#LOST IN RANDOM PHYSICAL EDITION SOFTWARE#

After the snapshot is created, snapshot software monitors and traces changes in the original data (write operations to the source data block), and copies data on the source data block to the new data block before the original data in the source data block is rewritten. Therefore, COW snapshot creation is very fast and can be completed instantly. When COW creates a snapshot, there is no physical data copying action, only the physical location metadata (pointer information) of the source data block where the original data resides is copied. The following figure shows the initial state of a snapshot.įigure 1-2 I/O process of writing data to a source volume Because no data is written to snapshot or source volumes, no data is recorded to the COW meta and data areas or to the snapshot meta and data volumes. The storage system allocates COW data space from the source LUN and generates a snapshot volume. In addition, if an application is prone to write hotspots, that is, only write data to a limited range of data, then the COW snapshot technology is recommended because multiple write operations to the same data will only lead to one COW operation.Īfter a snapshot is created and activated, a data copy that is identical to the source volume is generated. Therefore, COW can be used when the number of read operations on a LUN is far more than that of write operations. It takes a lot of I/Os for frequent writing. When data is first written to a location, COW completes a read operation (reading the original data from the location) and two write operations (writing the original data to the snapshot space and writing new data to the location). No COW operation is required for next write operations on this location. In a COW technology, when new data is written to a specific storage location for a first time, original content is first read and written to the COW data space, and then the new data is written to the storage device.

A private mapping table, which is stored in the snapshot meta volume of each snapshot LUN, records the mapping relationships between data written into the snapshot LUN and data locations in the snapshot LUN.

A shared mapping table, which is stored in the COW meta area, records the mapping relationships between COW data and COW data locations in the COW data area.

Mapping tables are classified into shared mapping tables and private mapping tables, where: It is used to record COW data if new data is written into the source LUN.Ī mapping table records the data changes on the source and snapshot LUNs at a point in time as well as the new locations of such data.

The COW data area corresponds to the COW meta area.

It is used to store the COW mapping information of the snapshot LUNs if new data is written into their source LUN, that is, the mapping relationships between COW data and data locations in the COW data area.

The COW meta area is shared by all snapshots in the source LUN.

A COW data space consists of a COW meta area and a COW data area, where: All the snapshot LUNs of the same source LUN share one COW data space.

The snapshot data volume corresponds to the snapshot meta volume and records new data written into the snapshot LUN.Īn amount of storage space dynamically allocated by the storage system from the storage pool where the source LUN resides to store COW data after a snapshot is generated and activated.

One snapshot has one snapshot meta volume.

The snapshot meta volume contains the snapshot metadata.

A snapshot LUN consists of a snapshot meta volume and a snapshot data volume, where:

The data volume records the service data saved in the source LUN.Ī logical data copy generated after a snapshot is created for a source LUN.

The meta volume records the location of the source data in the source LUN.

A source LUN consists of a meta volume and a data volume, where:

Data is read and written in units of extent.Ī volume that stores the source data for a snapshot and is represented as a source LUN to users. A data volume stores the actual user data.

Each meta volume is arranged in a tree structure.

A meta volume records the data organization form and attributes of a LUN.

Each LUN created in a storage pool is composed of both a meta volume and a data volume, where: Storage systems use virtual storage technology.

Lost in random physical edition