[linux-lvm] LVM and striping, defragmentation

[Heinz J. Mauelshagen]

On Mon, Feb 26, 2001 at 08:33:58PM +0000, Joe Thornber wrote:
> On Mon, Feb 26, 2001 at 05:04:41PM +0100, Urs Thuermann wrote:
> > I'd like to experiment with LVM and striping.  The HOWTO at
> > www.linuxdoc.org says in section 8.3 LVM native striping
> > 
> >     Performance notices
> > 
> >     The performance 'gain' may well be very negative if you stripe over 2
> >     partitions of the same disk - take care to prevent that.
> > 
> > 
> > How can I take care of it?  I've seen no option in lvcreate to specify
> > where the PE should be taken from.  If I have a VG with only two PVs on
> > two different disks, will LVM automatically choose PEs from the two
> > PVs instead of one PV?  And what if I have a VG with several PVs on
> > disk A and several PVs on disk B.  How can I "take care to prevent"
> > that LVM does not choose two PEs from the same disk for striping?

Having more than one PV mapped to a physical device is *not* the recommended
production configuration. The only reason why we build in that option is
flexibility in test configurations.

> 
> You have highlighted an area where we want to do more work.
> Specifically we want to write a tool that allows you more control over
> the LE->PE mapping.  The striping allocation will try and allocate
> LE's from two different PV's in the VG.  When you create the striped
> LV just try not to have two PV's on the same disk.  You can always
> disable allocation on an individual PV basis.

Check the lvcreate man page, please.
You can add a list of PV device specials to allocate the PEs from to the
lvcreate command line or use pvchange in order to prohibit allocation on
that PV before using lvcreate.

> 
> > When I resize my LVs in a VG multiple times, I can get some
> > fragmentation, i.e. LVs in the VG are not contiguous.  Is there a tool
> > to defragment LVs, i.e. to swap PEs so that all LVs in the VG are
> > contiguous again.  I read in the man page that the allocation policy
> > can be set to "contiguous", but how can I make LVs contiguous after
> > some creating, removing, and resizing of LVs?
> 
> At the moment it's not easy, you might want to look at the pvmove
> documentation.
> 
> > How much performance loss is to be expected in case of non-contiguous LVs?
> 
> It shouldn't be large, the LE's are normally 4 meg so the seeks to
> different parts of the disk won't be that frequent.  To contrast with
> stripes where the chunk size defaults to 64K, which can cause a lot of
> thrashing if the stripes are on the same disk.
> 
> - Joe
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-- 

Regards,
Heinz    -- The LVM Guy --

*** Software bugs are stupid.
    Nevertheless it needs not so stupid people to solve them ***

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Heinz Mauelshagen                                 Sistina Software Inc.
Senior Consultant/Developer                       Am Sonnenhang 11
                                                  56242 Marienrachdorf
                                                  Germany
Mauelshagen at Sistina.com                           +49 2626 141200
                                                       FAX 924446
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