Considering The System Software; Buffering The I/O; Aligning Host I/O With Raid Striping - Dell PowerVault MD3000i Tuning Manual

Dell™ PowerVault MD3000 and MD3000i Array Tuning Best Practices
used at certain times, providing a point-to-point raw wire speed of up to 12Gb/s.
Please note that this raw speed does not take into account the transmission
overhead or device spenditures on either side of the SAS wide link and is purely
a cached I/O operation.
Additionally, care must be taken of which buses are being used within the host.
Installing the HBAs that use the same bus will hamper data transfer rate. Ensure
that all HBAs installed in the host are on a different bus (see note in 5.1)

5.2 Considering the System Software

5.2.1 Buffering the I/O

The type of I/O, buffered or un-buffered, provided by the operating system to the
application is an important factor in analyzing storage performance issues.
Un-buffered I/O (also known as raw or direct I/O) moves data directly between
the application and the drive devices. Buffered I/O is a service provided by the
operating system or by the file system. Buffering improves application
performance by caching write data in a file system buffer, which the operating
system or file system periodically flushes to non-volatile storage.
Buffered I/O is generally preferred for shorter and more frequent transfers. File
system buffering might change the I/O patterns generated by the application.
That is, writes might coalesce so that the pattern seen by the storage system is
more sequential and more write-intensive than the application I/O itself. Direct I/O
is preferred for larger, less frequent transfers and for applications that provide
their own extensive buffering, such as Oracle. The I/O performance tool Iometer
is another application that can operate un-buffered to test a more raw
performance. Regardless of I/O type, I/O performance generally improves when
the storage system is kept busy with a steady supply of I/O requests from the
host application. Become familiar with the parameters that the operating system
provides for controlling I/O, such as maximum transfer size.

5.2.2 Aligning Host I/O with RAID Striping

Most host operating systems require or benefit from varying degrees of partition
I/O alignment and avoiding performance-degrading segment crossings. That is,
I/Os should not span a segment boundary. Matching of I/O size (commonly, by a
power-of-two) to disk group layout helps align I/Os across the entire drive.
However, this is only true if the starting sector is properly aligned to a segment
boundary. Segment crossing is often seen in the Microsoft Windows operating
system, where partitions created by Microsoft Windows 2000 or Microsoft
Windows 2003 start at the 64th sector. Starting at the 64th sector causes
misalignment with the underlying RAID striping and allows the possibility for a
single I/O operation to span multiple segments. Newer versions of the Microsoft
Windows OS have a default alignment typically of 2048 blocks that still may
require adjustment depending on the environment.
December 2008 – Revision A01 Page 25