapshots are kept that are at least <time period> apart. For Example 7d means the user wants to keep 7 snapshots that are at least one day (but potentially longer) apart from each other. The following time periods are recognized: h(our), d(ay), w(eek), m(onth), y(ear) and n. The latter is a special modifier where e.g. 10n means keep the last 10 snapshots regardless of timing,

All subcommands take optional fs and subvol arguments to specify paths in multi-fs setups and FS volumes and subvolumes managed setups. If not passed fs defaults to the first file system listed in the fs_map, subvolume defaults to nothing. When using FS volumes and subvolumes the argument fs is equivalent to a volume.

When a timestamp is passed (the start argument in the add, remove, activate and deactivate subcommands) the ISO format %Y-%m-%dT%H:%M:%S will always be accepted. When either python3.7 or newer is used or https://github.com/movermeyer/backports.datetime_fromisoformat is installed, any valid ISO timestamp that is parsed by python’s datetime.fromisoformat is valid.

When no subcommand is supplied a synopsis is printed:

#> ceph fs snap-schedule
no valid command found; 8 closest matches:
fs snap-schedule status [<path>] [<subvol>] [<fs>] [<format>]
fs snap-schedule list <path> [<subvol>] [--recursive] [<fs>] [<format>]
fs snap-schedule add <path> <snap_schedule> [<start>] [<fs>] [<subvol>]
fs snap-schedule remove <path> [<repeat>] [<start>] [<subvol>] [<fs>]
fs snap-schedule retention add <path> <retention_spec_or_period> [<retention_count>] [<fs>] [<subvol>]
fs snap-schedule retention remove <path> <retention_spec_or_period> [<retention_count>] [<fs>] [<subvol>]
fs snap-schedule activate <path> [<repeat>] [<start>] [<subvol>] [<fs>]
fs snap-schedule deactivate <path> [<repeat>] [<start>] [<subvol>] [<fs>]
Error EINVAL: invalid command

Inspect snapshot schedules

The module offers two subcommands to inspect existing schedules: list and status. Bother offer plain and json output via the optional format argument. The default is plain. The list sub-command will list all schedules on a path in a short single line format. It offers a recursive argument to list all schedules in the specified directory and all contained directories. The status subcommand prints all available schedules and retention specs for a path.

Examples:

ceph fs snap-schedule status /
ceph fs snap-schedule status /foo/bar --format=json
ceph fs snap-schedule list /
ceph fs snap-schedule list / --recursive=true # list all schedules in the tree

Add and remove schedules

The add and remove subcommands add and remove snapshots schedules respectively. Both require at least a path argument, add additionally requires a schedule argument as described in the USAGE section.

Multiple different schedules can be added to a path. Two schedules are considered different from each other if they differ in their repeat interval and their start time.

If multiple schedules have been set on a path, remove can remove individual schedules on a path by specifying the exact repeat interval and start time, or the subcommand can remove all schedules on a path when just a path is specified.

Examples:

ceph fs snap-schedule add / 1h
ceph fs snap-schedule add / 1h 11:55
ceph fs snap-schedule add / 2h 11:55
ceph fs snap-schedule remove / 1h 11:55 # removes one single schedule
ceph fs snap-schedule remove / 1h # removes all schedules with --repeat=1h
ceph fs snap-schedule remove / # removes all schedules on path /

Add and remove retention policies

The retention add and retention remove subcommands allow to manage retention policies. One path has exactly one retention policy. A policy can however contain multiple count-time period pairs in order to specify complex retention policies. Retention policies can be added and removed individually or in bulk via the forms ceph fs snap-schedule retention add <path> <time period> <count> and ceph fs snap-schedule retention add <path> <countTime period>[countTime period]

Examples:

ceph fs snap-schedule retention add / h 24 # keep 24 snapshots at least an hour apart
ceph fs snap-schedule retention add / d 7 # and 7 snapshots at least a day apart
ceph fs snap-schedule retention remove / h 24 # remove retention for 24 hourlies
ceph fs snap-schedule retention add / 24h4w # add 24 hourly and 4 weekly to retention
ceph fs snap-schedule retention remove / 7d4w # remove 7 daily and 4 weekly, leaves 24 hourly

Active and inactive schedules

Snapshot schedules can be added for a path that doesn’t exist yet in the directory tree. Similarly a path can be removed without affecting any snapshot schedules on that path. If a directory is not present when a snapshot is scheduled to be taken, the schedule will be set to inactive and will be excluded from scheduling until it is activated again. A schedule can manually be set to inactive to pause the creating of scheduled snapshots. The module provides the activate and deactivate subcommands for this purpose.

Examples:

ceph fs snap-schedule activate / # activate all schedules on the root directory
ceph fs snap-schedule deactivate / 1d # deactivates daily snapshots on the root directory

Limitations

Snapshots are scheduled using python Timers. Under normal circumstances specifying 1h as the schedule will result in snapshots 1 hour apart fairly precisely. If the mgr daemon is under heavy load however, the Timer threads might not get scheduled right away, resulting in a slightly delayed snapshot. If this happens, the next snapshot will be schedule as if the previous one was not delayed, i.e. one or more delayed snapshots will not cause drift in the overall schedule.

In order to somewhat limit the overall number of snapshots in a file system, the module will only keep a maximum of 50 snapshots per directory. If the retention policy results in more then 50 retained snapshots, the retention list will be shortened to the newest 50 snapshots.

Data storage

The snapshot schedule data is stored in a rados object in the cephfs metadata pool. At runtime all data lives in a sqlite database that is serialized and stored as a rados object.