http://partition.radified.com/partitioning_2.htm

System and program files that wind up at the far end of the drive take 
longer to access, and are transferred at a slower rate, which translates 
into a less-responsive system. If you look at the graph of sustained 
transfer rates (STRs) from the HD Tach benchmark posted here, you'll see 
clearly that the outermost sectors of the drive transfer data the fastest.


On Thu, 10 May 2007, Liang Yang wrote:

Could you please give me more details about this? What do you mean the fast 
end and slow end part of disk? Do you mean the location in each disk platter?

Thanks,

Liang


----- Original Message ----- From: "Justin Piszcz" [off-list ref]
To: "Liang Yang" <redacted>
Cc: <redacted>
Sent: Thursday, May 10, 2007 2:33 PM
Subject: Re: Questions about the speed when MD-RAID array is being 
initialized.

quoted

On Thu, 10 May 2007, Liang Yang wrote:

quoted

Hi,

I created a MD-RAID5 array using 8 Maxtor SAS Disk Drives (chunk size is 
256k). I have measured the data transfer speed for single SAS disk drive 
(physical drive, not filesystem on it), it is roughly about 80~90MB/s.

However, I notice MD also reports the speed for the RAID5 array when it is 
being initialized (cat /proc/mdstat). The speed reported by MD is not 
constant which is roughly from 70MB/s to 90MB/s (average is 85MB/s which 
is very close to the single disk data transfer speed).

I just have three questions:
1. What is the exact meaning of the array speed reported by MD? Is that 
mesured for the whole array (I used 8 disks) or for just single underlying 
disk? If it is for the whole array, then 70~90B/s seems too low 
considering 8 disks are used for this array.

2. How is this speed measured and what is the I/O packet size being used 
when the speed is measured?

3. From the beginning when MD-RAID 5 array is initialized to the end when 
the intialization is done, the speed reports by MD gradually decrease from 
90MB/s down to 70MB/s. Why does the speed change? Why does the speed 
gradually decrease?

Could anyone give me some explanation?

I'm using RHEL 4U4 with 2.6.18 kernel. MDADM version is 1.6.

Thanks a lot,

Liang




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For no 3. because it starts from the fast end of the disk and works its way 
to the slower part (slower speeds).

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