[linux-lvm] Taking it off list - Re: copy lv from lv?

[Ray Morris]

> If you are experiencing significant performance degradation with
> usage of single  'dd'  with two different driver compared with 2
> piped 'dd' - you should probably make a report on lkml list - There
> is something broken if you gain 'hours' like you said by piping 2 dd
> commands.

I'm taking this off list as off topic, so if anyone else is interested
please let me know and I'll update you on what we find.
-- 
Ray Morris
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On Tue, 05 Apr 2011 23:10:20 +0200
Zdenek Kabelac <zkabelac at redhat.com> wrote:

> Dne 5.4.2011 18:25, Ray Morris napsal(a):
> >> Isn't this already done in a better way by 'dd' using 'zero' copy
> >> mechanism ?? (assuming it is already using either  'mmap()' ->
> >> 'write()' or 'splice()'  as your proposed piped copy is doing a lot
> >> of pointless memory copies and cache trashing.
> > 
> > No, with directio there isn't "a lot of pointless memory copies 
> > and cache trashing". There are in fact fewer mempoy copies than 
> > there would be with a simple:
> > dd if=thing of=thing
> > 
> > See:
> > http://www.ukuug.org/events/linux2001/papers/html/AArcangeli-o_direct.html
> > 
> 
> Seem like 10 years old paper for 2.4.6 kernel - so I do not think it
> still applies to upstream 2.6.38 - thought I've not made any recent
> measuring myself.
> 
> > The proof of the pudding is in the eating, and in fact that 
> > recipe is extremely fast, for this application (sequential 
> > copy of GBs from one device to another). I spent a couple 
> > of hours testing before spending hundreds of hours copying.
> > My bottom line was speed, so I didn't even look at memory
> > usage or CPU.  I would't mind using an extra 64 MB in order
> > to get my copying done 30 hours faster.
> 
> The kernel should be able to use as much free memory as you have - and
> when it's properly configured - disk flushing running in separate
> thread should ensure maximal throughput.
> 
> If you are experiencing significant performance degradation with
> usage of single  'dd'  with two different driver compared with 2
> piped 'dd' - you should probably make a report on lkml list - There
> is something broken if you gain 'hours' like you said by piping 2 dd
> commands.
> 
> (I don't have fast enough arrays myself for such playing)
> 
> Zdenek
> 
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