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
System and Storage Seminar 2005, Haifa
© 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
SOURCE
HOSTPROCESSORs
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Bit Map
System and Storage Seminar 2005, Haifa
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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
System and Storage Seminar 2005, Haifa
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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
System and Storage Seminar 2005, Haifa
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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
System and Storage Seminar 2005, Haifa
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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
System and Storage Seminar 2005, Haifa
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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
System and Storage Seminar 2005, Haifa
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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
System and Storage Seminar 2005, Haifa
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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
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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
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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
System and Storage Seminar 2005, Haifa
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Usage
� Expected use of space efficient volume is as a target for a Point-In-Time Copy relation
System and Storage Seminar 2005, Haifa
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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
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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
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Questions ?