Skip to main content
  • Place orders quickly and easily
  • View orders and track your shipping status
  • Enjoy members-only rewards and discounts
  • Create and access a list of your products
  • Manage your Dell EMC sites, products, and product-level contacts using Company Administration.

Article Number: 000064210


Data Domain: How to calculate the maximum size of disks which can be added to the active tier of a Data Domain Virtual Edition (DDVE) appliance

Summary: This article describes how to calculate the maximum amount of physical disk which can be added to the active tier of a Data Domain Virtual Edition (DDVE) appliance

Article Content


Symptoms



Data Domain Virtual Edition (DDVE) is a new product which, as of DDOS 5.7, allows Data Domain Restorers (DDRs) to be deployed as virtual machines in VMware ESXi or Microsoft Hyper-v environments.

Once deployed data disks must be added to the active tier of the DDVE appliance to be used for data storage by the Data Domain File System (DDFS). Note that the maximum possible size of DDFS on a DDVE appliance is dictated by both:
  • The configuration of the DDVE virtual machine
  • The size of capacity license added to the DDVE appliance
For example, the following DDVEs virtual machine is configured such that it is a 4TiB appliance (i.e. unless the CPU/RAM configuration is changed this virtual machine can never support a DDFS file system which is > 4TiB in size):

# system vresource show current
...
This is a 4TiB configuration - Supports 0.5 TiB up to 4 TiB Data Domain Filesystem (DDFS) capacity.


In addition it has a capacity license which is 3.63TiB (i.e. the usable size of the DDFS file system cannot exceed 3.63Tb):

sysadmin@localhost# elicense show
...
Capacity licenses:
##   Feature    Capacity   Mode              Expiration Date
--   --------   --------   ---------------   ---------------
1    CAPACITY   3.63 TiB   permanent (int)   n/a            
--   --------   --------   ---------------   ---------------


Note, however, that some of the space on data disks added to the active tier of the DDVE is used for internal data such as:
  • DDFS metadata
  • Ext3 file systems
  • RAID on LUN (ROL) parity information
This data is known as 'overhead' and is not counted towards used/licensed capacity. 'Overhead' differs depending on active tier size/system configuration hence it can be difficult to determine how much physical disk space to add to the active tier to use all licensed capacity - for example:
  • If 3.63TiB of storage is added to the active tier of the above DDVE and a file system created the resulting usable capacity of the file system will be less than 3.63TiB leaving a small amount of unused but licensed capacity
  • Conversely if 4TiB storage is attempted to be added to the active tier the DDVE may error indicating that this exceeds licensed capacity and will not allow the add to complete
This article discusses how to determine the maximum size of data disks which can be added to the active tier of a DDVE appliance given system and capacity license sizes.

Resolution

When adding capacity to the active tier of a DDVE appliance the system it is first necessary to determine the maximum supported capacity of the active tier - this will either be:
  • The configured size of the DDVE appliance
  • The licensed capacity of the active tier (i.e. the size of capacity license installed)
The smaller of the two values should be used - for example the following system is a 4TiB configuration with 3.63TiB licensed capacity:

sysadmin@localhost# system vresource show current
...
This is a 4TiB configuration - Supports 0.5 TiB up to 4 TiB Data Domain Filesystem (DDFS) capacity.

sysadmin@localhost# elicense show
...
Capacity licenses:
##   Feature    Capacity   Mode              Expiration Date
--   --------   --------   ---------------   ---------------
1    CAPACITY   3.63 TiB   permanent (int)   n/a            
--   --------   --------   ---------------   ---------------


As a result the system will have maximum active tier capacity/size of 3.63TiB - this is referred to below as 'target' capacity.

