IBM System Storage DS3500 Introduction And Implementation Manual page 77

7914DS3KPlanning_090710.fm
Draft Document for Review March 28, 2011 12:24 pm
RAID 3: Data striping with a dedicated parity drive
A RAID 3 array uses data striping with a dedicated parity drive. Similar to RAID 0 data
striping, information written to disk is split into chunks (a fixed amount of data), and each
chunk is written out to the same physical position on separate disks (in parallel). This
architecture requires parity information to be written for each stripe of data; RAID 3 uses a
dedicated physical drive for storing parity data. If any one disk drive in the array fails, the array
and associated logical drives become degraded, but all data is still accessible by the host
application.
However, with RAID 3, the dedicated parity drive is a performance bottleneck during writes.
Because each write operation requires the parity to be re-computed and updated, this means
that the parity drive is accessed every time a block of data is written to the array. Because of
this, RAID 3 is rarely used today in the industry and RAID 5 has taken it's place. The DS3500
storage subsystem supports a maximum of 30 drives in a RAID 3 array.
RAID 5: Data striping with distributed parity
Like RAID Level 3, RAID Level 5 also uses parity for data protection but unlike RAID 3 it does
not use a dedicated parity drive. Instead, the parity blocks are evenly distributed across all
physical disk drives in the array, as shown in Figure 3-16. The failure of a single physical drive
in a RAID 5 array will cause the array and associated logical drives to be degraded, but all the
data will remain accessible to the host application. This level of data redundancy is known as
n+1 redundancy because the data remains accessible after a single drive failure. When you
create a RAID 5 array, the capacity of the array is reduced by the equivalent capacity of one
drive (for parity storage). The DS3500 storage subsystem requires a minimum of three and
supports a maximum of 30 drives in a RAID 5 array.
Figure 3-16 RAID 5
RAID Level 5 is best used in environments requiring high availability and fewer writes than
reads.
RAID Level 5 can be good for multi-user environments, such as database or file system
storage where the typical I/O size is small and there is a high proportion of read activity.
Applications with a low read percentage and a high random write percentage may not perform
53
Chapter 3. IBM System Storage DS3500 Storage System planning tasks