Provisioning Space in Single Track ... - IBM Research · Implementation and Usage Rivka Matosevich...

System and Storage Seminar 2005, Haifa

© 2005 IBM Corporation

Provisioning Space in Single Track Granularity: Implementation and Usage

Rivka MatosevichIBM Haifa Research Lab

[email protected] | December 2005


© 2005 IBM Corporation2 Space Efficient Point-In-Time Copy | [email protected]

Point-In-Time Copy Quick Overview

� Establish a relationship between a source and target volume� Relationship recorded in non volatile

storage

� Set up bitmap to know whether to take target track from source or target device

� Return

� Target is a point-in-time copy of source

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Bit Map



Point-In-Time Copy Quick Overview

� Target volume available for read/write

� After the Establish operation completes– Attempts to process data not yet copied to the target are

intercepted

– Data is copied on demand or read from source as needed

� Optionally background copy can be requested. When the background copy completes the target is a physical copy of the source



Space Efficient Point-In-Time Copy Motivation

� Target volume is needed

� Regular volumes are fully-provisioned

� Many point-in-time implementations never access most of the target space



Space Efficient Design Points

� Space efficiency is an attribute of the volume and not of a point-in-time relation

� A single repository volume is used as a backing store for a group of space efficient volumes

� Space is allocated on destage

� The allocation unit is a single track



Mapping�The track id is used as a key to the data, which is the address of the track on the repository

Space Efficient ArchitectureRepository Volume

Space efficient volumes with no allocated storage

Free Space management

MappingSpace Efficient Volume�Space efficiency attribute is defined when the volume is created

�Has a size

�Does not have allocated storage

�Is associated with a repository volume

Repository Volume�Holds the physical storage for many space efficient volumes

Free Space Management

�Handles allocation and deallocation of space on the repository volume



Staging and Destaging an existing track�Lookup is performed

�Stage or destage is redirected to the repository

Staging and Destaging to a Space Efficient VolumeRepository Volume

Space efficient volumes with no allocated storage

Free Space management

Mapping

Destage a new track�Space is allocated on the repository�Destage is redirected to the allocated repository track

�Mapping is updated



Implementation Characteristics

� Allocate space solely as a function of host write activity

� From the user perspective a space efficient volume is no different then a regular volume

� Space efficient volume may suffer performance degradation

� A Space Efficient volume needs to be explicitly created



Space Efficient Point-In-Time Copy

Target

Address

Repository

Address

3 25

� Read track which was not copied

� Read redirected to source

Read

� Read track which was copied

� Read redirected to the repository

source repositorytarget



Space Management

� In conventional volumes all space is allocated upfront

� In Space Efficient volumes space is allocated on destage

� The allocation unit is a single track

� Repository is shared among many Space Efficient volumes– More flexibility in space utilization



Performance of Point-in-Time copy Using Space Efficient Target

� Consider the backup to tape example:– Reads on the target of data not yet copied are redirected to the

source

– Destage on the source waits for destage of point-in-time data on the target

– Reads on the target of data already copied have additional overhead of lookup in the mapping



Usage

� Expected use of space efficient volume is as a target for a Point-In-Time Copy relation



Point-In-Time Copy vs. Space Efficient Point-In-Time Copy

� Point-In-Time Copy has better performance

� Space Efficient Point-In-Time Copy has better space utilization



Benefits of Space Efficient Point-In-Time Copy

� Better utilization of disk space

� More flexibility with space usage

� Overhead of creating many copies for backup and for application development is greatly reduced



Questions ?

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