Partition Alignment - Revision history

Leo at 18:28, 6 April 2021

2021-04-06T18:28:43Z

← Older revision		Revision as of 18:28, 6 April 2021
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	Partitions should be aligned as follows:		Partitions should be aligned as follows:
	{\| class="wikitable"		{\| class="wikitable"
			\|+
			* 1MB is common. To be safe, check the block size of your SSD and align your partition based on that.
	!Media Type		!Media Type
	!Alignment		!Alignment
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	\|-		\|-
	\|Solid State Drives		\|Solid State Drives
	\|1MB ?		\|1MB*
	\|}		\|}<br />

	==Background==		==Background==
	Hard drives store data on platters that are subdivided into tracks and each track divided into sectors. On most older hard drives under, each sector holds 512 bytes of data while newer hard drives supporting using advanced format (AF) stores 4096 bytes per sector. Every time a contents of a sector is changed, the entire sector must be read, modified, then re-written.		Hard drives store data on platters that are subdivided into tracks and each track divided into sectors. On most older hard drives under, each sector holds 512 bytes of data while newer hard drives supporting using advanced format (AF) stores 4096 bytes per sector. Every time a contents of a sector is changed, the entire sector must be read, modified, then re-written.

Leo at 18:26, 6 April 2021

2021-04-06T18:26:20Z

← Older revision		Revision as of 18:26, 6 April 2021
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			Partitions should be aligned as follows:
			{\| class="wikitable"
			!Media Type
			!Alignment
			\|-
			\|Hard Drive (less than 2TB, without AF)
			\|512 bytes
			\|-
			\|Hard Drive (greater than 2TB, with AF)
			\|4096 bytes
			\|-
			\|Solid State Drives
			\|1MB ?
			\|}

	~~Storage devices work with an internal page size varying between~~ 512 bytes~~, to~~ 4096 bytes ~~(or higher for SSDs)~~. ~~Proper partition alignment ensures data written to~~ a ~~storage device do not straddle between these internal pages. If partitions are misaligned~~, ~~a piece of data could cause multiple write or~~ read ~~operations~~, ~~resulting in reduced performance~~.		==Background==
			Hard drives store data on platters that are subdivided into tracks and each track divided into sectors. On most older hard drives under, each sector holds 512 bytes of data while newer hard drives supporting using advanced format (AF) stores 4096 bytes per sector. Every time a contents of a sector is changed, the entire sector must be read, modified, then re-written.

	~~== Background ==~~		Solid state devices on the other hand stores data in banks of memory cells and groups them into pages ranging from 4 to 16KB. These pages are then further grouped into a block that ranging from 128 to 512 pages, or 512KB to 8MB. Likewise to a sector, every time a contents of a block is changed, the entire block must be read by the controller, modified, then re-written as a whole.
	~~On legacy hard disks~~, ~~the first partition typically began at LBA 63~~. ~~With each physical~~ sector ~~containing 512 bytes~~, ~~such~~ a ~~partition~~ is ~~offset~~ by ~~32256 bytes (32KB - 512B). This is fine on legacy drives because~~ the ~~disk worked with 512 byte data. However~~, ~~with modern hard disks using 4K physical sector sizes~~, ~~using such~~ a ~~partition scheme would cause the partition to straddle between two physical sectors (as the 32256 offset isn't divisible by 4K)~~.

	~~When creating a~~ partition ~~on a modern Linux system, this~~ is ~~not something you need to concern yourself with as <code>fdisk</code> has sensible defaults when DOS compatibility mode is disabled, resulting~~ in ~~new~~ partition ~~aligned at the 1MB boundary (2048~~ sectors).		A partition should be aligned such that its offset is divisible by the sector size in the case of hard drives and the block size in the case of SSDs. A misaligned partition would result write amplification, as everything that's written will cause two sectors or pages to be modified. What should have been a change to one sector now becomes two. Consequently, a misaligned partition would results in lower throughput and increases wear on SSDs

	== Legacy Systems ==		==Legacy Systems==
	On older systems, you need to ~~be mindful of~~ the units that are reported by <code>fdisk</code>. Ensure that partitions are aligned properly by specifying the units with the <code>fdisk -lu</code> option. ~~When creating~~ a partition, ~~you~~ can ~~specify~~ 32 sectors by 64 heads ~~(for 2048 sectors, with each being~~ 512 bytes ~~results in a~~ 1MB ~~alignment) with~~ the <code>fdisk</code> options <code>-S 32 -H 64</code>. For example:		This section only applies to systems with an older versions of <code>fdisk</code>. On older systems, you need to pay special attention to the units that are reported by <code>fdisk</code>. as they are most likely reported as Cylinder-Head-Sector (CHS) units. Ensure that partitions are aligned properly by specifying the units with the <code>fdisk -lu</code> option.

			To create a partition with a 2048 sector or 1MB offset, we can call for 32 sectors by 64 heads. We can calculate the offset thusly: 32 * 512 bytes * 64 heads = 1MB.

