In previous HBase releases, the only way to a back up or to clone a table was to use CopyTable or ExportTable, or to copy all the hfiles in HDFS after disabling the table. These methods have disadvantages:

• CopyTable and ExportTable can degrade RegionServer performance.
• Disabling the table means no reads or writes; this is usually unacceptable.

HBase snapshots allow you to clone a table without making data copies, and with minimal impact on RegionServers. Exporting the table to another cluster does not have any impact on the RegionServers.

Snapshots are on by default; to disable them, set the hbase.snapshot.enabled property in hbase-site.xml to false:

<property>
   <name>hbase.snapshot.enabled</name>
   <value>
      false
   </value>
</property>

To enable snapshots after you have disabled them, set hbase.snapshot.enabled to true.

  Note:

If you have taken snapshots and then decide to disable snapshots, you must delete the snapshots before restarting the HBase master; the HBase master will not start if snapshots are disabled and snapshots exist.

Snapshots do not affect HBase performance if they are not used.

You can manage snapshots by using the HBase shell or the HBaseAdmin Java API.

The following table shows actions you can take from the shell.

Action	Shell command	Comments
Take a snapshot of tableX called snapshotX	snapshot 'tableX', 'snapshotX'	Snapshots can be taken while a table is disabled, or while a table is online and serving traffic. If a table is disabled (using disable <table>), an offline snapshot is taken. This snapshot is managed by the master and fully consistent with the state when the table was disabled. This is the simplest and safest method, but it involves a service interruption because the table must be disabled to take the snapshot. In an online snapshot, the table remains available while the snapshot is taken, and incurs minimal performance degradation of normal read/write loads. This snapshot is managed by the master and run on the RegionServers. The current implementation—simple-flush snapshots—provides no causal consistency guarantees. Despite this shortcoming, it offers the same degree of consistency as CopyTable and is a significant improvement.
Restore snapshot snapshotX (replaces the source table content)	restore_snapshot ‘snapshotX’	For emergency use only; see Restrictions. Restoring a snapshot replaces the current version of a table with different version. To run this command, you must disable the target table. The restore command takes a snapshot of the table (appending a timestamp code), and then clones data into the original data and removes data not in the snapshot. If the operation succeeds, the target table is enabled.   Warning: If you use coprocessors, the coprocessor must be available on the destination cluster before restoring the snapshot.
List all available snapshots	list_snapshots
List all available snapshots starting with ‘mysnapshot_’ (regular expression)	list_snapshots ‘my_snapshot_.*’
Remove a snapshot called snapshotX	delete_snapshot ‘snapshotX’
Create a new table tableY from a snapshot snapshotX	clone_snapshot ‘snapshotX’, ‘tableY’	Cloning a snapshot creates a new read/write table that serves the data kept at the time of the snapshot. The original table and the cloned table can be modified independently; new data written to one table does not show up on the other.

#!/bin/bash
# Take a snapshot of the table passed as an argument
# Usage: snapshot_script.sh table_name
# Names the snapshot in the format snapshot-YYYYMMDD

# Parse the arguments
if [ -z $1 ]||[$1 == '-h' ]; then
        echo "Usage: $0 <table>"
        echo "       $0 -h"
        exit 1
fi

# Modify to suit your environment
export HBASE_PATH=/home/user/hbase
export DATE=`date +"%Y%m%d"`
echo "snapshot '$1', 'snapshot-$DATE'" | $HBASE_PATH/bin/hbase shell -n 
status=$?
if [$status -ne 0]; then
        echo "Snapshot may have failed: $status"
fi
exit $status  

HBase Shell returns an exit code of 0 on successA non-zero exit code indicates the possibility of failure, not a definite failure. Your script should check to see if the snapshot was created before taking the snapshot again, in the event of a reported failure.

You can export any snapshot from one cluster to another. Exporting the snapshot copies the table's hfiles, logs, and the snapshot metadata, from the source cluster to the destination cluster. Specify the -copy-from option to copy from a remote cluster to the local cluster or another remote cluster. If you do not specify the -copy-from option, the hbase.rootdir in the HBase configuration is used, which means that you are exporting from the current cluster. You must specify the -copy-to option, to specify the destination cluster.

