Partition Alignment for SSD’s

Microsoft Support: Article ID: 929491 – Last Review: June 8, 2009 – Revision: 4.0 (Summary)

“Disk performance may be slower than expected when you use multiple disks in Microsoft Windows Server 2003, in Microsoft Windows XP, and in Microsoft Windows 2000. For example, performance may slow when you use a hardware-based redundant array of independent disks (RAID) or a software-based RAID.

This issue may occur 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. A volume cluster may be created over a stripe unit boundary instead of next to the stripe unit boundary. This is because Windows uses a factor of 512 bytes to create volume clusters. This behavior causes a misaligned partition. Two disk groups are accessed when a single volume cluster is updated on a misaligned partition.

To verify that an existing partition is aligned, divide the size of the stripe unit by the starting offset of the RAID disk group. Use the following syntax:

((Partition offset) * (Disk sector size)) / (Stripe unit size)

Example of alignment calculations in bytes for a 256-KB stripe unit size:

(63 * 512) / 262144 = 0.123046875
(64 * 512) / 262144 = 0.125
(128 * 512) / 262144 = 0.25
(256 * 512) / 262144 = 0.5
(512 * 512) / 262144 = 1

Example of alignment calculations in kilobytes for a 256-KB stripe unit size:

(63 * .5) / 256 = 0.123046875
(64 * .5) / 256 = 0.125
(128 * .5) / 256 = 0.25
(256 * .5) / 256 = 0.5
(512 * .5) / 256 = 1

These examples shows that the partition is not aligned correctly for a 256-KB stripe unit size until the partition is created by using an offset of 512 sectors (512 bytes per sector).”

SMART Modular Technologies (author: Esther Spanjer), April 2010

“NAND flash devices are divided into erasable blocks composed of multiple pages (up to 256 pages per block, and up to 8KB per page). A flash block must be fully erased prior to re-writing, and a single-block erase process can take up to several milliseconds. The write speed may suffer a great deal if the SSD controller has to perform unnecessary block erase operations due to partition misalignment. Proper partition alignment is one of the most critical attributes that can greatly boost the I/O performance of an SSD due to reduced read modify‐write operations.

Windows XP or Windows Server 2000/2003 start partition offset at 31.5KB (32,256 bytes). Due to this misalignment, clusters of data are spread across physical memory block boundaries, incurring read-modify-write penalty. As a result, the host ends up writing at least 2X more I/O for every write as illustrated in this figure:

Alignment
When choosing a partition starting offset, SMART Modular recommends system integrators to correlate the partition offset with the RAID stripe size and cluster size to achieve the optimal SSD I/O performance. The figure shows an example of a misaligned partition offset and an example of an aligned partition offset for Windows Server.”

Chatwin says: Based on the conclusions as mentioned above, I would recommend the following alignment and cluster settings for NAND flash devices (unit = 512 bytes):

((Partition offset / Flash unit) * (Disk sector size)) / (Erase block)

((2048 / 2) * .5) / 512K = 1
((4096 / 4) * .5) / 512K = 1

With a flash unit of 2 for SLC flash (aka. 1K, 1 byte/cell group [32×32]) and 4 for MLC flash (aka. 2K, 2 bytes/cell group [32×32]), this results in an offset of 1 MB (2048) for SSD’s with Single Layer Cell memory, and 2 MB (4096) for Multi Layer Cell memory, when you want to align your partitions on erase block boundaries. The best cluster size (allocation unit) can be calculated with this formula:

((Single erase block / Flash unit) * (Disk sector size)) = Cluster Size

((128 / 2) * .5) = 32K (SLC),
((512 / 4) * .5) = 64K (MLC, “The Force“)

With 8K as the ideal sector size for flash memory (4K SLC), this is in my opinion the best choice when you want the fastest performance for your SSD (Indilinx, Samsung, JMicron controllers), unless you’re more concerned about maximizing your storage capacity at any price. And not to forget: a longer lifespan, for reducing a lot of overhead and fragmentation compared to the standard NTFS formatting of 4096 bytes. Keep in mind that this setting was default in a time that hard disks were limited to 2-4 GB, instead of 2 TB in this time and age.

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