Using the target capacity and knowledge of how overhead for active tier storage is calculated it is possible to determine the total size of disk which can be added to the active tier to use all of the licensed capacity. Note that overhead is generated in two ways depending on the Data Domain Operating System (DDOS) version running on the appliance:
  • Computational overhead (calculated in DDOS 5.7 and 6.0):
First the amount of overhead used for DDFS metadata must be determined from the following charts. For 5.7 (the following is correct as of 5.7.32.0):

DDVE RAM SIZE (GB)   METADATA SIZE (GB)   CONFIGURATION TYPE
6                    102                  4TiB (2 CPUs/512MiB NVRAM)
8                    104                  8TiB (2 CPUs/512MiB NVRAM)
16                   101                  16TiB (4 CPUs/512MiB NVRAM)


For 6.0 (the following is correct as of DDOS 6.0.0.20): 
 
DDVE RAM SIZE (GB)   METADATA SIZE (GB)   CONFIGURATION TYPE
6                    107                  4TiB (2 CPUs/512MiB NVRAM)
8                    109                  8TiB (2 CPUs/512MiB NVRAM)
16                   107                  16TiB (4 CPUs/512MiB NVRAM)
24                   118                  32TiB (4 CPUs/1GiB NVRAM)
36                   177                  48TiB (4 CPUs/1GiB NVRAM)
48                   231                  64TiB (8 CPUs/1GiB NVRAM)
64                   320                  96TiB (8 CPUs/2GiB NVRAM)

 
In addition to this:

12.9Gb of space is used for ext3 file systems
5.6% of space (excluding that used for ext3 file systems) is used for RAID on LUN parity information

As a result computational overhead can be calculated using the following equation:
 
Overhead = ((Target capacity - 12.9) * 0.056) + metadata overhead + 12.9
  • Percentage overhead (calculated in DDOS 6.0):
Overhead is calculated as being 10% of resulting active tier size, i.e.:
 
Overhead = Target capacity * 0.1

Unfortunately, as overhead is directly dependent on size of storage, it is very difficult to quickly calculate the amount of disk which is required on a given system for a given licensed capacity. To assist with this, however, the following two equations can be used:

Disk size in GiB using computational overhead = (Target capacity in GiB + 12.1776 + DDFS metadata size in GiB) * 0.944

Disk size in GiB using percentage overhead = Target capacity in GiB * 0.9

If using DDOS 5.7 then only the value using computational overhead should be calculated

If using DDOS 6.0 then both the value using computational overhead and percentage based overhead should be calculated then the larger of the two sizes used

Note that to ensure storage is below the maximum limit any computed values should be rounded down - for example if it is calculated that storage of size 4.027TiB can be added to a system this should be rounded down to 4.02TiB rather than rounded up to 4.03TiB. Even with this done computed sizes can still be slightly larger than the actual supported capacity of the system (as described in the following examples).

Example 1:

Initially lets consider the above system - this is configured/licensed as follows:
  • DDOS version: 6.0.0.20
  • Target capacity (the smaller of licensed capacity and system configuration): 3.63TiB (3717GiB)
  • DDFS metadata size (for 4TiB configuration): 107GiB
Storage size to use all target capacity can be computed as follows - note that as this is DDOS 6.0 both computational and percentage based calculations should be used:

Disk size in GiB = (3717 + 12.1776 + 107) * 0.944 = 3621.3516544GiB  (3.53Tib)
Disk size in GiB = 3717 * 0.9 = 3345.3GiB (3.26TiB)

3621GiB (3.53TiB) is the larger of the two computed sizes hence this is the maximum size of storage which can be added to the active tier of this system. In practice a maximum of 3.53TiB was able to be added to such a system.

Example 2:

A DDVE appliance running 6.0.0.20 is provisioned which is an 8TiB configuration and has a licensed active tier capacity of 5.45TiB:

sysadmin@localhost# system vresource show current
...
This is a 8TiB configuration - Supports 0.5 TiB up to 8 TiB Data Domain Filesystem (DDFS) capacity.


sysadmin@localhost# elicense show
...
Capacity licenses:
##   Feature    Capacity   Mode              Expiration Date
--   --------   --------   ---------------   ---------------
1    CAPACITY   5.45 TiB   permanent (int)   n/a            
--   --------   --------   ---------------   ---------------


As a result:
  • DDOS version: 6.0.0.20
  • Target capacity (the smaller of licensed capacity and system configuration): 5.45TiB (5580GiB)
  • DDFS metadata size (for 8TiB configuration): 109GiB
Storage size to use all target capacity can be computed as follows - note that as this is DDOS 6.0 both computational and percentage based calculations should be used:

Disk size in GiB = (5580 + 12.1776 + 109) * 0.944 =  5381.9116544GiB (5.25TiB)
Disk size in GiB = 5580 * 0.9 = 5022GiB(4.90TiB)

5381GiB (5.25TiB) is the larger of the two computed sizes hence this is the maximum size of storage which can be added to the active tier of this system. In practice a maximum of 6.06TiB was able to be added to such a system.

