The (up to) 30% percent figure is mentioned here:
http://insights.oetiker.ch/linux/raidoptimization.html
On http://forums.storagereview.net/index.php?showtopic=25786:
This user writes about the problem:
XP, and virtually every O/S and partitioning software of XP's day, by default
places the first partition on a disk at sector 63. Being an odd number, and
31.5KB into the drive, it isn't ever going to align with any stripe size. This
is an unfortunate industry standard.
Vista on the other hand, aligns the first partition on sector 2048 by default
as a by-product of it's revisions to support large-sector sized hard drives. As
RAID5 arrays in write mode mimick the performance characteristics of
large-sector size hard drives, this comes as a great if not inadvertent
benefit. 2048 is evenly divisible by 2 and 4 (allowing for 3 and 5 drive arrays
optimally) and virtually every stripe size in common use. If you are however
using a 4-drive RAID5, you're SOOL.
Page 9 in this PDF (EMC_BestPractice_R22.pdf) shows the problem graphically:
http://bbs.doit.com.cn/attachment.php?aid=6757
------
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
---
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?
What is the best (and correct) way to calculate stripe-alignment on the
RAID5 device itself?
---
The EMC paper recommends:
Disk partition adjustment for Linux systems
In Linux, align the partition table before data is written to the LUN, as
the partition map will be rewritten
and all data on the LUN destroyed. In the following example, the LUN is
mapped to
/dev/emcpowerah, and the LUN stripe element size is 128 blocks. Arguments
for the fdisk utility are
as follows:
fdisk /dev/emcpowerah
x # expert mode
b # adjust starting block number
1 # choose partition 1
128 # set it to 128, our stripe element size
w # write the new partition
---
Does this also apply to Linux/SW RAID5? Or are there any caveats that are
not taken into account since it is based in SW vs. HW?
---
What it currently looks like:
Command (m for help): x
Expert command (m for help): p
Disk /dev/sdc: 255 heads, 63 sectors, 18241 cylinders
Nr AF Hd Sec Cyl Hd Sec Cyl Start Size ID
1 00 1 1 0 254 63 1023 63 293041602 fd
2 00 0 0 0 0 0 0 0 0 00
3 00 0 0 0 0 0 0 0 0 00
4 00 0 0 0 0 0 0 0 0 00
Justin.
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