[linux-lvm] Snapshot Problem JFS
Heinz J . Mauelshagen
mauelshagen at sistina.com
Mon Jun 23 09:24:02 UTC 2003
On Fri, Jun 20, 2003 at 12:29:15PM -0500, David Sornig wrote:
> Heinz,
>
> Thank you for your help. Just want to make sure I understand perfectly what should happen.
>
> I have a Volume group with 4.2 T of space. In this Volume Group I have a Logical Volume called LogVol00 that is 1.4 T in size. Currently the LogVol has 400G of Data on it. In order to allow for approx 5% growth I need a snapshot of 420G or 1.470T. This is were I am confused.
>
If you want to allow for 5% changes on the original LV (1.4 TB), you
want a snapshot size of 70 GB. Please remember: the snapshot only stores
the changes to the original LV.
> so I think I need the follwoing command. I am not in front of the system to test this so I am asking before I go forward since our live data is on this server.
>
> lvcreate - L 1.470T -s -n snap00 /dev/Volume00/LogVol00
lvcreate -L 70G -s -n snap00 /dev/Volume00/LogVol00
>
> The reason I am trying to clarify is that I will be creating and destroying these using scripts and I do not want to have aproblem down the road without fully understanding.
>
You can check with "lvdisplay /dev/Volume00/snap00" hw much space is
already used and if space isn't sufficient, you can grow it with lvextend.
Regards,
Heinz -- The LVM Guy --
> Thanks again for all of your help and sorry for being such a stickler for details.
>
> David
>
> -----Original Message-----
> From: Heinz J . Mauelshagen [mailto:mauelshagen at sistina.com]
> Sent: Fri 6/20/2003 9:50 AM
> To: linux-lvm at sistina.com
> Cc:
> Subject: Re: [linux-lvm] Snapshot Problem JFS
>
>
>
> On Fri, Jun 20, 2003 at 06:43:40AM -0500, David Sornig wrote:
> > Heinz,
> >
> > So to make a snapshot of the 1.4T LV which currently has a 400G of space being used by data I need to run the lvcreate command like this:
> >
> > lvcreate -L1000M -s -n snap00 /dev/Volume00/LogVol00
>
> Yes, if you want to have a snapshot with a 1000MB exception store.
>
> >
> > I guess I thought the -L needed to be the same size as the current logical volume?
>
> No, it doesn't need unless you have an update ratio of 100% to the original
> during the lifetime of the snapshot. As a rule of thumb you can assume 5%
> (which would be ~71g in your case) but that heavily depends on your
> usage of the filesystem for obvious reasons.
>
> You can use lvdisplay on the snapshot to retrieve how much space is in use
> and if it becomes full, you can lvextend it.
>
> Regards,
> Heinz -- The LVM Guy --
>
>
> >
> > Thanks,
> >
> > David
> >
> >
> > -----Original Message-----
> > From: Heinz J . Mauelshagen [mailto:mauelshagen at sistina.com]
> > Sent: Fri 6/20/2003 4:09 AM
> > To: linux-lvm at sistina.com
> > Cc:
> > Subject: Re: [linux-lvm] Snapshot Problem JFS
> >
> >
> >
> > On Thu, Jun 19, 2003 at 09:07:30PM -0500, David Sornig wrote:
> > > I am running RedHat 9.0
> > > Kernel 2.4.20-8
> > > LVM Version 1.0.5+(22/07/2002)
> > >
> > > I have a 1.4T LogVol with 400G of space being used on a JFS file system.
> > >
> > > The LVM is working excellent. However, I cannot create a snapshot. I run the following lvcreate command as per the man page and get the following error:
> > >
> > > lvcreate --error "Cannot allocate memory" creating VGDA for "/dev/Volume00/snap01" in kernel
> > >
> >
> > David,
> >
> > memory allocated to the snapshot needs free physical RAM.
> >
> > With 400GB and default snapshot extent size we're talking about ~1GB
> > (using virtual memory for snapshot exception tables so that the table doesn't
> > need to be in RAM completely any longer is a work item for LVM2).
> >
> > Do you really expect that much change to your 1.4TB LV during the lifetime of
> > the snapshot ?
> > If you use it for a backup, you might get along with a couple of GB allocated
> > to the snapshot reducing exception table size drastically.
> >
> >
> > > Not sure why this is happening. I have read the list and maybe I just don't understand the problem. These servers have 4 Gigs of RAM.
> > >
> > > Regards,
> > >
> > > David
> >
> > --
> >
> > Regards,
> > Heinz -- The LVM Guy --
> > _______________________________________________
> > linux-lvm mailing list
> > linux-lvm at sistina.com
> > http://lists.sistina.com/mailman/listinfo/linux-lvm
> > read the LVM HOW-TO at http://tldp.org/HOWTO/LVM-HOWTO/
> >
>
> _______________________________________________
> linux-lvm mailing list
> linux-lvm at sistina.com
> http://lists.sistina.com/mailman/listinfo/linux-lvm
> read the LVM HOW-TO at http://tldp.org/HOWTO/LVM-HOWTO/
>
>
*** 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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