CentOS - Oct 2009 - Stripe vs Cylinder alignement...

Hi,

I modified my kickstart to do some custom partioning and formating in a
pre-install script.
I am trying to align the partitions on the RAID stripe (and format with a
correct stride).
But, sfdisk complains that it does not start/end on a cylinder boundary (used -L
option to limit complaining).
Since the cylinder size is not a multiple of the stripe size, I cannot align on
both.
I tried to align the begining on the stripe and the end on the end of a
cylinder, but sfdisk still compains...
Basicaly, I have a 128KB (256 sectors) stripe, and 255*32 = 8160 sectors
cylinders.
What I am doing is:
begin = ( begin / 256 ) * 256
end = ( end / 8160 ) * 8160 -1
So, for my first partition (96MB):
  begin=256
  size = ( ( 96 * 1024 * 2 ) / 8160 ) * 8160 = 195840 sectors
  end = 195840 - 1 - 256 = 195583
Any idea what I am doing wrong in my calculations or logic?

Thx,
JD

> So, for my first partition (96MB):
>   begin=256
>   size = ( ( 96 * 1024 * 2 ) / 8160 ) * 8160 = 195840 sectors
>   end = 195840 - 1 - 256 = 195583
Oops, I meant:

  begin = 256
  size = 96 * 1024 * 2 = 196608 sectors
  end = ( ( begin + size ) / 8160 ) * 8160 - 1 = 195839
  aligned_size = end - begin + 1 = 195584

Maybe... I am a bit confused  ^_^

JD

Cylinders are largely logical - even in magnetic disks you have no way 
of knowing if the logical cylinder matches up with the physical 
construct of a cylinder on the disk medium - in any modern(15 years ?) 
disk they won't. Don't worry about cylinders, just align your fs to the 
stripe/sector.

Obviously, the concept of a cylinder starts to go out the window with 
RAID, SSD's, etc.

More information about partition alignment here - 
http://www.ocztechnologyforum.com/forum/showpost.php?p=335049&postcount=134

--Blake

-------- Original Message  --------
Subject: [CentOS] Stripe vs Cylinder alignement...
From: John Doe <jdmls at yahoo.com>
To: centos at centos.org
Date: Friday, October 30, 2009 9:45:59 AM
> Hi,
>
> I modified my kickstart to do some custom partioning and formating in a
pre-install script.
> I am trying to align the partitions on the RAID stripe (and format with a
correct stride).
> But, sfdisk complains that it does not start/end on a cylinder boundary
(used -L option to limit complaining).
> Since the cylinder size is not a multiple of the stripe size, I cannot
align on both.
> I tried to align the begining on the stripe and the end on the end of a
cylinder, but sfdisk still compains...
> Basicaly, I have a 128KB (256 sectors) stripe, and 255*32 = 8160 sectors
cylinders.
> What I am doing is:
> begin = ( begin / 256 ) * 256
> end = ( end / 8160 ) * 8160 -1
> So, for my first partition (96MB):
>   begin=256
>   size = ( ( 96 * 1024 * 2 ) / 8160 ) * 8160 = 195840 sectors
>   end = 195840 - 1 - 256 = 195583
> Any idea what I am doing wrong in my calculations or logic?
>
> Thx,
> JD
>
>
>       
> _______________________________________________
> CentOS mailing list
> CentOS at centos.org
> http://lists.centos.org/mailman/listinfo/centos
>

On 10/30/09, John Doe <jdmls at yahoo.com> wrote:
> Hi,
>
> I modified my kickstart to do some custom partioning and formating in a
> pre-install script.
> I am trying to align the partitions on the RAID stripe (and format with a
> correct stride).
> But, sfdisk complains that it does not start/end on a cylinder boundary
> (used -L option to limit complaining).
> Since the cylinder size is not a multiple of the stripe size, I cannot
align
> on both.
> I tried to align the begining on the stripe and the end on the end of a
> cylinder, but sfdisk still compains...
> Basicaly, I have a 128KB (256 sectors) stripe, and 255*32 = 8160 sectors
> cylinders.
> What I am doing is:
> begin = ( begin / 256 ) * 256
> end = ( end / 8160 ) * 8160 -1
> So, for my first partition (96MB):
>   begin=256
>   size = ( ( 96 * 1024 * 2 ) / 8160 ) * 8160 = 195840 sectors
>   end = 195840 - 1 - 256 = 195583
> Any idea what I am doing wrong in my calculations or logic?
>
Logic!  Well not logic but knowledge about the physical layer inside the disk.

Most disks today do not have a constant number of blocks
per track/ cylinder.   Most disk partitioning tools just guess
and do some rounding.   There can be one, two or even three
zones on the surface with different blocks per track on each.

One strategy is to look at the buffer setup and size of a disk
and match stripe sizes to work with memory not blocks per cylinder.
Note that a drive with an 8MB buffer uses some for read and some
for write.   Some research or testing can expose the ratio.

Another strategy is to partition devices exactly the same.

LBA (Logical block addressing) hides the physical layer....




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