Partition Alignment

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Partition alignment is understood to mean the proper alignment of partitions to the reasonable boundaries of a data storage device (such as a hard disk, solid-state drive (SSD) or RAID volume). Proper partition alignment ensures ideal performance during data access. Incorrect partition alignment will cause reduced performance, especially with regard to SSDs (with an internal page size of 4,096 or 8,192 bytes, for example), hard disks with four-kilobyte (4,096 byte) sectors and RAID volumes.

Contents

A History of Partitions

In the past, the first partition always began at LBA Address 63, which corresponds to the sixty-fourth sector (see also CHS and LBA hard disk addressing). Such (logical) sectors had a size of 512 bytes. This was acceptable for normal hard disks (with a physical sector size of 512 bytes). Newer hard disks with a physical sector size of 4,096 bytes (four kilobytes) are really emulating a sector size of 512 bytes as far as external access is concerned, however internally they are working with 4,096 bytes. Even SSDs work with page sizes of four or eight kilobytes. Partitioning beginning at LBA Address 63 as such is a problem for these new hard disk and SSDs.

If partitions are formatted with a file system with a typical block size of four kilobytes, the four-kilobyte blocks for the file system will not directly fit into the four-kilobyte sectors for a hard disk or the four-, or eight-, kilobyte pages for an SSD. When a four-kilobyte file system block is written, two four-kilobyte sectors or pages will have to be modified. The fact that the respective 512-byte blocks must be maintained simply adds to the already difficult situation, meaning that a Read/Modify/Write process will have to be performed. A reduction in writing performance of up to a factor of 25 is the consequence during smaller data access attempts.[1]

Problems with incorrect partition alignment

Proper Partition Alignment

To avoid these problems, alignment at one-megabyte boundaries is recommended, which is a conservative approach over the long term. With the current addressing system divided in logical sectors of 512 bytes, doing so would correspond to 2,048 sectors.

Proper partition alignment ensures somewhat increased performance for SSDs

Virtualized Systems

The article, File System Alignment in Virtualized Environments, contains information about virtualized systems.

Windows

Newer version of Windows (Windows Vista, Windows 7 and Windows Server 2008) perform the following, reasonable alignment:[2][3]

  • Disk sizes less than or equal to four gigabytes should be aligned on sixty-four-kilobyte boundaries
  • Disk sizes larger than four gigabytes should be aligned on one-megabyte boundaries

Older versions of Windows will require manual alignment.[4]

Linux

fdisk (Older Versions)

With older version of fdisk, manual alignment can be achieved using the -S and -H flags. There are a variety of recommendations with regards to the specific number of sectors per track determined by the -S flag and the number of heads determined by the -H flag.[5][6][7] The -S 32 -H 64 flags will however definitely align along the one megabyte boundary (32 sectors per track by 64 heads by 512 bytes equals 1,048,576 bytes or 1 megabyte). Thereby, creation of the first partition will begin with Cylinder 2. Misalignment will occur using fdisk, if special parameters are not used.

fdisk from Version 2.17.1

With regard to util-linux-ng versions after 2.17.1, fdisk will align on the one megabyte boundary, if DOS compatibility mode has been disabled.[8]

The recommended settings for newer versions of fdisk are:[9]

  • Use the fdisk utility from util-linux-ng versions 2.17.2 or later
  • Read the fdsik warnings.
  • Deactivate DOS compatibility mode (the -c flag).
  • Use sectors as display units (the -u flag).
  • Use the +size(M, G) option in order to specify the end of a partition.

Incorrect Alignment Example

The following example shows an incorrect alignment. It is caused by DOS compatibility mode.

root@ubuntu-10-04:~# fdisk /dev/sdb
Device contains neither a valid DOS partition table, nor Sun, SGI or OSF disklabel
Building a new DOS disklabel with disk identifier 0xe4909079.
Changes will remain in memory only, until you decide to write them.
After that, of course, the previous content won't be recoverable.

