Rebuild Impact On Performance; Write Working Set - HP Surestore Disk Array 12h - And FC60 Service Manual

Concepts and Management
Optimizing Performance

Rebuild Impact on Performance

Because the rebuild process uses internal array resources, it may decrease array performance while a
rebuild is in progress. This is particularly true if the host is issuing many I/Os to the disk array.
The disk array uses a rebuild priority that allows you to determine which is more important: system
performance or data redundancy. A high rebuild priority ensures the rebuild will be completed as quickly as
possible, but at the expense of disk array performance. A low rebuild priority maintains disk array
performance but delays completion of the rebuild, leaving your data vulnerable for a longer period of time.

Write Working Set

A key factor in maintaining disk array performance is the balance between the RAID 0/1 space and the
write working set. The write working set is the number of unique data blocks written to the disk array over
a period of time. The disk array monitors the number of write I/Os to create the write working set size
parameter. As long as the amount of RAID 0/1 space available is equal to or greater than the write working
set size, maximum performance is maintained.
On newly installed disk arrays there is enough RAID 0/1 space for the write working set and performance
is not a problem. However, as more data is stored on the disk array and the amount of RAID 5 space
increases, the RAID 0/1 space available for the write working set decreases. If a point is reached where the
write working set exceeds the size of the available RAID 0/1 space, some writes will have to be serviced
from RAID 5, and then there may be a performance degradation.
To maintain performance, the disk array reserves at least 10% of the array capacity as RAID 0/1 for the
write working set. This value was chosen after careful analysis of many diverse operating environments
and application loads. It makes the assumption that in most situations 90% of the data on the disk remains
essentially unchanged. That is, it is only read or not accessed at all and can therefore be stored in RAID 5
without impacting performance. Thus 10% of the capacity should be large enough to service the remaining
data that is frequently updated, and performance can be maintained.
However, if your applications make heavy write demands on the disk array, the write working set may
exceed the 10% RAID 0/1 minimum. In this case, the array must begin servicing write I/Os from RAID 5,
and the performance of the array begins to degrade. Large data base applications that frequently update
many records may have a write working set large enough to exceed the 10% RAID 0/1 space.
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