[linux-lvm] Migrating LVM

[Ray Morris]

> Oops, my eyes missed the pipe and second dd when I made my previous
> comments.  That is pretty good for different disks then yes... not
> so good for same physical disk.

    Indeed my tests were done copying from the "old" disk
to the "new" disk, as the OP is doing, I believe.

> I actually have some old hacked up dd code I made once to use 16
> concurrent aio requests with O_DIRECT.  I need to clean it up a
> bit but it showed great promise.

    Considering how often "dd" is used for copying large amounts
of data, even a modest improvement could save many thousands of
hours of admin time.  I would like to encourage you to do any
needed cleanup and make it available, preferably integrated with
GNU dd - it could save hundreds of thousands of dollars worth of
time.
--
Ray Morris
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On 04/21/2010 11:27:22 AM, Phillip Susi wrote:
> On 4/21/2010 11:36 AM, malahal at us.ibm.com wrote:
> > Interesting! You are doing direct I/O to avoid copying from cache  
> to user
> > buffer for read and vice-versa for write, but you are losing the  
> ability
> > to do them parallel! You are doing the next best, that is creating  
> two
> > "dd" threads -- one for reading and another for writing. Since the  
> pipe
> > is really implemented in memory, why should this be faster than  
> normal
> > "dd" that uses page cache? Likely that kswapd is not kicking early
> > enough?
> 
> Oops, my eyes missed the pipe and second dd when I made my previous
> comments.  That is pretty good for different disks then yes... not so
> good for same physical disk.
> 
> > Enhancing "dd" to create a reader and a writer thread would really
> > help, I believe.
> 
> I actually have some old hacked up dd code I made once to use 16
> concurrent aio requests with O_DIRECT.  I need to clean it up a bit  
> but
> it showed great promise.
> 
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> 
>