  Note: Snapshots must be enabled on the destination cluster. See Configuring and Enabling Snapshots.

  Warning: If you use coprocessors, the coprocessor must be available on the destination cluster before restoring the snapshot.

The ExportSnapshot tool executes a MapReduce Job similar to distcp to copy files to the other cluster. It works at file-system level, so the HBase cluster can be offline.

Run ExportSnapshot as the hbase user or the user that owns the files. If the user, group, or permissions need to be different on the destination cluster than the source cluster, use the -chuser, -chgroup, or -chmod options as in the second example below, or be sure the destination directory has the correct permissions. In the following examples, replace the HDFS server path and port with the appropriate ones for your cluster.

To copy a snapshot called MySnapshot to an HBase cluster srv2 (hdfs://srv2:8020/hbase) using 16 mappers:

hbase org.apache.hadoop.hbase.snapshot.ExportSnapshot -snapshot MySnapshot -copy-to hdfs://srv2:<hdfs_port>/hbase -mappers 16

To export the snapshot and change the ownership of the files during the copy:

hbase org.apache.hadoop.hbase.snapshot.ExportSnapshot -snapshot MySnapshot -copy-to hdfs://srv2:<hdfs_port>/hbase -chuser MyUser -chgroup MyGroup -chmod 700 -mappers 16

hbase org.apache.hadoop.hbase.snapshot.ExportSnapshot -Dsnapshot.export.default.map.group=10 -snapshot MY_SNAPSHOT -copy-to hdfs://cluster2/hbase

To export from one remote cluster to another remote cluster, specify both -copy-from and -copy-to parameters.

hbase org.apache.hadoop.hbase.snapshot.ExportSnapshot -snapshot snapshot-test -copy-from hdfs://machine1/hbase -copy-to hdfs://machine2/my-backup

hbase org.apache.hadoop.hbase.snapshot.ExportSnapshot -snapshot snapshot-test -copy-from hdfs://machine1/hbase -copy-to hdfs://machine2/my-backup -target new-snapshot

  Warning:

Do not use merge in combination with snapshots. Merging two regions can cause data loss if snapshots or cloned tables exist for this table.

The merge is likely to corrupt the snapshot and any tables cloned from the snapshot. If the table has been restored from a snapshot, the merge may also corrupt the table. The snapshot may survive intact if the regions being merged are not in the snapshot, and clones may survive if they do not share files with the original table or snapshot. You can use the Snapinfo tool (see Information and Debugging) to check the status of the snapshot. If the status is BROKEN, the snapshot is unusable.

• If you have enabled the AccessController Coprocessor for HBase, only a global administrator can take, clone, or restore a snapshot, and these actions do not capture the ACL rights. This means that restoring a table preserves the ACL rights of the existing table, and cloning a table creates a new table that has no ACL rights until the administrator adds them.
• Do not take, clone, or restore a snapshot during a rolling restart. Snapshots require RegionServers to be up; otherwise, the snapshot fails.
    Note: This restriction also applies to a rolling upgrade, which can be done only through Cloudera Manager.

If you are using HBase Replication and you need to restore a snapshot:

  Important:

Snapshot restore is an emergency tool; you need to disable the table and table replication to get to an earlier state, and you may lose data in the process.

If you are using HBase Replication, the replicas will be out of sync when you restore a snapshot. If you need to restore a snapshot, proceed as follows:

1. Disable the table that is the restore target, and stop the replication.
2. Remove the table from both the master and worker clusters.
3. Restore the snapshot on the master cluster.
4. Create the table on the worker cluster and use CopyTable to initialize it.

  Note:

If this is not an emergency (for example, if you know exactly which rows you have lost), you can create a clone from the snapshot and create a MapReduce job to copy the data that you have lost.

In this case, you do not need to stop replication or disable your main table.

Region moves, splits, and other metadata actions that happen while a snapshot is in progress can cause the snapshot to fail. The software detects and rejects corrupted snapshot attempts.

You can use the SnapshotInfo tool to get information about a snapshot, including status, files, disk usage, and debugging information.

Examples:

Use the -h option to print usage instructions for the SnapshotInfo utility.

