HP StoreOnce 4430 HP StoreOnce Backup System Concepts and Configuration Guidel - Page 67

Rotation schemes and retention policy, Retention policy, Rotation scheme

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implementation, by default, does not allow block sizes that are greater than 256 KB. To use a block size greater than this you need to modify the following registry setting: nl nl HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Control\Class\ nl {4D36E97B-E325-11CE-BFC1-08002BE10318}\0000\Parameters Change the REG_DWORD MaxTransferLength to "80000" hex (524,288 bytes), and restart the media server - this will restart the iSCSI initiator with the new value. Rotation schemes and retention policy Retention policy The most important consideration is the type of backup rotation scheme and associated retention policy to employ. With data deduplication there is little penalty for using a large number of virtual cartridges in a rotation scheme and therefore a long retention policy for cartridges because most data will be the same between backups and will therefore be deduplicated. A long retention policy provides a more granular set of recovery points with a greater likelihood that a file that needs to be recovered will be available for longer and in many more versions. Rotation scheme There are two aspects to a rotation scheme which need to be considered: • Full versus Incremental/Differential backups • Overwrite versus Append of media Full versus Incremental/Differential backups The requirement for full or incremental backups is based on two factors, how often offsite copies of virtual cartridges are required and speed of data recovery. If regular physical media copies are required, the best approach is that these are full backups on a single cartridge. Speed of data recovery is less of a concern with a virtual library appliance than it is with physical media. For example, if a server fails and needs to be fully recovered from backup, this recovery will require the last full backup plus every incremental backup since (or the last differential backup). With physical tape it can be a time consuming process to find and load multiple physical cartridges, however, with virtual tape there is no need to find all of the pieces of media and, because the data is stored on disk, the time to restore single files is lower due to the ability to randomly seek within a backup more quickly and to load a second cartridge instantly. Overwrite versus append of media Overwriting and appending to cartridges is also a concept where virtual tape has a benefit. With physical media it is often sensible to append multiple backup jobs to a single cartridge in order to reduce media costs; the downside of this is that cartridges cannot be overwritten until the retention policy for the last backup on that cartridge has expired. The diagram below shows cartridge containing multiple appended backup sessions some of which are expired and other that are valid. Space will be used by the StoreOnce appliance to store the expired sessions as well as the valid sessions. Moving to an overwrite strategy will avoid this. With virtual tape a large number of cartridges can be configured for "free" and their sizes can be configured so that they are appropriate to the amount of data stored in a specific backup. Appended backups are of no benefit because media costs are not relevant in the case of VTL. Rotation schemes and retention policy 67

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implementation, by default, does not allow block sizes that are greater than 256 KB. To use a
block size greater than this you need to modify the following registry setting:
nl
nl
HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Control\Class\
nl
{4D36E97B-E325-11CE-BFC1-08002BE10318}\0000\Parameters
Change the
REG_DWORD MaxTransferLength
to “80000” hex (524,288 bytes), and restart
the media server – this will restart the iSCSI initiator with the new value.
Rotation schemes and retention policy
Retention policy
The most important consideration is the type of backup rotation scheme and associated retention
policy to employ. With data deduplication there is little penalty for using a large number of virtual
cartridges in a rotation scheme and therefore a long retention policy for cartridges because most
data will be the same between backups and will therefore be deduplicated.
A long retention policy provides a more granular set of recovery points with a greater likelihood
that a file that needs to be recovered will be available for longer and in many more versions.
Rotation scheme
There are two aspects to a rotation scheme which need to be considered:
•
Full versus Incremental/Differential backups
•
Overwrite versus Append of media
Full versus Incremental/Differential backups
The requirement for full or incremental backups is based on two factors, how often offsite copies
of virtual cartridges are required and speed of data recovery. If regular physical media copies are
required, the best approach is that these are full backups on a single cartridge. Speed of data
recovery is less of a concern with a virtual library appliance than it is with physical media. For
example, if a server fails and needs to be fully recovered from backup, this recovery will require
the last full backup plus every incremental backup since (or the last differential backup). With
physical tape it can be a time consuming process to find and load multiple physical cartridges,
however, with virtual tape there is no need to find all of the pieces of media and, because the
data is stored on disk, the time to restore single files is lower due to the ability to randomly seek
within a backup more quickly and to load a second cartridge instantly.
Overwrite versus append of media
Overwriting and appending to cartridges is also a concept where virtual tape has a benefit. With
physical media it is often sensible to append multiple backup jobs to a single cartridge in order
to reduce media costs; the downside of this is that cartridges cannot be overwritten until the retention
policy for the last backup on that cartridge has expired. The diagram below shows cartridge
containing multiple appended backup sessions some of which are expired and other that are valid.
Space will be used by the StoreOnce appliance to store the expired sessions as well as the valid
sessions. Moving to an overwrite strategy will avoid this.
With virtual tape a large number of cartridges can be configured for “free” and their sizes can be
configured so that they are appropriate to the amount of data stored in a specific backup. Appended
backups are of no benefit because media costs are not relevant in the case of VTL.
Rotation schemes and retention policy
67