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How cephfs snapshots works

This document (000019627) is provided subject to the disclaimer at the end of this document.

Environment

SUSE Enterprise Storage 6

Situation

Customer needs to implement cephfs snapshots and basic understanding of snapshots. 

Resolution

How cephfs snapshots works:
 
cephfs snapshots are managed in the ".snap" subdirectory.
Snapshots can be created/deleted as often as desired. (Hourly, Daily, Weekly, etc...)  
 
Example how to create cephfs snapshots:
Note: Each snapshot is a read only view of the parent directory at the time the snapshot directory was created:
Create file “test-1” and “snapshot01”:
    cephfs-client:/mnt/cephfs/compression # ll
    total 0
    cephfs-client:/mnt/cephfs/compression # touch test01
    cephfs-client:/mnt/cephfs/compression # cd .snap
    cephfs-client:/mnt/cephfs/compression/.snap # mkdir snapshot01
    cephfs-client:/mnt/cephfs/compression/.snap # ll snapshot01/
    total 0
    -rw-r--r-- 1 root root 0 May 15 09:19 test01
 
Create file “test-2” and “snapshot02”:
    cephfs-client:/mnt/cephfs/compression # touch test02
    cephfs-client:/mnt/cephfs/compression # ll
    total 0
    -rw-r--r-- 1 root root 0 May 15 09:19 test01
    -rw-r--r-- 1 root root 0 May 15 09:20 test02
    cephfs-client:/mnt/cephfs/compression/.snap # mkdir snapshot02
    cephfs-client:/mnt/cephfs/compression/.snap # ll snapshot02/
    total 0
    -rw-r--r-- 1 root root 0 May 15 09:19 test01
    -rw-r--r-- 1 root root 0 May 15 09:20 test02
    cephfs-client:/mnt/cephfs/compression/.snap # ll snapshot01/
    total 0
    -rw-r--r-- 1 root root 0 May 15 09:19 test01
 
Create file “test-3” and “snapshot03”:
    cephfs-client:/mnt/cephfs/compression # touch test03
    cephfs-client:/mnt/cephfs/compression # ll
    total 0
    -rw-r--r-- 1 root root 0 May 15 09:19 test01
    -rw-r--r-- 1 root root 0 May 15 09:20 test02
    -rw-r--r-- 1 root root 0 May 15 09:22 test03
    cephfs-client:/mnt/cephfs/compression/.snap # mkdir snapshot03
    cephfs-client:/mnt/cephfs/compression/.snap # ll snapshot03/
    total 0
    -rw-r--r-- 1 root root 0 May 15 09:19 test01
    -rw-r--r-- 1 root root 0 May 15 09:20 test02
    -rw-r--r-- 1 root root 0 May 15 09:22 test03
    cephfs-client:/mnt/cephfs/compression/.snap # ll snapshot02/
    total 0
    -rw-r--r-- 1 root root 0 May 15 09:19 test01
    -rw-r--r-- 1 root root 0 May 15 09:20 test02
    cephfs-client:/mnt/cephfs/compression/.snap # ll snapshot01/
    total 0
    -rw-r--r-- 1 root root 0 May 15 09:19 test01
 
Example how to recover data from snapshots:
Now deleting “test01” and creating "snapshot04":
    cephfs-client:/mnt/cephfs/compression # rm test01
    cephfs-client:/mnt/cephfs/compression # ll
    total 0
    -rw-r--r-- 1 root root 0 May 15 09:20 test02
    -rw-r--r-- 1 root root 0 May 15 09:22 test03
 
    cephfs-client:/mnt/cephfs/compression/.snap # mkdir snapshot04
    cephfs-client:/mnt/cephfs/compression/.snap # ll snapshot04/
    total 0
    -rw-r--r-- 1 root root 0 May 15 09:20 test02
    -rw-r--r-- 1 root root 0 May 15 09:22 test03
 
Note: “test01 was deleted, but still exists in each of the older snapshots.
    cephfs-client:/mnt/cephfs/compression/.snap # ll snapshot03/
    total 0
    -rw-r--r-- 1 root root 0 May 15 09:19 test01
    -rw-r--r-- 1 root root 0 May 15 09:20 test02
    -rw-r--r-- 1 root root 0 May 15 09:22 test03
    cephfs-client:/mnt/cephfs/compression/.snap # ll snapshot02/
    total 0
    -rw-r--r-- 1 root root 0 May 15 09:19 test01
    -rw-r--r-- 1 root root 0 May 15 09:20 test02
    cephfs-client:/mnt/cephfs/compression/.snap # ll snapshot01/
    total 0
    -rw-r--r-- 1 root root 0 May 15 09:19 test01
 
