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Date: Fri, 18 Oct 2002 16:03:37 +0100
To: christophe.varoqui@free.fr
Cc: linux-kernel@vger.kernel.org
Subject: Re: block allocators and LVMs
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From: Joe Thornber <joe@fib011235813.fsnet.co.uk>
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On Fri, Oct 18, 2002 at 03:04:24PM +0200, christophe.varoqui@free.fr wrote:
> I realize I didn't pick the right words (from my poor English 
> dictionnary) : I meant an extend remapper rather than a block remapper. 

extent remapper.

> As far as I can see, this task can be done entirely from userland : 
>  
> o per-extend IO counters exported from kernel-space can be turned into 
>   a list of extends sorted by activity 
>  
> o lvdisplay-like tool gives the mapping extend<->physical blocks 
>  
> o a scheduled job in user-space should be able to massage this info to 
>   decide where to move low-access-rate-extends to the border of the 
>   platter and pack high-access-rate-extends together ... all in one run 
>   that can be scheduled at low activity period (cron defrag way) 
>  
> The algorithm could be something along the line of : 
>  
> while top_user_queue_not_empty 
> do 
>   extend = dequeue_lowest_user_extend 
>   if extend_in_good_spot 
>   then 
>     move_extend_to_corner_destination 
>     find_highest_user_extend_in_bad_spot 
>     move_this_extend_to_freed_good_spot 
>   fi 
> done 

What you describe could be very beneficial, especially if you start
striping the high bandwidth areas.  However in no way could this be
described as 'online FS defragmentation'.

- Joe
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