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" <redacted>
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.

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).