Warning: invalid flag 0x0000 of partition table 4 will be corrected by w(rite)

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): p

Disk /dev/sdb: 160.0 GB, 160041885696 bytes
255 heads, 63 sectors/track, 19457 cylinders
Units = cylinders of 16065 * 512 = 8225280 bytes
Sector size (logical/physical): 512 bytes / 512 bytes
I/O size (minimum/optimal): 512 bytes / 512 bytes
Disk identifier: 0xe4909079

   Device Boot      Start         End      Blocks   Id  System

Command (m for help): n
Command action
   e   extended
   p   primary partition (1-4)
p
Partition number (1-4): 1
First cylinder (1-19457, default 1): 
Using default value 1
Last cylinder, +cylinders or +size{K,M,G} (1-19457, default 19457): +10G

Command (m for help): u
Changing display/entry units to sectors

Command (m for help): p

Disk /dev/sdb: 160.0 GB, 160041885696 bytes
255 heads, 63 sectors/track, 19457 cylinders, total 312581808 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: 0xe4909079

   Device Boot      Start         End      Blocks   Id  System
/dev/sdb1              63    20980889    10490413+  83  Linux

Command (m for help): 

Proper Alignment Example using Older Versions of fdisk

Partitions can be properly aligned using the -S 32 -H 64 flags, if the second cylinder is used as the starting point.

debian:~# fdisk -S 32 -H 64 /dev/sdc

The number of cylinders for this disk is set to 65536.
There is nothing wrong with that, but this is larger than 1024,
and could in certain setups cause problems with:
1) software that runs at boot time (e.g., old versions of LILO)
2) booting and partitioning software from other OSs
   (e.g., DOS FDISK, OS/2 FDISK)

Command (m for help): p

Disk /dev/sdc: 68.7 GB, 68719476736 bytes
64 heads, 32 sectors/track, 65536 cylinders
Units = cylinders of 2048 * 512 = 1048576 bytes
Disk identifier: 0x5a3b93b6

   Device Boot      Start         End      Blocks   Id  System

Command (m for help): n
Command action
   e   extended
   p   primary partition (1-4)
p
Partition number (1-4): 1
First cylinder (1-65536, default 1): 2
Last cylinder or +size or +sizeM or +sizeK (2-65536, default 65536): 
Using default value 65536

Command (m for help): p

Disk /dev/sdc: 68.7 GB, 68719476736 bytes
64 heads, 32 sectors/track, 65536 cylinders
Units = cylinders of 2048 * 512 = 1048576 bytes
Disk identifier: 0x5a3b93b6

   Device Boot      Start         End      Blocks   Id  System
/dev/sdc1               2       65536    67107840   83  Linux

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

Calling ioctl() to re-read partition table.
Syncing disks.
debian:~# fdisk -lu /dev/sdc

Disk /dev/sdc: 68.7 GB, 68719476736 bytes
64 heads, 32 sectors/track, 65536 cylinders, total 134217728 sectors
Units = sectors of 1 * 512 = 512 bytes
Disk identifier: 0x5a3b93b6

   Device Boot      Start         End      Blocks   Id  System
/dev/sdc1            2048   134217727    67107840   83  Linux
debian:~# 

Proper Alignment Example using fdisk Versions 2.17.1 or later

Proper alignment can be achieved by deactivating DOS compatibility mode and setting the sector unit (the partition will start at the LBA Address 2,048. In the case of an SSD with a page size of four kilobytes, there will be 256 empty pages at the beginning of the disk. The partition will begin precisely at the start of Page 257).

root@ubuntu-10-04:~# fdisk -c -u /dev/sdb
Device contains neither a valid DOS partition table, nor Sun, SGI or OSF disklabel
Building a new DOS disklabel with disk identifier 0xfae13403.
Changes will remain in memory only, until you decide to write them.
After that, of course, the previous content won't be recoverable.

Warning: invalid flag 0x0000 of partition table 4 will be corrected by w(rite)

Command (m for help): p

Disk /dev/sdb: 160.0 GB, 160041885696 bytes
255 heads, 63 sectors/track, 19457 cylinders, total 312581808 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: 0xfae13403

   Device Boot      Start         End      Blocks   Id  System

Command (m for help): n
Command action
   e   extended
   p   primary partition (1-4)
p
Partition number (1-4): 1
First sector (2048-312581807, default 2048): 
Using default value 2048
Last sector, +sectors or +size{K,M,G} (2048-312581807, default 312581807): +10G

Command (m for help): p

Disk /dev/sdb: 160.0 GB, 160041885696 bytes
255 heads, 63 sectors/track, 19457 cylinders, total 312581808 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: 0xfae13403

   Device Boot      Start         End      Blocks   Id  System
/dev/sdb1            2048    20973567    10485760   83  Linux

Command (m for help): 

Testing the Alignment

Either of the two following commands will test alignment under Linux (replace /dev/sdX with the name of the device, such as /dev/sda).

