[linux-lvm] Snapshot Problem JFS

[David Sornig]

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.  
 
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
 
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.  
 
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/
	>      
	
	
	*** 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 at Sistina.com                           +49 2626 141200
	                                                       FAX 924446
	=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-
	
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