Recover “test01” from the most resent snapshot.  I would assume that this would be the most typical use case, but the could be reasons for choosing  the file from a previous snapshot as well.
    cephfs-client:/mnt/cephfs/compression/.snap/snapshot03 # ll
    total 0
    -rw-r--r-- 1 root root 0 May 15 09:19 test01
    -rw-r--r-- 1 root root 0 May 15 09:20 test02
    -rw-r--r-- 1 root root 0 May 15 09:22 test03
    cephfs-client:/mnt/cephfs/compression/.snap/snapshot03 # cp test01 /mnt/cephfs/compression/
 
Note: “test01” was recovered from the snapshot.
    cephfs-client:/mnt/cephfs/compression # ll
    total 0
    -rw-r--r-- 1 root root 0 May 15 09:27 test01
    -rw-r--r-- 1 root root 0 May 15 09:20 test02
    -rw-r--r-- 1 root root 0 May 15 09:22 test03
 
 
Snapshotting will use extra disk space as it keep versions of the files:
Example: Add data to “test01”
    cephfs-client:/mnt/cephfs/compression # echo "this is a test" > test01
    cephfs-client:/mnt/cephfs/compression/.snap # mkdir snapshot05
    cephfs-client:/mnt/cephfs/compression/.snap # cd snapshot05
    cephfs-client:/mnt/cephfs/compression/.snap/snapshot05 # ll
    total 1
    -rw-r--r-- 1 root root 15 May 15 09:42 test01
    -rw-r--r-- 1 root root  0 May 15 09:20 test02
    -rw-r--r-- 1 root root  0 May 15 09:22 test03
    cephfs-client:/mnt/cephfs/compression/.snap/snapshot05 # cat test01
    this is a test
 
Now add more data to “test01” and create "snapshot06"
    cephfs-client:/mnt/cephfs/compression # echo "this is another test" >> test01
    cephfs-client:/mnt/cephfs/compression # cat test01
    this is a test
    this is another test
 
Note: Each snapshot has its own version of “tes01” with it unique content.
    cephfs-client:/mnt/cephfs/compression/.snap # mkdir snapshot06
    cephfs-client:/mnt/cephfs/compression/.snap # cd snapshot06
    cephfs-client:/mnt/cephfs/compression/.snap/snapshot06 # cat test01
    this is a test
    this is another test
    
    cephfs-client:/mnt/cephfs/compression/.snap # cd snapshot05
    cephfs-client:/mnt/cephfs/compression/.snap/snapshot05 # cat test01
    this is a test
 
How to delete a snapshot:
    cephfs-client:/mnt/cephfs/compression/.snap # ll
    total 0
    drwxr-xr-x 2 root root 1 May 15 09:19 snapshot01
    drwxr-xr-x 2 root root 2 May 15 09:20 snapshot02
    drwxr-xr-x 2 root root 3 May 15 09:22 snapshot03
    drwxr-xr-x 2 root root 2 May 15 09:24 snapshot04
    drwxr-xr-x 2 root root 3 May 15 09:27 snapshot05
    drwxr-xr-x 2 root root 3 May 15 09:27 snapshot06
    
    cephfs-client:/mnt/cephfs/compression/.snap # rmdir snapshot01
    cephfs-client:/mnt/cephfs/compression/.snap # ll
    total 0
    drwxr-xr-x 2 root root 2 May 15 09:20 snapshot02
    drwxr-xr-x 2 root root 3 May 15 09:22 snapshot03
    drwxr-xr-x 2 root root 2 May 15 09:24 snapshot04
    drwxr-xr-x 2 root root 3 May 15 09:27 snapshot05
    drwxr-xr-x 2 root root 3 May 15 09:27 snapshot06

Status

Top Issue

Additional Information

Also see:
Managing CephFS Snapshots in the SES6 Administration Guide:
https://documentation.suse.com/ses/6/single-html/ses-admin/#cephfs-snapshots

Disclaimer

This Support Knowledgebase provides a valuable tool for SUSE customers and parties interested in our products and solutions to acquire information, ideas and learn from one another. Materials are provided for informational, personal or non-commercial use within your organization and are presented "AS IS" WITHOUT WARRANTY OF ANY KIND.

  • Document ID:000019627
  • Creation Date: 15-May-2020
  • Modified Date:23-Oct-2020
    • SUSE Enterprise Storage

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