2
2-6
EMC Fibre Channel Storage System Model FC4700 Configuration Planning Guide
RAID Types and Tradeoffs
proceed from the first disk to the second, third, and fourth, then the
first, and so on.
Figure 2-3 RAID 3 Group
RAID 3 differs from RAID 5 in several important ways. First, in a
RAID 3 Group the hardware processes disk requests serially; whereas
in a RAID 5 Group the hardware can interleave disk requests. Second,
with a RAID 3 Group, the parity information is stored on one disk;
with a RAID 5 Group, it is stored on all disks. Finally, with a RAID 3
Group, the I/O occurs in small units (one sector) to each disk. A
RAID 3 Group works well for single-task applications that use I/Os
of blocks larger than 64 Kbytes.
Each RAID 3 Group requires some dedicated SP memory (6 Mbytes
recommended per group). This memory is allocated when you create
the group, and becomes unavailable for storage-system caching. For
top performance, we suggest that you do not use RAID 3 Groups
with RAID 5, RAID 1/0, or RAID 0 Groups, since SP processing
power and memory are best devoted to the RAID 3 Groups. RAID 1
mirrored pairs and individual units require less SP processing power,
and therefore work well with RAID3 Groups.
EMC1816
Second disk
Third disk
Fourth disk
Fifth disk
Stripe
element
size
Stripe
size
…
…
…
…
…
First disk
User data
Parity data
2048-2559 4096-4607 6144-6655 8192-8603
0-511
512-1023 2560-3071 4608-5119 6656-7167 8604-9115
5120-56311024-1535 3072-3583 7168-76799116-9627
3584-40951536-2047 5632-6143 7680-81919628-10139
Parity Parit y Parity Pa rity Pari ty
Data block
Bytes