IBM N Series Hardware Manual page 193

All 24 available disks are now allocated:
Spare disk drive: 2 (1 per controller)
RAID parity disks: 2 (2 per controller)
Data disks: 18 (9 per controller)
About 25% of the raw capacity is used by hardware protection. This amount varies depending
on the ratio of data disks to protection disks. The remaining usable capacity becomes less
deterministic from this point because of ever increasing numbers of variables, but a few firm
guidelines are still available.
Right-sizing
A commonly misunderstood memory requirement is that imposed by the right-sizing process.
This overhead is because of three main factors:
Block leveling
– Disks from different batches (or vendors) can contain a slightly different number of
addressable blocks. Therefore, the N series controller assigns a common maximum
capacity across all drives of the same basic type. For example, this process makes all
"1 TB" disks exactly equal.
– Block leveling has a negligible memory requirement because disks of the same type
are already similar.
Decimal to binary conversion
– Because disk vendors measure capacity in decimal units and array vendors usually
work in binary units, the stated usable capacity differs.
– However no capacity is really lost because both measurements refer to the same
number of bytes. For example, 1000 GB decimal = 1000000000000 bytes = 931 GB
binary.
Checksums for data integrity
– Fibre Channel (FC) disks natively use 520 byte sectors, of which only 512 bytes are
used to store user data. The remaining 8 bytes per sector are used to store a
checksum value. This imposes a minimal capacity overhead.
– SATA disks natively use 512 byte sectors, all of which is used to store user data.
Therefore one sector per eight blocks is reserved to store the checksum value. This
imposes a higher capacity overhead than for FC disks.
Table 14-2 Right-sized disk capacities
Disk Type Capacity
(decimal GB)
FC 72
144
300
600
Capacity GB Checksum
(binary GB) Type
68 512/520 Block
(approximately
136
2.4%)
272
Chapter 14. Designing an N series solution
Right-sized Cap.
(binary GB)
66
132
265
173