Re: slow mdadm reshape, normal?

[Michael Ole Olsen <gnu@gmx.net>]

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Thanks

I was just hoping for something like 127MB/s divided by 3 = 42MB/s which 
is the practical limit of the pci bus assuming each disk gets its equal
share.

6MB/s is only 14% of that, so I wonder where the lost bandwidth goes :)

The reshape takes only 10-15% cpu or so on my 3ghz cpu so dont think that is
the bottleneck.

Might be interesting to test a pci network controller at the same time had I
installed one before the build, that way i could see if the pci bus really got
congested.


On Thu, 11 Jun 2009, Robin Hill wrote:

> On Thu Jun 11, 2009 at 01:36:59AM +0200, Michael Ole Olsen wrote:
> 
> > the slow disks (them with 85MB/s) in the posting are the one on the pci 4x sata controller
> > 
> > the 32bit pci sata (sata_sil) controller has 3 disks connected
> > 
> > the onboard sata2 6x sata connectors have 6 disks connected 
> > (p5n32-e sli motherboard), i believe all these are sata2 and not shared with
> > an ide controller.
> > 
> > so only the pci controller should have bus limitations, but i wonder why it is
> > that slow on reshape compared to build.
> > 
> The build was reading (sequentially) from all drives and writing
> (sequentially) only to one, whereas a reshape is reading from and
> writing to all the drives (including seeking between reads & writes).
> This means it is inevitably a lot slower.  In addition, you're now using
> 3 rather than 2 drives on the PCI bus, which will increase the
> congestion.
> 
> Cheers,
>     Robin
> -- 
>      ___        
>     ( ' }     |       Robin Hill        <robin@robinhill.me.uk> |
>    / / )      | Little Jim says ....                            |
>   // !!       |      "He fallen in de water !!"                 |



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