IBM Midrange System DS4000 Series Hardware Manual page 139

Segment Size = I/O block size request
It is very difficult to align segment size with block size, so this option is not practical to
implement, so use the other two scenarios.
Every request transaction uses exactly one disk operation.
There is high IOPS.
This option is ideal for random I/O requests, such as the database file system.
Segment Size < I/O block size request
This is the best option to obtain MBps performance and low IOPs, which is the norm for
multimedia applications.
More disks are used or requested.
There is higher throughput (Mbps).
It is ideal for sequential writes, such as a large multimedia application.
It is optimized when a single I/O request can be serviced with a single or exact multiple
data stripes (the segment size multiplied by the number of drives in the array that are used
for I/O). In this case, multiple disks are used for the same request, but each disk is only
accessed once, or the exact number of times if the block size is too big and you have to
use multiple stripes.
To calculate the best segment size in this environment, consider the RAID protection and
number of drives participating in the array by using the following formula:
Segment size = (Block size / X)
Where X is the number of drives used in the array for I/O and where:
– RAID 0=Number of drives
– RAID 1 or 10=Number of drives / 2
– RAID 5= Number of drives - 1
– RAID 6= Number of drives - 2
If the resulting Segment Size is >512 KB, then divide it by an integer number staring with 2
to obtain multiple data stripes of each block requested.
Tips: The possible segment sizes available are 8 KB, 16 KB, 32 KB, 64 KB, 128 KB, 256
KB, and 512 KB.
Storage Manager sets a default block size of 128 KB for every logical volume, except
for RAID 3 volumes, which are set to 256 KB.
For database applications, block sizes between 32–128 KB have shown to be more
effective.
In a large file environment, such as media streaming or CAD, 128 KB or more are
recommended.
For a Web server or file and print server, the range should be between 16–64 KB.
The performance monitor can be used to evaluate how a given segment size affects the
workload.
Chapter 4. IBM System Storage DS planning and configuration
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