			Use the <code>fdisk</code> options <code>-S 32 -H 64</code>. For example:

	{{highlight\|lang=terminal\|code=		{{highlight\|lang=terminal\|code=
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	Device Boot Start End Blocks Id System		Device Boot Start End Blocks Id System
	/dev/sdd1 2048 3907028991 1953513472 83 Linux		/dev/sdd1 2048 3907028991 1953513472 83 Linux
	}}		}}<br />

	==See Also==		==See Also==

Leo: clear up language and added background

2020-06-14T23:18:53Z

clear up language and added background

← Older revision		Revision as of 23:18, 14 June 2020
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	Proper disk partition alignment ~~using {{~~code\|fdisk}}		Storage devices work with an internal page size varying between 512 bytes, to 4096 bytes (or higher for SSDs). Proper partition alignment ensures data written to a storage device do not straddle between these internal pages. If partitions are misaligned, a piece of data could cause multiple write or read operations, resulting in reduced performance.

			== Background ==
			On legacy hard disks, the first partition typically began at LBA 63. With each physical sector containing 512 bytes, such a partition is offset by 32256 bytes (32KB - 512B). This is fine on legacy drives because the disk worked with 512 byte data. However, with modern hard disks using 4K physical sector sizes, using such a partition scheme would cause the partition to straddle between two physical sectors (as the 32256 offset isn't divisible by 4K).

			When creating a partition on a modern Linux system, this is not something you need to concern yourself with as <code>fdisk</code> has sensible defaults when DOS compatibility mode is disabled, resulting in new partition aligned at the 1MB boundary (2048 sectors).

			== Legacy Systems ==
			On older systems, you need to be mindful of the units that are reported by <code>fdisk</code>. Ensure that partitions are aligned properly by specifying the units with the <code>fdisk -lu</code> option. When creating a partition, you can specify 32 sectors by 64 heads (for 2048 sectors, with each being 512 bytes results in a 1MB alignment) with the <code>fdisk</code> options <code>-S 32 -H 64</code>. For example:

	{{highlight\|lang=terminal\|code=		{{highlight\|lang=terminal\|code=
	~~root@nas mnt]~~# fdisk -S 32 -H 64 /dev/sdd		# fdisk -S 32 -H 64 /dev/sdd

	WARNING: DOS-compatible mode is deprecated. It's strongly recommended to		WARNING: DOS-compatible mode is deprecated. It's strongly recommended to
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	## Verify that the partition starts on 2048		## Verify that the partition starts on 2048
	~~[root@nas mnt]~~# fdisk -lu /dev/sdd		# fdisk -lu /dev/sdd

	Disk /dev/sdd: 2000.4 GB, 2000398934016 bytes		Disk /dev/sdd: 2000.4 GB, 2000398934016 bytes
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	}}		}}

	== See Also ==		==See Also==
	* https://www.thomas-krenn.com/en/wiki/Partition_Alignment
			*https://www.thomas-krenn.com/en/wiki/Partition_Alignment

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Leo: Created page with " Proper disk partition alignment using {{code|fdisk}} {{highlight|lang=terminal|code= root@nas mnt]# fdisk -S 32 -H 64 /dev/sdd WARNING: DOS-compatible mode is deprecated. I..."

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Created page with " Proper disk partition alignment using {{code|fdisk}} {{highlight|lang=terminal|code= root@nas mnt]# fdisk -S 32 -H 64 /dev/sdd WARNING: DOS-compatible mode is deprecated. I..."

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Proper disk partition alignment using {{code|fdisk}}

{{highlight|lang=terminal|code=
root@nas mnt]# fdisk -S 32 -H 64 /dev/sdd

WARNING: DOS-compatible mode is deprecated. It's strongly recommended to
switch off the mode (command 'c') and change display units to
sectors (command 'u').

Command (m for help): n
Command action
e extended
p primary partition (1-4)
p
Partition number (1-4): 1
First cylinder (1-1907729, default 1): 2
Last cylinder, +cylinders or +size{K,M,G} (2-1907729, default 1907729):
Using default value 1907729

Command (m for help): p

Disk /dev/sdd: 2000.4 GB, 2000398934016 bytes
64 heads, 32 sectors/track, 1907729 cylinders
Units = cylinders of 2048 * 512 = 1048576 bytes
Sector size (logical/physical): 512 bytes / 512 bytes
I/O size (minimum/optimal): 512 bytes / 512 bytes
Disk identifier: 0x0003522f

Device Boot Start End Blocks Id System
/dev/sdd1 2 1907729 1953513472 83 Linux

Command (m for help): w
The partition table has been altered!

Calling ioctl() to re-read partition table.
Syncing disks.

## Verify that the partition starts on 2048
[root@nas mnt]# fdisk -lu /dev/sdd

Disk /dev/sdd: 2000.4 GB, 2000398934016 bytes
64 heads, 32 sectors/track, 1907729 cylinders, total 3907029168 sectors
Units = sectors of 1 * 512 = 512 bytes
Sector size (logical/physical): 512 bytes / 512 bytes
I/O size (minimum/optimal): 512 bytes / 512 bytes
Disk identifier: 0x0003522f

Device Boot Start End Blocks Id System
/dev/sdd1 2048 3907028991 1953513472 83 Linux
}}

== See Also ==
* https://www.thomas-krenn.com/en/wiki/Partition_Alignment

[[Category:Todo]]
[[Category:Linux]]