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176 DS8000 Series: Concepts and Architecture
Figure 9-8 FB RAID rank capacity
For example, if you configure a RAID-5 rank with 146 GB DDMs with a spare disk in open
system environments, the capacity of the rank totals 779 extents (779 GB).
9.4.2 Sparing rules
To estimate your usable storage capacity, it is helpful to understand the rules of sparing since
the capacity of the rank is different with or without spare disks in the rank. The sparing rules
for the DS8000 are as follows:
A minimum of one spare is required for each array site defined until the following
considerations are met:
– Minimum of 4 spares per DA pair.
The spares are balanced between the two device interfaces.
– Minimum of 4 spares per the largest capacity array site on the DA pair.
The spares are balanced between the two device interfaces.
– Minimum 2 spares of capacity and RPM greater than or equal to the fastest array site
of any given capacity on the DA pair.
The spares are balanced between the two device interfaces.
Spares are not necessarily allocated on the first arrays to be formatted.
Intermix configuration complicates the sparing rules. You should use Capacity Magic for
these calculations.
Note: In the DS8000, extent size is specified with binary size, not decimal size. For
example, 1 GB in binary is described as 1024 x 1024 x 1024, and 1GB in decimal is
described as 1000 x 1000 x 1000. Operating systems adopt the binary format for
calculating their storage.
FB RAID rank capacity
Notes
Device sizes that are not a multiple of 1 binary GB will use additional space on the
disk subsystem as the allocation unit is an extent of this size
Rank
Spare /
No spare Extents
Binary
GB
Decimal
GB
RAID10 73GB Spare
192 192 206.16
RAID10 73GB No spare
256 256 274.88
RAID10 146GB Spare
386 386 414.46
RAID10 146GB No spare
519 519 557.27
RAID10 300GB Spare
785 785 842.89
RAID10 300GB No spare
1,048 1,048 1,125.28
RAID5 73GB Spare
386 386 414.46
RAID5 73GB No spare
450 450 483.18
RAID5 146GB Spare
779 779 836.44
RAID5 146GB No spare
909 909 976.03
RAID5 300GB Spare
1,582 1,582 1,698.66
RAID5 300GB No spare
1,844 1,844 1,979.98