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Re: [linux-lvm] Snapshot Problem JFS



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 sistina com] 
> 	Sent: Fri 6/20/2003 9:50 AM 
> 	To: linux-lvm 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 sistina com]
> 	>       Sent: Fri 6/20/2003 4:09 AM
> 	>       To: linux-lvm 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 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 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 Sistina com                           +49 2626 141200
                                                       FAX 924446
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