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Re: [linux-lvm] Why can't I get more bandwidth by adding more disks?



You've probably saturated the SCSI bus the disks are connected too. With
the newer disks, they are fully capable of flooding a bus with sequential
access. 

On a side note, a more useful test would be to take those same disks and
scatter the I/O across the RAID set randomly. This way you should see
nearly linear scaling of the bandwidth (depending on the block size of
your disk).

	-- Cheers
	-- James

On Fri, 10 Aug 2001, Xiaoxiang Liu wrote:

> I have tried to upgrade LVM to beta8 on kernel 2.4.7 and it
> works well. Then I am trying to do some raw io test to see
> how much bandwidth I can get. My machine is HP kayak and connected
> with a MTI RAID by fiber channel.
> 
> If I do sequence rawio on one scsi disk in MTI, the bandwidth is 14MB/s.
> Then I create a LV on two disks with striping size 4k, the bandwidth
> becomes 28MB/s and I really feel excited for this result. :)
> But when I create LV on more disks (3,4,5 or 8 disks), the bandwidth
> can't increase any more. I always get 28MB/s or less. I do 1G-byte sequence 
> rawio and my buffer size is 32k. The maximum bandwidth of fiber channel is
> 100MB/s and we could actually get 75MB/s in other tests.
>  
> I don't know what's the bottleneck here? If I create LV on 4 disks with
> striping size 4K and do 32k rawio, does and how LVM do parallel io on
> those 4 disks? Why can't I increase the bandwidth by 4 times? 
> Please help me! Your suggestions will be highly appreciated!
> 
> 
> Xiang
> 
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> 



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