Specifications
© IBM Copyright, 2012 Version: January 26, 2012
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Summary of Best Practices for Storage Area Networks
turns out to be less critical based on the caching it provides, and less variation in its
I/O profile used to access back-end disks.
For the SVC the maximum destage size is 32k and thus it is not possible to achieve
full stride writes for random workload. For the SVC the only opportunity for full stride
writes occurs with large sequential workloads, and in that case the larger the
segment size, the better. Larger segment sizes can have an adverse effect on
random I/O however. It turns out that the SVC and controller cache does a good job
of hiding the RAID5 write penalty for random I/O and therefore larger segment sizes
can be accommodated.
The main consideration for selecting segment size is to ensure that a single host I/O
will fit within a single segment to prevent accessing multiple physical drives. Testing
has shown that the best compromise for handling all workloads is to use a segment
size of 256k.
For the earlier models of DS4000 using the 2 Gbps FC adapters the 4k block size
performed better for random I/O and 16k performs better for sequential I/O.
However, since most workloads contain a mix of random and sequential, the default
values have proven to be the best choice. For the higher performing DS4000
storage systems, such as the DS4700 and DS4800, the 4k cache block size
advantage for random I/O is less obvious. Since most customer workloads involve at
least some sequential workload the best overall choice for these models is the 16k
block size.
4.5 SVC
The arrangement of RAID arrays into mDisks and mDisk groups is one of the most
troublesome parts of SVC Installation Architecture, and unfortunately the proper
method to create RAID arrays and arrange them into mDisk groups varies widely by
vendor, applications, necessary capacity, and the layout of the particular disk array
in use.
Some “rules of thumb” for mapping back-end arrays to managed disks (MDisk) and
MDisk Groups:
a) Any given MDisk Group should only contain disks of a single size and speed.