Managing GlusterFS Volume Snapshots
This section describes how to perform common GlusterFS volume snapshot management operations
GlusterFS volume snapshot feature is based on thinly provisioned LVM snapshot. To make use of snapshot feature GlusterFS volume should fulfill following pre-requisites:
- Each brick should be on an independent thinly provisioned LVM.
- Brick LVM should not contain any other data other than brick.
- None of the brick should be on a thick LVM.
- gluster version should be 3.6 and above.
Details of how to create thin volume can be found at the following link. https://access.redhat.com/documentation/en-US/Red_Hat_Enterprise_Linux/7/html/Logical_Volume_Manager_Administration/LV.html#thinly_provisioned_volume_creation
Few features of snapshot are:
when a snapshot is taken at a particular point-in-time, it is made sure that the taken snapshot is crash consistent. when the taken snapshot is restored, then the data is identical as it was at the time of taking a snapshot.
When the snapshot is being taken the file system and its associated data continue to be available for the clients.
The quorum feature ensures that the volume is in good condition while the bricks are down. Quorum is not met if any bricks are down in a n-way replication where n <= 2. Quorum is met when m bricks are up, where m >= (n/2 + 1) where n is odd, and m >= n/2 and first brick is up where n is even. snapshot creation fails if quorum is not met.
During snapshot creation some of the fops are blocked to guarantee crash consistency. There is a default time-out of 2 minutes, if snapshot creation is not complete within that span then fops are unbarried. If unbarrier happens before the snapshot creation is complete then the snapshot creation operation fails. This to ensure that the snapshot is in a consistent state.
Syntax : gluster snapshot create <snapname> <volname> [no-timestamp] [description <description>] [force]
Details : Creates a snapshot of a GlusterFS volume. User can provide a snap-name and a description to identify the snap. The description cannot be more than 1024 characters.
Snapshot will be created by appending timestamp with user provided snap name. User can override this behaviour by giving no-timestamp flag.
NOTE : To be able to take a snapshot, volume should be present and it should be in started state.
Syntax : gluster snapshot clone <clonename> <snapname>
Details : Creates a clone of a snapshot. Upon successful completion, a new GlusterFS volume will be created from snapshot. The clone will be a space efficient clone, i.e, the snapshot and the clone will share the backend disk.
NOTE : To be able to take a clone from snapshot, snapshot should be present and it should be in activated state.
Syntax : gluster snapshot restore <snapname>
Details : Restores an already taken snapshot of a GlusterFS volume. Snapshot restore is an offline activity therefore if the volume is online (in started state) then the restore operation will fail.
Once the snapshot is restored it will not be available in the list of snapshots.
Syntax : gluster snapshot delete (all | <snapname> | volume <volname>)
Details : If snapname is specified then mentioned snapshot is deleted. If volname is specified then all snapshots belonging to that particular volume is deleted. If keyword all is used then all snapshots belonging to the system is deleted.
Listing of available snaps
Syntax: gluster snapshot list [volname]
Details: Lists all snapshots taken. If volname is provided, then only the snapshots belonging to that particular volume is listed.
Information of available snaps
Syntax: gluster snapshot info [(snapname | volume <volname>)]
Details: This command gives information such as snapshot name, snapshot UUID, time at which snapshot was created, and it lists down the snap-volume-name, number of snapshots already taken and number of snapshots still available for that particular volume, and the state of the snapshot.
Status of snapshots
Syntax: gluster snapshot status [(snapname | volume <volname>)]
Details: This command gives status of the snapshot. The details included are snapshot brick path, volume group(LVM details), status of the snapshot bricks, PID of the bricks, data percentage filled for that particular volume group to which the snapshots belong to, and total size of the logical volume.
If snapname is specified then status of the mentioned snapshot is displayed. If volname is specified then status of all snapshots belonging to that volume is displayed. If both snapname and volname is not specified then status of all the snapshots present in the system are displayed.
Configuring the snapshot behavior
snapshot config [volname] ([snap-max-hard-limit
Details: Displays and sets the snapshot config values.
snapshot config without any keywords displays the snapshot config values of all volumes in the system. If volname is provided, then the snapshot config values of that volume is displayed.
