2
RAID Types
2-5
RAID Types and Tradeoffs
Figure 2-2 RAID 5 Group
RAID 5 Groups offer excellent read performance and good write
performance. Write performance benefits greatly from
storage-system caching.
RAID 3 Group (Parallel Access Array)A RAID 3 Group consists of fiv e or more disks. The hardwa re always
reads from or writes to all the disks. A RAID 3 Group uses disk
striping. To maintain the RAID 3 performance, you can create only
one LUN per RAID 3 group.
The storage system writes parity information that lets the Group
continue operating if a disk fails. When you replace the failed disk,
the SP rebuilds the group using the information stored on the
working disks. Performance is degraded while the SP rebuilds the
group. However, the storage system continues to function and gives
users access to all data, including data stored on the failed disk.
The following figure shows user and parity data with a data block
size of 2 Kbytes in a RAID 3 Group. Notice that the byte addresses
EMC1815
Second disk
128-255 640-767 1152-1279 Parity 2048-2175
Parity
Third disk
256-383 768-895 1664-17912176-2303
Parity
Fourth disk
384-511 1280-1407 1792-1919 2304-2431
Parity
Fifth disk
896-1023 1408-1535 1920-2047 2432-2559
Stripe
Stripe
element
size
Stripe
size
First disk
512-639 1024-11511536-1663 Parity
0-127
Blocks
User data
Parity data
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