[linux-lvm] pvmove doesn't work the way the man page says

[linux-lvm] pvmove doesn't work the way the man page says

[Phillip Susi]

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LVM 2.02.66-4ubuntu7 on Ubuntu 12.04 LTS:

The man page for pvmove says it creates many temporary mirror segments
to sync each segment then breaks the mirror and mirrors the next
segment, thus allowing the move to be resumed after a reboot
approximately where it left off.

This does not appear to be the case.  Looking at dmsetup table during
a move of two logical volumes, there is one mirror segment for the
entire logical volume being moved, and a linear for the next logical
volume.  So it appears that logical volumes are copied as a whole unit
and a reboot will restart the copy from the beginning.

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Re: [linux-lvm] pvmove doesn't work the way the man page says

[Spelic]

On 02/13/13 16:43, Phillip Susi wrote:
> there is one mirror segment for the
> entire logical volume being moved, and a linear for the next logical
> volume.

 >  2.  Every  logical  volume in the volume group is searched for 
contiguous data that need moving according to the command line arguments.
 >       For each piece of data found, a new segment is added to the end 
of the pvmove LV.  This segment takes the form of a temporary mirror

so it DOES behave according to the manpage

breaking it up in smaller subsegments would probably have been wiser, 
except that you risk to induce fragmentation if other LVM operations 
happen during the move.

Re: [linux-lvm] pvmove doesn't work the way the man page says

[Alasdair G Kergon]

On Thu, Feb 14, 2013 at 01:45:35AM +0100, Spelic wrote:
> breaking it up in smaller subsegments would probably have been wiser,  

The plan was always for more frequent checkpoints, but it involves more
complicated code and there's simply been no need to do this: given how rarely
pvmove is used, the current approach seems to be good enough and nobody's
written the extra code yet.

Alasdair

Re: [linux-lvm] pvmove doesn't work the way the man page says

[Marian Csontos]

On 02/14/2013 01:58 AM, Alasdair G Kergon wrote:
> On Thu, Feb 14, 2013 at 01:45:35AM +0100, Spelic wrote:
>> breaking it up in smaller subsegments would probably have been wiser,
>
> The plan was always for more frequent checkpoints, but it involves more
> complicated code and there's simply been no need to do this: given how rarely
> pvmove is used, the current approach seems to be good enough and nobody's
> written the extra code yet.
>
> Alasdair

Should not then the man page be fixed to match actual behavior (and not 
only at this place)? Otherwise it is a wish list.

Bugzilla is IMO more appropriate tracker for RFEs.

-- Marian


>
> _______________________________________________
> linux-lvm mailing list
> linux-lvm@redhat.com
> https://www.redhat.com/mailman/listinfo/linux-lvm
> read the LVM HOW-TO at http://tldp.org/HOWTO/LVM-HOWTO/

Re: [linux-lvm] pvmove doesn't work the way the man page says

[Alasdair G Kergon]

On Thu, Feb 14, 2013 at 02:25:46PM +0100, Marian Csontos wrote:
> Should not then the man page be fixed to match actual behavior (and not  

As already stated on this thread, the man page does describe what
the code currently does.

Alasdair

Re: [linux-lvm] pvmove doesn't work the way the man page says

[Marian Csontos]

On 02/14/2013 02:41 PM, Alasdair G Kergon wrote:
> On Thu, Feb 14, 2013 at 02:25:46PM +0100, Marian Csontos wrote:
>> Should not then the man page be fixed to match actual behavior
>> (and not
>
> As already stated on this thread, the man page does describe what
> the code currently does.

I must apologize! Excuse me for being an ... (donkey?).

I had been reading it just few days ago, and got confused as well and 
would be almost willing to testify...

Rereading it carefully again, knowing what it is supposed to say, it 
really does say so.

-- Marian

>
> Alasdair
>
> _______________________________________________ linux-lvm mailing
> list linux-lvm@redhat.com
> https://www.redhat.com/mailman/listinfo/linux-lvm read the LVM
> HOW-TO at http://tldp.org/HOWTO/LVM-HOWTO/

Re: [linux-lvm] pvmove doesn't work the way the man page says

[Lars Ellenberg]

On Thu, Feb 14, 2013 at 12:58:54AM +0000, Alasdair G Kergon wrote:
> On Thu, Feb 14, 2013 at 01:45:35AM +0100, Spelic wrote:
> > breaking it up in smaller subsegments would probably have been wiser,  
> 
> The plan was always for more frequent checkpoints, but it involves more
> complicated code and there's simply been no need to do this: given how rarely
> pvmove is used, the current approach seems to be good enough and nobody's
> written the extra code yet.

Also, if you want more frequent checkpoints,
you can do that by calling pvmove appropriately:
just don't move the whole thing at once,
but repeatedly call pvmove with an explicit extent range.

	Lars

Re: [linux-lvm] pvmove doesn't work the way the man page says

[Phillip Susi]

Please use reply-to-all on public mailing lists; I did not see there had 
been any replies until now because I was not Cc'd.

On 02/13/2013 07:45 PM, Spelic wrote:
> breaking it up in smaller subsegments would probably have been wiser,
> except that you risk to induce fragmentation if other LVM operations
> happen during the move.

There's no risk of fragmentation because you still allocate the whole 
thing at once, you just set it up as (linear, mirror, linear) instead of 
one big mirror, and the mirror portion slides over when synced.