Snapshot config command along with keywords can be used to change the existing config values. If volname is provided then config value of that volume is changed, else it will set/change the system limit.
snap-max-soft-limit and auto-delete are global options, that will be inherited by all volumes in the system and cannot be set to individual volumes.
The system limit takes precedence over the volume specific limit.
When auto-delete feature is enabled, then upon reaching the soft-limit, with every successful snapshot creation, the oldest snapshot will be deleted.
When auto-delete feature is disabled, then upon reaching the soft-limit, the user gets a warning with every successful snapshot creation.
Upon reaching the hard-limit, further snapshot creations will not be allowed.
activate-on-create is disabled by default. If you enable activate-on-create, then further snapshot will be activated during the time of snapshot creation.
Activating a snapshot
Syntax: gluster snapshot activate <snapname>
Details: Activates the mentioned snapshot.
Note : By default the snapshot will not be activated during snapshot creation.
Deactivate a snapshot
Syntax: gluster snapshot deactivate <snapname>
Details: Deactivates the mentioned snapshot.
Accessing the snapshot
Snapshots can be accessed in 2 ways.
1) Mounting the snapshot:
The snapshot can be accessed via FUSE mount (only fuse). To do that it has to be mounted first. A snapshot can be mounted via fuse by below command
mount -t glusterfs
i.e. say "host1" is one of the peers. Let "vol" be the volume name and "my-snap" be the snapshot name. In this case a snapshot can be mounted via this command
mount -t glusterfs host1:/snaps/my-snap/vol /mnt/snapshot
2) User serviceability:
Apart from the above method of mounting the snapshot, a list of available snapshots and the contents of each snapshot can be viewed from any of the mount points accessing the glusterfs volume (either FUSE or NFS or SMB). For having user serviceable snapshots, it has to be enabled for a volume first. User serviceability can be enabled for a volume using the below command.
gluster volume set
Once enabled, from any of the directory (including root of the filesystem) an access point will be created to the snapshot world. The access point is a hidden directory cding into which will make the user enter the snapshot world. By default the hidden directory is ".snaps". Once user serviceability is enabled, one will be able to cd into .snaps from any directory. Doing "ls" on that directory shows a list of directories which are nothing but the snapshots present for that volume. Say if there are 3 snapshots ("snap1", "snap2", "snap3"), then doing ls in .snaps directory will show those 3 names as the directory entries. They represent the state of the directory from which .snaps was entered, at different points in time.
NOTE: The access to the snapshots are read-only. The snapshot needs to be activated for it to be accessible inside .snaps directory.
Also, the name of the hidden directory (or the access point to the snapshot world) can be changed using the below command.
gluster volume set
3) Accessing from windows: The glusterfs volumes can be made accessible by windows via samba. (the glusterfs plugin for samba helps achieve this, without having to re-export a fuse mounted glusterfs volume). The snapshots of a glusterfs volume can also be viewed in the windows explorer.
There are 2 ways:
Give the path of the entry point directory
NOTE: From the explorer, snapshot world can be entered via entry point only from the root of the samba share. If snapshots have to be seen from subfolders, then the path should be provided in the run command window.
For snapshots to be accessible from windows, below 2 options can be used. A) The glusterfs plugin for samba should give the option "snapdir-entry-path" while starting. The option is an indication to glusterfs, that samba is loading it and the value of the option should be the path that is being used as the share for windows. Ex: Say, there is a glusterfs volume and a directory called "export" from the root of the volume is being used as the samba share, then samba has to load glusterfs with this option as well.
ret = glfs_set_xlator_option(fs, "*-snapview-client", "snapdir-entry-path", "/export");
The xlator option "snapdir-entry-path" is not exposed via volume set options, cannot be changed from CLI. Its an option that has to be provided at the time of mounting glusterfs or when samba loads glusterfs. B) The accessibility of snapshots via root of the samba share from windows is configurable. By default it is turned off. It is a volume set option which can be changed via CLI.
gluster volume set
Only when both the above options have been provided (i.e snapdir-entry-path contains a valid unix path that is exported and show-snapshot-directory option is set to true), snapshots can accessed via windows explorer.
If only 1st option (i.e. snapdir-entry-path) is set via samba and 2nd option (i.e. show-snapshot-directory) is off, then snapshots can be accessed from windows via the run command window, but not via the explorer.