PowerEdge: What is a Snapshot and How To Manage

Snapshot (Otherwise known as a "Point in time copy" or "replay") is a "capture" of a volume and its data at a specific point of time, either created manually or scheduled through a Snapshot Profile. Customers can set an internal policy on the frequency of snapshots and how long a snapshot is kept/expired depending on the recovery point objective (RPS) and recovery time objective (RTO).

When a Snapshot is created from a volume the exact contents of the volume at block level are captured at the moment of snapshot creation when created from Dell Storage Manager (DSM) or the integrated Storage Center Web UI (SCOS 6.x and earlier). Creating a snapshot does not interrupt IO or preventing access to the volume.

When implementing snapshots, there are two techniques:

1.The original storage is never modified. When a write request is made, it is redirected away from the original data into a new storage area. (called "Redirect-on-write" or ROW)

2.When a write request is made, the data are copied into a new storage area, and then the original data are modified. (called "Copy-on-write" or COW)

Despite their names, copy-on-write usually sees the first technique. COW does two data writes compared to ROW's one; it is difficult to implement efficiently and thus used infrequently.

The copy-on-write technique can be used to emulate a read/write storage on media that require wear leveling or are physically write once read many. Dell Compellent array use ROW technology

Snapshots created using DSM are considered crash consistent. If all the interrelated data components are as they were (write-order consistent) at the instant of the crash. To better understand this type of consistency, imagine the status of the data on your personal computer’s hard drive after a power outage or similar event. A crash-consistent backup is sufficient for non-database operating systems and applications like file servers, DHCP servers, print servers, and so on.

For application consistent snapshots - A backup or snapshot is application consistent if, in addition to being write-order consistent, running applications complete all their operations and flush their buffers to disk (application quiescing). Application-consistent backups are recommended for database operating systems and applications such as SQL, Oracle, and Exchange and Dell Compellent provides a replay manager to orchestrate allowing host servers to create snapshots on the Dell Compellent array.

How Snapshot work？

Data Instant Replay Benefits

Benefits include:

• Unlimited, space-efficient, time-based replays at any time interval

   

• Instantaneous recovery from data hazards by restoring a replay

   

• Minimize system downtime

   

• Reduce dependence on tape-based backups

   

• Intuitive profiles and scheduling mechanism

   

• Replays can also be used to test new applications on data without risk of data loss or corruption

  
  Data Instant Replay Data Pages

   Data in various pages have different accessibility characteristics. Their interaction describes the process of the DIR.

   

• Active Pages

  Data that is Read/Writable. This is the newest or most current data.

   

• Accessible Data Populated in R-O Pages

  This data is Read-Only, but is accessible. It is from an earlier time mark, but has not been changed.

   

• Inaccessible Data Populated R-O Pages

  Data that has had changes to them. This data is Read-Only and can only be accessed through recovering a view volume. 

  A snapshot (or replay) is a point-in-time copy (PITC) of a volume to provide fast recovery from hazards such as viruses, power outages, hardware failure, or human error. Restoring a snapshot can recover lost data or revert the volume to a previous point in time.

  Dell's snapshots differ from the traditional PITC concept in that the blocks or pages are stopped responding and not copied. No user data is moved, making the process efficient in both the time taken to complete the snapshot and the space used by snapshots.

  
  Managing Data Instant Replay
   

  When using the Dell Storage Center's Data Instant Replay feature, the user must consider several factors when scheduling snapshots.

   

• How often should snapshots be scheduled? 

   

• Determine Recovery Point Objective (RPO) for each volume.

   

• Schedule snapshots based on the backup policy established to maximize storage space utilization.

   

• Expire snapshots when they are no longer useful:

   

• This frees up storage.

   

• When views are no longer needed, unmap and delete them.

   

• Snapshots can be scheduled to occur:

   

• Automatically by applying snapshot (replay) profiles

   

• On demand, created manually

   

  View Snapshot Profile

  By default, Storage Center provides two standard Snapshot Profiles that cannot be deleted.

  Daily: Creates a snapshot every day at 12:01 AM, and expires the snapshot in one week

  Sample: Applies three schedule rules:

   

• Creates a snapshot every 12 hours between 12:05 AM and 6 PM, expiring in five days

   

• Creates a snapshot on the first day of every month at 11:30 PM, expiring in 26 weeks

   

• Creates a snapshot every Saturday at 11:30 PM, expiring in 5 weeks

    

  From the figure, click Snapshot Profile. This example shows the two default Snapshot Profiles: Daily and Sample

   

  View Snapshot Profile Volume Usage
   

  Create a Snapshot Profile

    

  Name the Snapshot Profile

    

  Add Rule

    

  Snapshot Creation Method

    

  From the Snapshot Creation Method drop-down menu, select an option to control how snapshots triggered by the Snapshot Profile are created.

  1.Standard - When selected, takes snapshots in series for all volumes associated with the snapshot.

  2.Parallel - When selected, takes snapshots simultaneously for all volumes associated with the snapshot.

  3.Consistent - When selected, stops IO and takes snapshots for all volumes associated with the snapshot. Provides options for timing out snapshot creation and expiring incomplete snapshots

  Apply a Snapshot Profile to Volumes

       
   

  View Snapshots Taken

    

  Create a Manual Snapshot

    

   

  Recover from Replay

    

    

   

  Map Volume to Server

    

  Mapping View

    

  Rescan Disk using Computer Management

    

  Disk Online

    

   

  Remove View Volume

    

  Remove Mapping

    

    

  Delete View Volume