Partition Alignment

2011-06-20
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update: cgdisk, when beginning with a new partition table, when creating the first partition, automatically creates a small empty partition before it, and thus is said to align everything properly. I haven't double-checked this.

ARGH, all of this is busted-ish. The newest efforts work fine, but Windows cannot access the Linux partitions using the ext2 driver. Boo.

Table of Contents [hide]

Propose adding recommendation of mke2fs ... -E stripe-width=128 for 512kB erase page block drives (vertex) or mke2fs ... -E stripe-width=32 for 128kB erase page block drives. (afaik everything else)

Steps I've taken ∞

  • cfdisk and create one large partition.

  • fdisk -lu /dev/sdg

    • Learned the cycles/heads/sectors.
    • Learned the starting sector.
   Device Boot      Start         End      Blocks   Id  System
/dev/sdg1              63  3907024064  1953512001   83  Linux

Find the starting sector using sudo fdisk -lu. If it starts on a value that are an even multiple of 1024. Then you're ready to proceed.

Huh?? I have "63".

sudo mke2fs -t ext4 -E stripe-width=128 /dev/sda1

Since the default file system sector size is 4kB, stripe-width=128 will result in 512kB alignment. The file system will now try to align each write at the start of an empty erase block.

sda1 indicates that we want to create our file system on the first partition on /dev/sda.

I experimented with fdisk -H 32 -S 32 /dev/sdg to get 512KB alignment. I was forced to start at cylinder 3. When goofing around some more, I was able to start at cylinder 2 with fdisk -H 8 -S 16 /dev/sdg.

Cylinder size (Times 512k):
 1024K (x2)
  -S 16 -H 128
  -S 32 -H 64
 512K (x1)
  -S 8 -H 128
  -S 16 -H 64
  -S 32 -H 32
 256K (x0.5)
  -S 4 -H 128
  -S 8 -H 64
  -S 16 -H 32
  -S 32 -H 16
 128K (x0.25)
  -S 2 -H 128
  -S 4 -H 64
  -S 8 -H 32
  -S 16 -H 16
  -S 32 -H 8
 64KB (recommended for Windows)
  -S 16 -H 8
\fdisk  -H 8  -S 16  /dev/sdg
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').
c
DOS Compatibility flag is not set
n
Command action
   e   extended
   p   primary partition (1-4)
p
Partition number (1-4): 
1
First cylinder (17-30523665, default 17): 
(enter)
Last cylinder, +cylinders or +size{K,M,G} (2-30523665, default 30523665): 
+8G
n
Command action
  e   extended
  p   primary partition (1-4)
e
Partition number (1-4): 
2
First cylinder (131089-30523665, default 131089): 
(enter)
Last cylinder, +cylinders or +size{K,M,G} (131089-30523665, default 30523665): 
30523665
n
Command action
  l   logical (5 or over)
  p   primary partition (1-4)
l

+128000

First cylinder (128034-30523665, default 128034): 
(enter)
Last cylinder, +cylinders or +size{K,M,G} (128034-30523665, default 30523665): 
+591949
n
Command action
  l   logical (5 or over)
  p   primary partition (1-4)
l
First cylinder (720000-30523665, default 720000): 
(enter)
Last cylinder, +cylinders or +size{K,M,G} (720000-30523665, default 30523665): 
+96001
n
l
First cylinder (816018-30523665, default 816018): 
(enter)
+128000
n
l
First cylinder (944035-30523665, default 944035): 
(enter)
Last cylinder, +cylinders or +size{K,M,G} (944035-30523665, default 30523665): 
+32753

+4577070

and then the rest...

w
The partition table has been altered!

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



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): c
DOS Compatibility flag is not set

Command (m for help): n
Command action
   e   extended
   p   primary partition (1-4)
p
Partition number (1-4): 1
First cylinder (17-30523665, default 17): 
Using default value 17
Last cylinder, +cylinders or +size{K,M,G} (17-30523665, default 30523665): +128000

Command (m for help): n
Command action
   e   extended
   p   primary partition (1-4)
e
Partition number (1-4): 2
First cylinder (128018-30523665, default 128018): 
Using default value 128018
Last cylinder, +cylinders or +size{K,M,G} (128018-30523665, default 30523665): 
Using default value 30523665

Command (m for help): n
Command action
   l   logical (5 or over)
   p   primary partition (1-4)
l
First cylinder (128034-30523665, default 128034): 
Using default value 128034
Last cylinder, +cylinders or +size{K,M,G} (128034-30523665, default 30523665): +591949

Command (m for help): n
Command action
   l   logical (5 or over)
   p   primary partition (1-4)
l
First cylinder (720000-30523665, default 720000): 
Using default value 720000
Last cylinder, +cylinders or +size{K,M,G} (720000-30523665, default 30523665): +96001

Command (m for help): n
Command action
   l   logical (5 or over)
   p   primary partition (1-4)
l
First cylinder (816018-30523665, default 816018): 
Using default value 816018
Last cylinder, +cylinders or +size{K,M,G} (816018-30523665, default 30523665): +128000
Command (m for help): n
Command action
   l   logical (5 or over)
   p   primary partition (1-4)
l
First cylinder (944035-30523665, default 944035): 
Using default value 944035
Last cylinder, +cylinders or +size{K,M,G} (944035-30523665, default 30523665):  +32753