Example 3:

A DDVE appliance running 6.0.0.20 is provisioned which is an 4TiB configuration and has a licensed active tier capacity of 0.90TiB:

sysadmin@localhost# system vresource show current
...
This is a 4TiB configuration - Supports 0.5 TiB up to 4 TiB Data Domain Filesystem (DDFS) capacity.

sysadmin@localhost# elicense show
...
Capacity licenses:
##   Feature    Capacity   Mode              Expiration Date
--   --------   --------   ---------------   ---------------
1    CAPACITY   0.90 TiB   permanent (int)   n/a            
--   --------   --------   ---------------   ---------------


As a result:
  • DDOS version: 6.0.0.20
  • Target capacity (the smaller of licensed capacity and system configuration): 0.90TiB (921GiB)
  • DDFS metadata size (for 8TiB configuration): 107GiB
Storage size to use all target capacity can be computed as follows - note that as this is DDOS 6.0 both computational and percentage based calculations should be used:
 
Disk size in GiB = (921 + 12.1776 + 107) * 0.944 =  981.9276544GiB(0.95TiB)
Disk size in GiB = 921 * 0.9 = 828.9Gib(0.80TiB)

981GiB (0.95TiB) is the larger of the two computed sizes hence this is the maximum size of storage which can be added to the active tier of this system. In practice a maximum of 0.95TiB was able to be added to such a system.

Example 4:

A DDVE appliance is running 5.7, is configured as an 8TiB configuration, and has a licensed capacity of 7.28TiB:

sysadmin@localhost# system vresource show current
...
This is a 8TiB configuration - Supports 0.5 TiB up to 8 TiB Data Domain Filesystem (DDFS) capacity.

sysadmin@localhost# elicense show
...
Capacity licenses:
##   Feature    Capacity   Mode              Expiration Date
--   --------   --------   ---------------   ---------------
1    CAPACITY   7.28 TiB   permanent (int)   n/a            
--   --------   --------   ---------------   ---------------


As a result:
  • DDOS version: 5.7.32.0
  • Target capacity (the smaller of licensed capacity and system configuration): 7.28TiB (7454GiB)
  • DDFS metadata size (for 4TiB configuration): 104GiB
As this DDVE is running DDOS 5.7 only the storage size derived from computational overhead need be calculated:
 
Disk size in GiB = (7454 + 12.1776 + 104) * 0.944 = 7146.2476544GiB (6.97TiB)

7146.24GiB (6.97TiB) is the maximum size of storage which can be added to the active tier of this system. In practice a maximum of 7.82TiB of storage was able to be added to such a system (i.e. computed size had to be reduced slightly to allow storage to be added).

Additional Information

If computed storage sizes are still too large for the systems configuration/licensed capacity an error such as the following will be displayed when storage is added to the active tier:

sysadmin@localhost# storage add dev9
Checking storage requirements...
**** Could not add storage: system capacity exceeds the limit allowable by the license.


In this case it is necessary to reduce the size of provisioned storage in the underlying hypervisor by a small amount (for example 0.01TiB). Once done the DDVE appliance should be forced to rescan provisioned storage and the 'storage add' retried after which it should succeed:
 
# disk rescan
# storage add [device]

Note also that the size of DDFS metadata/ext3 file systems may change in future releases. If this happens then this document will be updated as necessary.

For any assistance with the procedure/calculations described in this document please contact your contracted support provider.

Article Properties


Affected Product

Data Domain

Product

Data Domain, Data Domain Virtual Edition

Last Published Date

20 Nov 2020

Version

2

Article Type

Solution