$ hbase org.apache.hadoop.hbase.snapshot.SnapshotInfo -h
Usage: bin/hbase org.apache.hadoop.hbase.snapshot.SnapshotInfo [options]
 where [options] are:
  -h|-help                Show this help and exit.
  -remote-dir             Root directory that contains the snapshots.
  -list-snapshots         List all the available snapshots and exit.
  -snapshot NAME          Snapshot to examine.
  -files                  Files and logs list.
  -stats                  Files and logs stats.
  -schema                 Describe the snapshotted table.

Use the -list-snapshots option to list all snapshots and exit.

$ hbase org.apache.hadoop.hbase.snapshot.SnapshotInfo -list-snapshots
SNAPSHOT             | CREATION TIME        | TABLE NAME
snapshot-test        |  2014-06-24T19:02:54 | test

Use the -remote-dir option with the -list-snapshots option to list snapshots located on a remote system.

$ hbase org.apache.hadoop.hbase.snapshot.SnapshotInfo -remote-dir s3a://mybucket/mysnapshot-dir -list-snapshots
SNAPSHOT |   CREATION TIME  | TABLE NAME
snapshot-test        2014-05-01 10:30    myTable

Use the -snapshot option to print information about a specific snapshot.

$ hbase org.apache.hadoop.hbase.snapshot.SnapshotInfo -snapshot test-snapshot
Snapshot Info
----------------------------------------
   Name: test-snapshot
   Type: DISABLED
  Table: test-table
Version: 0
Created: 2012-12-30T11:21:21
**************************************************************

$ hbase org.apache.hadoop.hbase.snapshot.SnapshotInfo -stats -snapshot snapshot-test
Snapshot Info
----------------------------------------
   Name: snapshot-test
   Type: FLUSH
  Table: test
 Format: 0
Created: 2014-06-24T19:02:54


1 HFiles (0 in archive), total size 1.0k (100.00% 1.0k shared with the source table)

$ hbase org.apache.hadoop.hbase.snapshot.SnapshotInfo  -schema -snapshot snapshot-test
Snapshot Info
----------------------------------------
   Name: snapshot-test
   Type: FLUSH
  Table: test
 Format: 0
Created: 2014-06-24T19:02:54

Table Descriptor
----------------------------------------
'test', {NAME => 'cf', DATA_BLOCK_ENCODING => 'FAST_DIFF', BLOOMFILTER => 'ROW', REPLICATION_SCOPE => '0', 
COMPRESSION => 'GZ', VERSIONS => '1', TTL => 'FOREVER', MIN_VERSIONS => '0', KEEP_DELETED_CELLS => 'false', 
BLOCKSIZE => '65536', IN_MEMORY => 'false', BLOCKCACHE => 'true'}

Use the -files option with the -snapshot option to list information about files contained in a snapshot.

$ hbase org.apache.hadoop.hbase.snapshot.SnapshotInfo -snapshot test-snapshot -files
Snapshot Info
----------------------------------------
   Name: test-snapshot
   Type: DISABLED
  Table: test-table
Version: 0
Created: 2012-12-30T11:21:21

Snapshot Files
----------------------------------------
   52.4k test-table/02ba3a0f8964669520cf96bb4e314c60/cf/bdf29c39da2a4f2b81889eb4f7b18107 (archive)
   52.4k test-table/02ba3a0f8964669520cf96bb4e314c60/cf/1e06029d0a2a4a709051b417aec88291 (archive)
   86.8k test-table/02ba3a0f8964669520cf96bb4e314c60/cf/506f601e14dc4c74a058be5843b99577 (archive)
   52.4k test-table/02ba3a0f8964669520cf96bb4e314c60/cf/5c7f6916ab724eacbcea218a713941c4 (archive)
  293.4k test-table/02ba3a0f8964669520cf96bb4e314c60/cf/aec5e33a6564441d9bd423e31fc93abb (archive)
   52.4k test-table/02ba3a0f8964669520cf96bb4e314c60/cf/97782b2fbf0743edaacd8fef06ba51e4 (archive)

6 HFiles (6 in archive), total size 589.7k (0.00% 0.0 shared with the source table)
0 Logs, total size 0.0