Command (m for help): n
Command action
   l   logical (5 or over)
   p   primary partition (1-4)
l
First cylinder (976805-30523665, default 976805): 
Using default value 976805
Last cylinder, +cylinders or +size{K,M,G} (976805-30523665, default 30523665):  +4577700

Command (m for help): n
Command action
   l   logical (5 or over)
   p   primary partition (1-4)
l
First cylinder (5554522-30523665, default 5554522): 
Using default value 5554522
Last cylinder, +cylinders or +size{K,M,G} (5554522-30523665, default 30523665): 
Using default value 30523665
w

[wrong] Formula for calculating cylinders. For example, I want an 8389.79MB partition, and I can mostly re-create it with:

 size_wanted * 1000 / cylinder_size
 8389.79*1000/64
 = 131090

this is too big

look at the top of cfdisk:

 Heads: 255   Sectors per Track: 63   Cylinders: 7783

[wrong] press 'u' to get your units to cylinders. I found 1020. Then do this math:

 cylinder_size_currently * cylinders / sylinder_size_wanted
 1020*7783/64
 = 124041

This is too small

I have no clue what I'm doing.

...

# sdg1 and sdg5 will become ntfs
\mkfs.ext4  -I 128 /dev/sdg6
\mkfs.ext4  -I 128 /dev/sdg7
\mkswap  /dev/sdg8
\mkfs.ext4  -I 128 /dev/sdg9
\mkfs.ext4  -I 128 /dev/sdg10

One guy did:

\mkfs.ext4  -O has_journal  -O dir_index  -O extents  -E stripe-width=128 /dev/sda1

Checking that it's aligned ∞

\fdisk  -ul /dev/sda

Example from fdisk (in Swedish):

Kommando (m för hjälp): p

Disk /dev/sda: 64,0 GB, 64023257088 byte
32 huvuden, 32 sektorer/spår, 122114 cylindrar
Enheter = cylindrar av 1024 · 512 = 524288 byte
Diskidentifierare: 0xca7fa911

    Enhet Start     Början        Slut     Block    Id  System
/dev/sda1               2       26626    13632000   83  Linux
/dev/sda2           26627      122114    48889856   83  Linux

Kommando (m för hjälp): u
Ändrar visnings-/inmatningsenheter till sektorer

Kommando (m för hjälp): p

Disk /dev/sda: 64,0 GB, 64023257088 byte
32 huvuden, 32 sektorer/spår, 122114 cylindrar, totalt 125045424 sektorer
Enheter = sektorer av 1 · 512 = 512 byte
Diskidentifierare: 0xca7fa911

    Enhet Start     Början        Slut     Block    Id  System
/dev/sda1            1024    27265023    13632000   83  Linux
/dev/sda2        27265024   125044735    48889856   83  Linux

Kommando (m för hjälp):

The response was:

It's the starting sector that really matters. Doing the maths yields

1024*512/(512*1024) = 1 -> integer (Correct write block alignment)
27265024*512/(512*1024) = 26626 -> integer (Correct write block alignment)

starting_sector*sector_size_in_bytes/(erase_block_size_kB*1024_bytes)

Simplified version:

1024/1024 = 1 -> integer (Correct write block alignment)
27265024/1024 = 26626 -> integer (Correct write block alignment)
\fdisk  -ul /dev/sda

Now check for a + next to any entry. This means that rounding has taken place.. which means it's not correctly done.

Windows Alignment with Linux ∞

List all connected disks and partitions. [sudo fdisk -l]

If your disk is located elsewhere, use the appropriate path instead of /dev/sda.

Each command should be followed by .

No changes will be made before you enter the command w.

Quit without saving. [q]

  • Start fdisk. [sudo fdisk /dev/sda]
  • On the fdisk prompt type u and enter to change units to sectors. [u]
  • Create a new empty dos partition table. [o]
  • Create a new partition. [n]
  • Make it an primary partition. [p]
  • Make it the first partition. [1]
  • Select starting offset in sectors. Change this value to adjust alignment. [128]
  • Press to select the maximum value or pick another one if you're going to use more than one partition.

  • If you're going to use this partition for Windows.

    • Change partition type. [t]
    • Select HPFS/NTFS. [7]
  • List and verify your partition table. [p]
  • Write your new partition table to the disk. Previous stored data will be lost. [w]

  • Done.

The alignment "128" is invalid for me. Sigh.

mkfs.ntfs ∞

\mkfs.ntfs  -I  -H 255  -S 63  -c /dev/sdxy

Hrm, I can't remember any of this shit.. so I'm not going to fiddle around.

Scheduler ∞

  • anticipatory

    • Default for for kernels 2.6 - 2.6.18
    • Do not use with SSDs.

  • CFQ

    • Default for kernels 2.6.19+.
    • Do not use with SSDs.

  • noop

    • Good if you only use SSDs.

