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RAID Array Stripe Alignment affect Performance

raid alignment gpt

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#1 klau



  • Joined: 13-November 01

Posted 02 November 2012 - 06:55

Just found out that RAID arrays can have alignment issues, separate from the well known 4K sector issue.

I suspect it to be the cause of slow transfer rates on my Adaptec RAID 3805 RAID6 (5x Hitachi 2TB 7200 RPM) array.

Nodesoft Disk Bench only gets 100MB/s writes and 80MB/s reads (these figures are more in line my Windows File Copy performance)

But I don't know why the following benchmarks report so much higher


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I think I have confirmed that my array is not aligned, but am not sure.

State of Array:
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Alignment Calculator:
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Alignment Checking Script:

Device ID: Disk #2, Partition #0
Total Volume Size: 5 TB
Starting Offset: 134235136 (131089 KB)
Partition Alignment: ** NOT ALIGNED!! **

VM Check Alignment Tool:
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My Calculation:
Formula: ((Partition offset) * (Disk sector size)) / (Stripe unit size)
Source: http://support.microsoft.com/kb/929491

Partition off set = 134235136b
Disk sector size = 512b
stripe unit size = 262144b
block size = 512b

answer = 262178

This is an integer, thus indicates the array is aligned. But this conflicts with the conclusions of the other calculators, so WTF? I have no idea what I'm doing lol :weep:

Array Info:
Adaptec RAID 3805 RAID6
HD 5x (Hitachi 2TB) Sector Size: 512Bytes
Stripe Size: 256K
Disk Format: GPT
Windows Disc mode, Basic or Dynamic: Basic

#2 +PeterUK


    Neowinian Senior

  • Tech Issues Solved: 5
  • Joined: 26-March 07

Posted 03 November 2012 - 00:28

I think you might be right as a GPT the Partition Starting Offset should be 135,266,304 bytes.