[linux-lvm] Red Hat Advanced Server + LVM

[Heinz J . Mauelshagen]

On Fri, Jun 20, 2003 at 04:29:54PM +0100, Geoff Dolman wrote:
> On Fri, 2003-06-20 at 12:02, Heinz J . Mauelshagen wrote:
> > On Fri, Jun 20, 2003 at 10:50:32AM +0100, Geoff Dolman wrote:
> > > Hi
> > > 
> > > I am soon to take delivery of a new Server: It's configuration is to be
> > > 
> > > 584GB disk (hardware raid5) 4 cpu (with Intel hyperthreading)
> > > 
> > > 32 GB RAM
> > > 
> > > 
> > > I intend to use RH Advanced Server 2.1 (whatever the latest kernel is at
> > > time of install).
> > > 
> > > I would ideally like to be able to snapshot dump this machine (the
> > > largest partition will use most of the available disk space (4-500GB)).
> > > 
> > > Is there any caveat or issue that I should be aware of with lvm in this
> > > configuration?
> > 
> > Snapshots trigger a virtual memory problem with highmem enabled in kernel 2.4 :(
> > 
> > The snapshot exception tables hold in RAM can grow quite big in case
> > of large snapshot sizes (which you might not need in case your filesystem(s)
> > are not heavily updated during the lifetime of the snapshot).
> > 
> 
> Hi Heinz
> 
> What do you recommend I do?

Because of the highmem problems which are not fixed in 2.4.21
TTBOMK (and I am afraid they wonÄt be before 2.6), you can run
snapshots without highmem enabled only, which for obvious
reasons can't be an option you prefer ;)

> What is (if any) the largest filesystem I
> can snapshot in this set-up?

Depends on your memory load and fs update ratio during a snapshots lifetime.
With 1GB of RAM used only, that would be up to ~512GB snapshot size (roughly
calculated) presuming you could use have of your RAM for the memory resident
snapshot excpetion table and go with 32k snapshot chunk size.

The ~512GB are the upper limit of changes to the original LV (which is a lot).

As a rule of thumb, you can calculate with ~5% changes to your original LV,
which means you'ld need a ~100GB snapshot for a 2TB original LV.

> 
> -- 
> Geoff Dolman <geoff.dolman at cimr.cam.ac.uk>
> 
> _______________________________________________
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> read the LVM HOW-TO at http://tldp.org/HOWTO/LVM-HOWTO/

-- 

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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