A snapshot represents a frozen image of a volume. The source of a snapshot is called an "original." When a snapshot is created, it looks exactly like the original at that point in time. As changes are made to the original, the snapshot remains the same and looks exactly like the original at the time the snapshot was created.

 

Snapshot technology is becoming prevalent to perform data protection and other tasks such as data mining and data cloning. Most leading storage hardware and software products such as AyRecovery Professional and AyRecovery Enterprise, provide snapshot support. The usage of snapshot technology for data protection offers critical business value, such as non-impact backup or no application downtime, frequent backups (for example, hourly) to reduce recovery time, efficient backup of large volumes of data, reduced exposure to data loss and instant recovery from snapshot. However, you must take careful consideration before selecting a solution that fits your needs and the environment.

 

The implementation of snapshots

There are different implementation approaches adopted by vendors to create snapshots, each with its own benefits and drawbacks. Therefore, it is important to understand snapshot implementations in order to be able to make effective data protection solutions and identify which functions are most critical for your organization to select the snapshot vendor accordingly.

The following are two commonly used methodologies for creating snapshots:

 

Copy-on-write:

A snapshot of a storage volume is created using the pre-designated space for the snapshot. When the snapshot is firstly created, only the meta-data where the original data is stored is copied. No physical copy of the data is done at the time the snapshot is created. Therefore, the creation of the snapshot is almost instantaneous. The snapshot copy then tracks the changing blocks on the original volume as writing to the original volume are performed. The original data that is being written to is copied into the designated storage pool that is set aside for the snapshot before the original data is overwritten, hence the name "copy-on-write".

Before a write is allowed to a block, copy-on-write moves the original data block to the snapshot storage. This keeps the snapshot data consistent with the exact time the snapshot was taken. Reading requests to the snapshot volume of the unchanged data blocks is redirected to the original volume, while reading requests to data blocks that have been changed is directed to the "copied" blocks in the snapshot. Snapshot contains the meta-data that describes the data blocks that have been changed since the snapshot was firstly created. Pay attention that the original data blocks are copied into the snapshot storage only once when the first writing request is received.

 

IBM FlashCopy® (NOCOPY), AIX® JFS2 snapshot, IBM TotalStorage® SAN File System snapshot, IBM General Parallel FIle System snapshot, Linux® Logical Volume Manager, and IBM Tivoli Storage Manager Logical Volume Snapshot Agent (LVSA) are all based on copy-on-write.

 

Redirect-on-write :

This method is quite similar to copy-on-write, without the double-writing penalty, and it offers storage space and performance efficient snapshots.

New writings to the original volume are redirected to another location set aside for snapshot. The advantage of redirecting the writing is that only one piece of writing takes place, whereas with copy-on-write, two writes occur (one to copy original data onto the storage space, the other to copy changed data).

However, with redirect-on-write, the original copy contains the point-in-time data, that is, snapshot, and the changed data reside on the snapshot storage. When a snapshot is deleted, the data from the snapshot storage must be reconciled back into the original volume. Furthermore, as multiple snapshots are created, accessing to the original data, tracking of the data in snapshots and original volume and reconciliation upon snapshot deletion are even more complicated. The snapshot relies on the original copy of the data and the original data set can quickly become fragmented.

 

IBM N series and the NetApp Filer snapshot implementation is based on redirect-on-write.

 

 

 

 

 

 

 

 

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