  • deadline

    • The best choice for mixed SSDs and HDDs.

anticipatory and cfq achieve a remarkable speedup on standard, platter hard drives by pausing after each read: usually after a process performs a read, it will read again (presumably grabbing another chunk of the same file) in quick succession. This trick will help alleviate seek times, which is the biggest detriment to hard drive performance.

noop performs no logic, and saves CPU cycles. If you use a secondary hard drive, then don't do this. You may end up disliking some of the jerkyness if you are writing and also trying to read.. some reading may end up waiting in line behind the writing.

deadline is suitable for SSDs. Read queues are given a higher priority, because processes usually block on read operations.

Learn what scheduler you're using:

\cat  /sys/block/sda/queue/scheduler

It probably says (notice the one with square brackets):

noop anticipatory deadline [cfq]
\su
\echo  deadline >! /sys/block/sda/queue/scheduler
\cat  /sys/block/sda/queue/scheduler
noop anticipatory [deadline] cfq

Make the change "permanent" by editing /etc/rc.local (rc.local) and adding the echo line.

If you use GRUB, you can edit /boot/grub/menu.lst and append elevator=deadline to the kernel line that you normally boot up with.

kernel (hd0,5)/boot/vmlinuz BOOT_IMAGE=foo root=UUID=foo
=>
kernel (hd0,5)/boot/vmlinuz BOOT_IMAGE=foo root=UUID=foo elevator=deadline

Speed testing ∞

hdparm:

\sudo  \hdparm  -t  /dev/sda
\sudo  \hdparm  -t  --direct  /dev/sda

dd:

\sudo  \dd  if=/dev/sda1  of=/dev/null  bs=4k  skip=0  count=51200

If you want to know how much I/O traffic you have on the SSD, you can check with iostat -m -d /dev/sda. This displays MB_read and MB_wrtn for the given drive (measured since system startup). iostat also works for individual partitions. It is provided by the *sysstat* package.

TODO: Check out iozone

\iozone  -T  -t 4 s 2g  -r 256k  -T  -I  -i0  -i1  -i2

Swap ∞

Kernel swappiness is a controversial topic. The standard setting (60) for kernel swappiness has the effect that inactive (but started) applications will be moved to swap after some time. If the swap partition is placed on a regular HDD in order to reduce wear of the SSD, then reloading the application from swap can take several seconds. This causes annoying delays and bad responsiveness of the system when you return from a coffee break, for example. The solution is setting kernel swappiness to zero, by adding a line vm.swappiness = 0 to /etc/sysctl.conf. A temporary change for trying out the effect is possible by calling echo 0 > /proc/sys/vm/swappiness (as root) from the command line.

Rotational ∞

\cat  /sys/block/sda/queue/rotational

Some SSDs don't identify themselves as being non-rotational.

\sudo  \echo 0 >! /sys/block/sdb/queue/rotational

Notes ∞

One guy said:

One last thing - lately I have started to make labels on all my partitions, and then have something like this in /etc/fstab (fstab): 

LABEL=root / ext3 noatime,minixdf,commit=120,errors=remount-ro 0 1
LABEL=home /home ext3 noatime,minixdf,commit=120,errors=remount-ro 0 2

and

# kopt=root=LABEL=root ro init=/sbin/preinit

.. in /boot/grub/menu.lst (/sbin/preinit is a script that mounts /var/log as an tmpfs before the logging starts + some more.)

This makes it easier to move disks around or replace disks. I'm no longer dependent on root being on /dev/sda1 and home on /dev/sdb2 or anything like that, and not dependent on this ID thing that's default on ubuntu.

Partitionless storage drive ∞

If you do not need several partitions on your SSD, and if you do not want to boot from it (or if you have another drive in your system to store the boot loader on), then explicit alignment can be avoided by creating a filesystem on the device as a whole. You don't need fdisk in this case. Just proceed as follows (with your preferred options to mke2fs, this is only an example):

sudo mke2fs -t ext2 /dev/sda
mke2fs 1.41.1 (01-Sep-2008)
/dev/sda is entire device, not just one partition!
Proceed anyway? (y,n) y
Filesystem label=
OS type: Linux
Block size=4096 (log=2)
...
This filesystem will be automatically checked every 20 mounts or
180 days, whichever comes first.  Use tune2fs -c or -i to override.

One suggestion is:

\mkfs.ext4  -E stripe-width=128  /dev/sda

Resources ∞

SSD ∞

--

notes ∞

When my own Vertex drive arrives, I'll be using fdisk -u to partition it, completely ignoring the antiquated CHS confusion. Much simpler that way, especially when there'll only be one or two partitions in total.

For a scheme with lots of partitions, it might be more convenient then to actually fiddle with the CHS parameters, though I doubt it.

Notes on Windows ∞

msinfo32 - components > storage > disks

  • Divisible by 4096 (partition starting offset)

    • Yes it was

Fresh install should be ok (yes it was)

  • But Linux / cfdisk cannot use the drive! boo!

Windows recovery console

  • Run map for name
  • fixmbr \Device\HardDisk0

  • fixboot C: (worked)

Align Partition without Data Loss ∞

There are some good search phrases like "Aligning Windows Partitions Without Losing Data".

Notes ∞

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