How available space is calculated in AdvancedDisk disk pools
| Article:TECH77600 | | | Created: 2009-01-04 | | | Updated: 2009-01-04 | | | Article URL http://www.symantec.com/docs/TECH77600 |
Problem
How available space is calculated in AdvancedDisk disk pools
Solution
The
Intelligent Capacity Management feature, introduced with Storage Lifecycle
Policies (SLPs) and AdvancedDisk in NetBackup 6.5, optimizes the process of
'staging' backups to disk before duplicating them to tape under SLP
control. It does this by checking not only the actual free space in a disk
pool but also the potential free space if images stored under capacity managed
retention are removed and the amount of space that may be required by other
backups running at the time a particular backup starts. This mechanism
ensures a high success rate of backups to disk by reducing the risk that a disk
within the pool will run out of space while the backup is in
progress.
This
document describes the variables used to calculate the amount of available space
in a NetBackup AdvancedDisk disk pool and how that figure is calculated and
updated.
The
variables and their associated values in bytes are displayed by the nbdevquery
-listdv -D command as follows:
total_capacity :
xxxxxxx
total_phys_capacity :
xxxxxxx
free_space :
xxxxxxx
free_phys_space :
xxxxxxx
potential_free_space:
xxxxxxx
committed_space :
xxxxxxx
precommitted_space :
xxxxxxx
Five of
these variables (total_capacity, free_space, potential_freespace,
committed_space and precommitted_space) are used to calculate available space in
AdvancedDisk disk pools. Two other variables (total_phys_capacity and
free_phys_space) are currently unused by NetBackup and are reserved for future
feature development.
Total_capacity
is the total size of the disk and free_space is the amount of free space on the
disk. These values are derived from the file system and are updated at one
minute intervals. For the purposes of AdvancedDisk, total_capacity
and total_phys_capacity should have the same values and free_space and
free_phys_space should have the same values.
Potential_free_space
is the sum of the sizes of all fragments on the disk that are in
EligibleForExpiration (EFE) state. This information is obtained from the
SLP information held in the EMM database and only applies when capacity managed
retention is used.
A
fragment enters EFE state when the copy it belongs to uses the Capacity Managed
retention type and has been successfully duplicated by the SLP duplication
manager. It leaves EFE state when the copy it belongs to has expired, at
which point the fragment moves to ToBeDeleted state.
There are
two points in NetBackup processing when the potential_free_space value will be
computed. Successful duplications will add EFE fragments so the SLP
duplication manager will compute the value for each disk at the end of each
duplication session. Successful expirations will remove EFE fragments so
the SLP expiration manager will compute the value for each disk at the end of
each expiration session.
The
algorithm to compute the value simply accumulates the sum of the sizes of all
the EFE fragments on the disk. The potential_free_space value for the disk
can become stale and may be incorrect by small amounts between computations due
to manual expirations, etc. but it is self-correcting. The next time the
computation is done, the correct value will be reestablished.
Committed_space
is the amount of data NetBackup has estimated as being written to the disk and
is the sum of all estimated 'in progress' backup sizes based on either 'last
backup + 20%' for full and incremental backups or 'total_capacity - high water
mark' for user backups.
Precommitted_space
is a helper value for committed_space and is decremented as a backup job
proceeds. Each time the capacity and free space information is updated
the value of committed_space is also updated to the current value of
precommitted_space. This ensures that all capacity related values are
updated at the same time to give the most accurate description of the state of
the disk.
NetBackup
uses free_space, potential_free_space and committed_space to determine how much
space is available on a disk according to the formula:
available
space = free_space + potential_free_space - committed_space
When a
backup starts NetBackup first compares the estimated size of the job against the
available space on all disks within the disk pool (including any disks that are
marked down) to determine if there is sufficient space for the backup. It
then selects the disk within the pool with the most available space and starts
writing the backup to that disk until the disk reaches its high water mark or
the backup completes.
When a
disk reaches its high water mark potential_free_space is released by expiring
images in accordance with the capacity managed retention rules. Once
those images are expired and removed from the disk potential_free_space
for that disk is recalculated. If high water mark is reached and there are
no images eligible for expiration on the disk, the backup will
continue. Should the disk run out of space before the backup completes, it
will then span to another disk within the disk pool. The choice of disk to
span to is again based on the most available space.
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