On Wed, 19 Dec 2007, Bill Davidsen wrote:

Justin Piszcz wrote:

quoted

On Wed, 19 Dec 2007, Mattias Wadenstein wrote:

quoted

On Wed, 19 Dec 2007, Justin Piszcz wrote:

quoted

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Now to my setup / question:

# fdisk -l /dev/sdc

Disk /dev/sdc: 150.0 GB, 150039945216 bytes
255 heads, 63 sectors/track, 18241 cylinders
Units = cylinders of 16065 * 512 = 8225280 bytes
Disk identifier: 0x5667c24a

  Device Boot      Start         End      Blocks   Id  System
/dev/sdc1               1       18241   146520801   fd  Linux raid 
autodetect

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If I use 10-disk RAID5 with 1024 KiB stripe, what would be the correct 
start and end size if I wanted to make sure the RAID5 was stripe aligned?

Or is there a better way to do this, does parted handle this situation 
better?

quoted

From that setup it seems simple, scrap the partition table and use the 

disk device for raid. This is what we do for all data storage disks (hw 
raid) and sw raid members.

/Mattias Wadenstein

Is there any downside to doing that?  I remember when I had to take my 
machine apart for a BIOS downgrade when I plugged in the sata devices again 
I did not plug them back in the same order, everything worked of course but 
when I ran LILO it said it was not part of the RAID set, because /dev/sda 
had become /dev/sdg and overwrote the MBR on the disk, if I had not used 
partitions here, I'd have lost (or more of the drives) due to a bad LILO 
run?

As other posts have detailed, putting the partition on a 64k aligned boundary 
can address the performance problems. However, a poor choice of chunk size, 
cache_buffer size, or just random i/o in small sizes can eat up a lot of the 
benefit.

I don't think you need to give up your partitions to get the benefit of 
alignment.

-- 
Bill Davidsen [off-list ref]
"Woe unto the statesman who makes war without a reason that will still
be valid when the war is over..." Otto von Bismark 

Hrmm..

I am doing a benchmark now with:

6 x 400GB (SATA) / 256 KiB stripe with unaligned vs. aligned raid setup.

unligned, just fdisk /dev/sdc, mkpartition, fd raid.
  aligned, fdisk, expert, start at 512 as the off-set

Per a Microsoft KB:

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).

So I should start at 512 for a 256k chunk size.

I ran bonnie++ three consecutive times and took the average for the 
unaligned, rebuilding the RAID5 now and then I will re-execute the test 3 
additional times and take the average of that.

Justin.