  • sfdisk -d /dev/sdX
  • fdisk -l -u /dev/sdX

This example shows a system with an Ubuntu 10.04 installation. In this case, the Ubuntu 10.04 installer has aligned both the primary and the logical partitions at the one-megabyte boundary. This is indicated by the fact that the start sector number for the partition can be divided by 2,048 (2,048 sectors * 512 bytes per sector equals 1,048,576 or one megabyte). The extended partition (in this example, /dev/sda2) has not been aligned at the one-megabyte boundary. However, this is not required, since that partition merely serves as a container for the logical partitions and the logical partitions themselves are aligned at the one-megabyte boundaries.

user@ubuntu-test:~$ sudo sfdisk -d /dev/sda
Warning: extended partition does not start at a cylinder boundary.
DOS and Linux will interpret the contents differently.
# partition table of /dev/sda
unit: sectors

/dev/sda1 : start=     2048, size= 39061504, Id=83, bootable
/dev/sda2 : start= 39065598, size=1761810434, Id= 5
/dev/sda3 : start=        0, size=        0, Id= 0
/dev/sda4 : start=        0, size=        0, Id= 0
/dev/sda5 : start= 39065600, size=  3997696, Id=82
/dev/sda6 : start= 43065344, size=1757810688, Id=83
user@ubuntu-test:~$ sudo fdisk -l -u /dev/sda

Disk /dev/sda: 1000.2 GB, 1000204886016 bytes
255 heads, 63 sectors/track, 121601 cylinders, total 1953525168 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: 0x000d8343

   Device Boot      Start         End      Blocks   Id  System
/dev/sda1   *        2048    39063551    19530752   83  Linux
/dev/sda2        39065598  1800876031   880905217    5  Extended
/dev/sda5        39065600    43063295     1998848   82  Linux swap / Solaris
/dev/sda6        43065344  1800876031   878905344   83  Linux
user@ubuntu-test:~$ 

Logical Volume Manager

If the Logical Volume Manager (LVM) is used, proper alignment will still be an important issue.

As a rule, alignment should be automatic. Older versions of the LVM align at the sixty-four-kilobyte boundary, which places the beginning of the physical volumes at the beginning of a page for SSDs as well. An August 2010 patch now ensures alignment at the one-megabyte boundary. The patch is included in the LVM Version 2.02.73 from August 18th 2010.

Software RAID

A Linux Software RAID array normally aligns properly. The superblock for RAID volumes using the older Superblock Metadata Version 0.90 is at the end of the device (in a block aligned on a sixty-four-kilobyte boundary). The RAID data starts at the beginning of the device. A software RAID array using the newer Superblock Metadata Version 1.* will align the data at the one megabyte boundary (see Linux Software RAID: Testing Alignment).

Table of References

  1. http://www.ibm.com/developerworks/linux/library/l-4kb-sector-disks/#benchmarks
  2. Disk Partition Alignment Best Practices for SQL Server (msdn.microsoft.com, May 2009)). In Windows Vista as well as Windows Server 2008, partition alignment is usually performed by default. The default for disks larger than 4 GB is 1 MB; the setting is configurable and is found in the registry at the following location: HKLM\SYSTEM\CurrentControlSet\Services\VDS\Alignment
  3. LUN Types and Linux partition alignment (blogs.netapp.com)
  4. Disk performance may be slower than expected when you use multiple disks in Windows Server 2003, in Windows XP, and in Windows 2000 (Microsoft KB)
  5. http://www.thinkpad-forum.de/software/linux/88045-umzug-von-hdd-zu-ssd-mit-minimalen-arbeitsaufwand/#post807878
  6. http://www.nuclex.org/blog/personal/80-aligning-an-ssd-on-linux
  7. http://www.ocztechnologyforum.com/forum/showthread.php?54379-Linux-Tips-tweaks-and-alignment&p=373224#post373224
  8. https://ata.wiki.kernel.org/index.php/ATA_4_KiB_sector_issues#L-2._Userspace_tools_status_.28thanks_to_Karel_Zak.5B13.5D.29
  9. http://karelzak.blogspot.com/2010/05/4096-byte-sector-hard-drives.html

Additional Information


Author: Werner Fischer


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