Disk Alignment

Disk Alignment

This is one of the most crucial pieces that we can talk about so far regarding performance. Having the disks that make up the LUN misaligned can be a performance impact of up to 30% on an application. The reason this occurs is because of the “Signature” or MetaData information that a host writes to the beginning of a LUN/Disk. To understand this we must first look at how the Clariion formats the LUNs.

In an earlier blog, we described how the Clariion formats the disks. The Clariion formats the disks in blocks of 128 per disk, which is equivalent to a 64 KB of data that is written to a disk from Cache. Why this is a problem, is that when an Operating System like Windows, grabs the LUN, it wants to initialize the disk, or write a disk signature. The size of this disk signature is 63 blocks, or 31 ½ KB of disk space. Because the Clariion formats the disks in 128 blocks, or 64 KB of disk space, that leaves 65 blocks, or 32 KB of disk space remaining on the first disk for the host to write data. The problem is that the host writes to Cache in whatever block size it does. Cache then holds the data a writes the data out to disk in a 64 KB Data Chunk. Because of the “Signature”, the 64 KB Data Chunk now has to go across two physical disks on the Clariion. Usually, we say that hitting more disks is better for performance. However, with this DISK CROSS, performance will go down on a LUN because Cache is now waiting for an acknowledgement from two disks instead of one disk. If one disk is overloaded with I/O, a disk is failing, etc…this will cause a delay in the acknowledgement back to the Storage Processor. This will be the case from now on when Cache writes every chunk of data out to this LUN. This will impact not only the LUN Cache is writing too, but to every LUN on the Raid Group may be affected.

By using an offset on a LUN from a Host Based Utility, ie Diskpart, or Diskpar for Windows, we are allowing the Clariion to write a 64 KB Data Chunk to one physical disk at a time. Essentially, what we are doing is giving up the remaining disk space on the first physical disk in the Raid Group as the illustration shows above. Window’s still writes it’s “Signature” to the first 63 blocks, but we use Diskpart, or Diskpar to offset the disk space the Clariion Writes to of the remaining space on the first disk. When Cache writes out to disk now, it will begin writing out to the first block on the second disk in the Raid Group, thereby giving the full 128 blocks/64 KB chunk that the Clariion hopes to write out to one physical disk.

The problem with all of this is that this offset or alignment needs to set on a Window’s Disk/LUN before any data is written to the LUN. Once there is data on the LUN, this cannot be done without destroying the existing LUN/data. The only way to now fix this problem is to create a new LUN on the Clariion, assign it to the host, set the offset/alignment, and do a host-based copy/migration. Again, a Clariion LUN Migration is a block for block LUN copy/move. All you are doing with a LUN Migration is moving the problem to a new location on the Clariion.
Windows has two utilities from the Command prompt that can be run to set the offset/alignment, Diskpar and Diskpart.

Diskpar is used for Window’s systems running Windows 2000, or 2003, not using at least Service Pack 1. Diskpar can be downloaded as part of the Resource Kit, and requires through its command line interface that the offset be equal to 128. Diskpar sets the offset in blocks, Since the Clariion formats the disks in 128 blocks, the Clariion will now offset writing to the LUN to block number 128, which is the first block on the second disk.

Diskpart is for Windows Systems running Windows 2003, service pack 1 and up. Diskpart sets the alignment in KiloBytes. Since the Clariion formats the disk in 64 KB, the Clariion will now align the writing to the LUN in 64 KB Chunks, or the first full 64 KB chunk, which is the second physical disk in the Raid Group.

This is also an issue with Linux servers, as an offset will need to set as well. Here again, the number to use is 128, because fdisk uses the number of blocks, not KiloBytes.
The following blog entry will list the steps for setting the offset for Windows 2003, as well as Linux servers.