Sunday, November 21, 2010

Partition Alignment

A hard drive can be dissected into different sections or partitions for many reasons – such as to segment data or because an operating system typically sits on one partition while applications and files sit on others. These partitions are positioned on the hard drive to optimize the way data is read and written to them. With new hard drives that rely on a different sector base than legacy drives, computers may not be able to recognize this change, effectively reducing the performance of hard drives and in some cases decreasing the lifespan of the drives dramatically.

To ensure the best performance of these drives, it’s important to make sure the partitions are all aligned properly and that data can be written and accessed without any issues

Why misaligned partitions are the problem for SAN and RAID
RAID is used to compose many hard disk drives or other storage devices into one large array of data. This array is seen as one large storage device in the system across which data is striped. The granularity at which data is stored on one drive of the array before subsequent data is stored on the next drive of the array (data striping) is based on the drive settings, none of which allow for adjusting for a newer 4K alignment.

System performance may slow when a hardware-based redundant array of independent disks (RAID) or a software-based RAID is used – or if the starting location of the partition is not aligned with a stripe unit boundary in the disk partition that is created on the RAID. In this case, one data operation will be multiplied over several RAID disks.

To resolve this issue, aligning the partitions properly maximizes the performance. All data operations become faster as there are no redundant disk operations.

Naturally, a SAN (Storage Area Network) is just a large RAID distributed over a local network or by Fibre Channel. Thus all issues of RAID partition alignment are the same for SAN.

Why misaligned partitions are the problem for virtual environments
Alignment in a virtual infrastructure is critical to performance, hardware lifecycle, and storage efficiencies. Misalignment results in retrieving more data from an underlying array than what the virtual machine is requesting, which causes inefficiencies requiring more storage hardware resources to serve a workload and slow down the process.

In the future, partition alignment issues for 4K HDD and SSD will lessen as 4K physical sectors and memory pages become visible and accessible on the operating system level, and the need for emulating becomes unnecessary.  However misalignment will continue to be a problem for RAID/SAN and virtual environments for the foreseeable future, and it is important to recognize and tackle this problem head-on.

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