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PowerMax systems offer various underlying RAID options: RAID 5 (single parity), RAID 6 (dual parity) and RAID 1 (dual drives). These options are described in the following section.
All data on PowerMax arrays is also managed as thin provisioned (128k) tracks within one (or more) Storage Resource Pools (SRPs). Each RAID group is divided into 64 logical blocks (TDATs), and each TDAT is configured to hold data at a given data reduction level, from 1:1 uncompressed data all the way to 16:1. The fixed, post-data reduction track size on a TDAT ranges from 8k to 128k, using every 8k increment except 120k.
As tracks are written to the array, the data reduction code will decide what size is needed to store the resulting track information, and then it selects a TDAT to store that data within (assuming it did not deduplicate). When the resulting size of a track changes or when snaps need to preserve the older version of a track, PowerMax writes the data to a new location rather than overwriting a prior version of the same data.
Due to SRP data management, there is no sequential mapping of host LUN data to tracks on TDATs. All data within a PowerMax is thin (single track) striped across all RAID groups in the SRP. As a result, PowerMax RAID types are all essentially “+0” with striping across RAID groups.
RAID 5 is an industry-standard data protection mechanism with rotating parity across all members of the RAID 5 set. If a hard drive failure occurs, the missing data is rebuilt by reading the remaining drives in the RAID group and performing XOR calculations.
PowerMax systems support two RAID 5 configurations:
RAID 6 enables the rebuilding of data if two drives fail within a RAID group. Our implementation of RAID 6 calculates two types of parity. This is important during events when two drives within the same RAID group fail, as it still allows the data in this scenario to be reconstructed. Horizontal parity is identical to RAID 5 parity, which is calculated from the data across all disks in the RAID group. Diagonal parity is calculated on a diagonal subset of data members. For applications without demanding performance needs, RAID 6 provides the highest data availability.
PowerMax systems support RAID 6 (6+2) with data striped across 8 drives (6 data, 2 parity).
RAID 1 is an industry-standard data protection consisting of two drives containing exact copies, or mirrors, of the data. There is no parity required as both RAID members have full copies of the data allowing the system to retrieve data from either disk. Dell PowerMax implements RAID 1 (1 + 1) mirroring the data across two drives.
RAID 1 is available on PowerMax arrays with the PowerMaxOS Q3 2020 release and later.