What is the usable capacity of drives in E-Series storage systems?
A portion of each drive in the storage array is reserved for storing configuration information, along with other types of information such as error logs and SANtricity OS (Controller Firmware) images. This reserved space, commonly referred to as the 'DACstore,' reduces the storage capacity available for creating volumes in volume groups and disk pools. Prior to the release of SANtricity OS 8.20, 500MB of space was reserved on each drive. Beginning with SANtricity OS 8.20, 5.5GB of space will be reserved on each drive. Additionally a allocateReservedSpace SMcli flag was added to VG/DDP creation in 8.20 so that system configurations can be created with the old 500MB extent. Consequently, the capacity available for storing user data will decrease by 5GB for each drive in each volume group or disk pool. The additional reserve capacity will accommodate the storage of increased amounts of information.
Note: The DACstore extent will be increased to 5.5GB only for drives in volume groups and disk pools. The DACstore extent will continue to be 500MB for drives used for SSD Cache.
- Large DACstore extent – a 5.5GB DACstore extent (5.5 x 1024 x 1024 x 1024 bytes)
- Legacy DACstore extent – a 500MB DACstore extent (500 x 1024 x 1024 bytes)
- SANtricity OS – storage controller firmware
References to 'SANtricity OS 8.20' implicitly refer to SANtricity OS 8.20 and higher versions. Similarly, references to 'SANtricity Storage Manager 11.20' implicitly refer to SANtricity Storage Manager 11.20 and higher versions.
All controller platforms supported by SANtricity OS 8.20:
- E5600/EF560 (only if an existing system is upgraded to E5600/EF560 controllers)
The DACstore extent decreases the usable capacity of each drive in the storage array. The usable capacity of each drive is reported in the Storage Array Profile, and is used by SANtricity Storage Manager to calculate the total unconfigured capacity reported for the storage array. Once a volume group or disk pool is created, the usable capacity from the member drives is used to report the free capacity for the volume group or disk pool. In SANtricity OS 8.20, the usable capacity for unassigned drives is always 500MB less than the raw capacity reported in the Storage Array Profile for each unassigned drive. The increased reserve capacity is not applied until a drive is included in a volume group or disk pool. When a volume group or disk pool is created, the usable capacity for each member drive decreases to 5.5GB less than its raw capacity. So, the free capacity for the volume group will be less than the total of the usable capacity of the unassigned drives. The usable capacity of drives in legacy volume groups and disk pools (that is, those that were created with pre-8.20 SANtricity OS) will be 500MB less than the raw capacity reported for each drive.
If you are upgrading from a previous (that is, pre-8.20) release of SANtricity OS, the capacity of any existing volume groups and disk pools will not be reduced. More specifically, the size of existing volumes will remain the same, and the free extent capacity in any existing volume groups and disk pools will remain the same. The DACstore extent will continue to be 500MB on drives in volume groups and disk pools that were present in the storage array prior to the upgrade. The DACstore extent will be 5.5GB on drives in volume groups or disk pools that are created after the upgrade occurs. So, it is possible to have a storage array where the DACstore extent is 500MB for drives in some volume groups and disk pools and 5.5GB for drives in other volume groups or disk pools. If an existing volume group or disk pool is deleted after the upgrade occurs, the DACstore extent will be increased to 5.5GB when those drives are subsequently added to a new volume group or disk pool.
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