Clariion LUN Alignment Offset (LBA) setting for Windows Host

[Clarksville]

I was told by someone much more experience than myself that when creating a new EMC Clariion LUN to be assigned to a Windows server, that the Alignment Offset (LBA) should be set to 63 (Even though it defaults to 0). Can anyone confirm or explain the necessity of this setting? thanks.

 

[Clarksville]

I found my own answer after posting. 63 is correct. BTW, you can find much helpful information about EMC products on Dell website. See below for explanation.

Binding LUNs: Alignment offset
The host via its operating system may record private information,
known as a signature block, at the start of a RAID-0, RAID-5, or
RAID-10 stripe. The alignment offset can be used to force the beginning
of usable LUN space to start past the signature block. The
signature block size, which is dictated by the storage array manufacturer
(EMC, in this case), consumes only 16 KB of space, but
the NT file system (NTFS)—a file system for Microsoft Windows®
operating systems—reserves 31.5 KB of space for the signature
block. This reservation of extra space at the beginning of the
LUN can increase the risk of disk crossings or stripe crossings.
Stripe crossings are similar in principle to disk crossings, but refer
to accessing two stripes instead of one. If administrators do not
adjust the NTFS signature block size from its default of 31.5 KB to
16 KB, the stripe will not align with the signature block and data
will be written to two stripes instead of one.
For example, because NTFS reserves 31.5 KB of signature space,
if a LUN has an element size of 64 KB with the default alignment
offset of 0 (both are default Navisphere settings), a 64 KB write to
that LUN would result in a disk crossing even though it would seem
to fit perfectly on the disk. A disk crossing can also be referred to as
a split I/O because the read or write must be split into two or more
segments. In this case, 32.5 KB would be written to the first disk
and 31.5 KB would be written to the following disk, because the
beginning of the stripe is offset by 31.5 KB of signature space. This
problem can be avoided by providing the correct alignment offset.
Each alignment offset value represents one block. Therefore,
EMC recommends setting the alignment offset value to 63, because
63 times 512 bytes is 31.5 KB. Contact Dell or EMC to determine
the latest recommended alignment offset value, because improper
use will degrade performance.2 If the alignment offset requirement
is uncertain, leave it at the Navisphere default.

 

[comtec17]

You should always set the alignment offset to 64k using windows disk utilities such as diskpar and diskpart. Never use the offset in Navisphere unless the LUN you are using will never be used with a layer application such as Snapview or SANCopy.