On Thu, 10 May 2007, Liang Yang wrote:

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).