[linux-lvm] snapshot with bigmem broken

[linux-lvm] snapshot with bigmem broken

[Lauro, John]

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Hello,

 

I did more testing, and it appears that snapshots in LVM will not work
with either 4GB or or 64GB support enabled in the kernel under 2.4.20.
This includes both LVM 1.0.7 or the latest LVM2.  

 

Is there anyone using LVM snapshots with 64GB RAM support (or even 4GB
RAM support, as snapshots would be good for at least our medium size
servers if not our large ones)?  Is so, let me know the kernel version
and LVM version you are using so that I can try it.

 

PS: As mentioned previously, everything else in LVM seems to be ok
with larger kernel memory models.  It's mainly just snapshots that
appear to be broken using the latest LVMs and kernel.

 


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Re: [linux-lvm] snapshot with bigmem broken

[Heinz J . Mauelshagen]

On Wed, May 14, 2003 at 06:50:46AM -0400, Lauro, John wrote:
> Hello,
> 
>  
> 
> I did more testing, and it appears that snapshots in LVM will not work
> with either 4GB or or 64GB support enabled in the kernel under 2.4.20.
> This includes both LVM 1.0.7 or the latest LVM2.  
> 

That's a known issue. You are faced with a VM flaw.

I presume that it's not LVM snapshots only bacause HIGHMEM is
basically broken.

>  
> 
> Is there anyone using LVM snapshots with 64GB RAM support (or even 4GB
> RAM support, as snapshots would be good for at least our medium size
> servers if not our large ones)?  Is so, let me know the kernel version
> and LVM version you are using so that I can try it.

Probably not, because the system lifelocks on bounce buffers (which is
the sympton of the principle HIGHMEM bug).
Obviously not a lot of HIGHMEM users out there raising priorities to fix this.

> 
>  
> 
> PS: As mentioned previously, everything else in LVM seems to be ok
> with larger kernel memory models.  It's mainly just snapshots that
> appear to be broken using the latest LVMs and kernel.
> 
>  
> 

-- 

Regards,
Heinz    -- The LVM Guy --

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

=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-

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

RE: [linux-lvm] snapshot with bigmem broken

[Lauro, John]

> > I did more testing, and it appears that snapshots in LVM will not
work
> > with either 4GB or or 64GB support enabled in the kernel under
2.4.20.
> > This includes both LVM 1.0.7 or the latest LVM2.
> >
> 
> That's a known issue. You are faced with a VM flaw.
> 
> I presume that it's not LVM snapshots only bacause HIGHMEM is
> basically broken.

Can you give more specifics/examples/web links/etc?  Does this mean I
should avoid LVM completely with HIGHMEM?  What if any other problems
should I expect with LVM (and/or other software)?

> > Is there anyone using LVM snapshots with 64GB RAM support (or even
4GB
> > RAM support, as snapshots would be good for at least our medium
size
> > servers if not our large ones)?  Is so, let me know the kernel
version
> > and LVM version you are using so that I can try it.
> 
> Probably not, because the system lifelocks on bounce buffers (which
is
> the sympton of the principle HIGHMEM bug).
> Obviously not a lot of HIGHMEM users out there raising priorities to
fix
> this.

Is this easy to reproduce?  If so, how?  Sounds like I am either a
lucky early adopter of a >4GB Libux system, and/or the problem is rare
even on HIGHMEM systems?

Thanks.

Re: [linux-lvm] snapshot with bigmem broken

[Heinz J . Mauelshagen]

On Fri, May 16, 2003 at 08:38:23AM -0400, Lauro, John wrote:
> > > I did more testing, and it appears that snapshots in LVM will not
> work
> > > with either 4GB or or 64GB support enabled in the kernel under
> 2.4.20.
> > > This includes both LVM 1.0.7 or the latest LVM2.
> > >
> > 
> > That's a known issue. You are faced with a VM flaw.
> > 
> > I presume that it's not LVM snapshots only bacause HIGHMEM is
> > basically broken.
> 
> Can you give more specifics/examples/web links/etc?

No. This is based on our test results.
I reported the highmem problem months ago on linux-kernel.

> Does this mean I
> should avoid LVM completely with HIGHMEM?

No. The problem seems to be limited to snapshots.

> What if any other problems
> should I expect with LVM (and/or other software)?

Any that we and the maintainers of other software are not aware of ;)

> 
> > > Is there anyone using LVM snapshots with 64GB RAM support (or even
> 4GB
> > > RAM support, as snapshots would be good for at least our medium
> size
> > > servers if not our large ones)?  Is so, let me know the kernel
> version
> > > and LVM version you are using so that I can try it.
> > 
> > Probably not, because the system lifelocks on bounce buffers (which
> is
> > the sympton of the principle HIGHMEM bug).
> > Obviously not a lot of HIGHMEM users out there raising priorities to
> fix
> > this.
> 
> Is this easy to reproduce?  If so, how?  Sounds like I am either a
> lucky early adopter of a >4GB Libux system, and/or the problem is rare
> even on HIGHMEM systems?
> 
> Thanks.
> 
> 
> _______________________________________________
> linux-lvm mailing list
> linux-lvm@sistina.com
> http://lists.sistina.com/mailman/listinfo/linux-lvm
> 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 ***

=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-

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

RE: [linux-lvm] snapshot with bigmem broken

[Andrew Rechenberg]

I have HIGHMEM enabled on my kernel but I have to limit the memory to
~6GB for snapshots to work.  I added mem=6000M to my kernel command line
and snapshots work fine.  

I also found this thread on the LKML that has a patch that may fix the
problem:

http://marc.theaimsgroup.com/?l=linux-kernel&m=104549874128521&w=2

The only machine we have that has >4GB RAM is a production box and I
really don't want to test something in production (call me crazy :) ).

Here are my specs:

Kernel 2.4.18-27.SHRbigmem
LVM 1.0.7

Hope this helps,
Andy.


On Fri, 2003-05-16 at 08:38, Lauro, John wrote:
> > > I did more testing, and it appears that snapshots in LVM will not
> work
> > > with either 4GB or or 64GB support enabled in the kernel under
> 2.4.20.
> > > This includes both LVM 1.0.7 or the latest LVM2.
> > >
> > 
> > That's a known issue. You are faced with a VM flaw.
> > 
> > I presume that it's not LVM snapshots only bacause HIGHMEM is
> > basically broken.
> 
> Can you give more specifics/examples/web links/etc?  Does this mean I
> should avoid LVM completely with HIGHMEM?  What if any other problems
> should I expect with LVM (and/or other software)?
> 
> > > Is there anyone using LVM snapshots with 64GB RAM support (or even
> 4GB
> > > RAM support, as snapshots would be good for at least our medium
> size
> > > servers if not our large ones)?  Is so, let me know the kernel
> version
> > > and LVM version you are using so that I can try it.
> > 
> > Probably not, because the system lifelocks on bounce buffers (which
> is
> > the sympton of the principle HIGHMEM bug).
> > Obviously not a lot of HIGHMEM users out there raising priorities to
> fix
> > this.
> 
> Is this easy to reproduce?  If so, how?  Sounds like I am either a
> lucky early adopter of a >4GB Libux system, and/or the problem is rare
> even on HIGHMEM systems?
> 
> Thanks.
> 
> 
> _______________________________________________
> linux-lvm mailing list
> linux-lvm@sistina.com
> http://lists.sistina.com/mailman/listinfo/linux-lvm
> read the LVM HOW-TO at http://tldp.org/HOWTO/LVM-HOWTO/
-- 
Andrew Rechenberg <arechenberg@shermfin.com>
Infrastrucutre Team, Sherman Financial Group