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:

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.

About Chatwin
This is my website:, dedicated to my personal designed computer with relevant technological know-how which I learned during my testing and fine tuning period. Concept PC “The Force” evolved in 21 months (minus 7 days) from only a slight idea, till an amazing workstation with remarkable capabilities. It wasn’t labor for me, nor suffering or hard work. It was fun.... If you’re interested in a custom build computer, based on your own preferences, contact me for further information. I only use the best hardware with an acceptable price level. I can assure the best performance within your budget and an unique (thoroughly tested) concept computer/server.

25 Responses to Partition Alignment for SSD’s

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  11. Buaya says:

    Kevin, Flash on PCIe can be asmwoee no doubt. But their are still some serious issues standing in the way of wide spread adoption. First is cost, SSD’s are priced around $650 for an intel 80GB drive, while it runs ( last time I checked ) $2400 for an 80GB fusion IO drive. Consumers can get 4 intel drives for the price of 1 fusion IO card, also getting 240GB of storage vs 80GB. The second thing to overcome is form factor. with 2.5Inch drives, you can pack a lot of the SSD drives in a single server. Depending on the form factor your PCIe slots are going to be extremely limited. For instance with 1u/2u servers you may get 2 or 3 Pcie slots, many clients I work with will take up 1 with an extra nic, sometimes 2. Even your bigger boxes end up with 5-7 PCIe slots. Also don’t Forget about All the blades being deployed today. While these maybe viable solutions for the extreme performance edge cases, I still think clients will look to a solution like violin memory or texas memory systems who make appliances that can be integrated into their environments before maxing out expansion cards. When your dealing with terrabyte size databases, and you only 5 PCIe spots, your not going to be able to pack enough density into your machine. While I understand and agree the price/size will come down the price of SSD drives will ( and is ) plummeting more quickly their are just too many vendors starting to play in this space. Competition will slowly bring the price/GB closer to disk. Also whether its SSD or FusionIO, many people running MySQL are still bound by their hosting providers if the hosting providers support it then great, if not all bets are off.

  12. Dizajn says:

    In my ernixeepce, there is still a definite difference between noatime’ and relatime’. relatime’ is doing a lot more writes and slowing things down (note: I’m only concerned with performance here, not the idea that I’m going to wear out the SSD too fast).I’m using the noop scheduler and it seems fine; much better than the default. I haven’t tried testing it against the deadline scheduler, though.I use ext2 as well. I rarely have unclean shutdowns, and hey, I used it for years before ext3 existed without a problem. Seems to me more likely that the SSD as a whole would fail than that I would lose data to an unclean shutdown, and I have full backups anyway.

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