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List:       evms-devel
Subject:    Re: [Evms-devel] 30 snapshot volume device node limit
From:       "Michael Yao" <m9230 () cn ! ee ! ccu ! edu ! tw>
Date:       2006-09-18 1:49:19
Message-ID: 20060918014622.M86955 () cn ! ee ! ccu ! edu ! tw
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On Mon, 18 Sep 2006 09:42:42 +0800, Michael Yao -«À§Ó¿² wrote
> -----Original Message-----
> From: evms-devel-bounces@lists.sourceforge.net [mailto:evms-devel-
> bounces@lists.sourceforge.net] On Behalf Of Michael Yao Sent: 
> Thursday, September 14, 2006 8:27 PM To: evms-devel@lists.sourceforge.net
> Subject: [Evms-devel] 30 snapshot volume device node limit
> 
> Hi, all
> 
> I created a lvm container and create a data volume on top of it.
> I take 30 snapshots, there are 30 device nodes in /dev/evms/
> Then I take 31st snapshot, there are still 30 device nodes in /dev/evms/
> It seems to have limitations on snapshot volume device nodes.
> 
> My question is:
> 
> Is there a limit to the snapshot device nodes?
> How to eliminate the limitation?
> How to activate the 31st snapshot volume so I can access the data?
> 
> These are the steps:
> 
> 1. Create a lvm or lvm2 container
> 2. Create a region then a volume on top of the container.
>   For example:
>     create: Region, LVM2={name=reg_volume1, size=20GB}, 
> lvm2/container/Freespace
>     create: Volume, lvm2/container/reg_volume1, Name=volume1
> 3. Create a region -> a feature object -> a volume
>   For example:
>     create: Region, LVM2={name=reg_snapshot, size=128MB}, 
> lvm2/container/Freespace 
>     create: Object, Snapshot={original=/dev/evms/volume1, 
> snapshot=snapshot_object}, lvm2/container/reg_snapshot 
>     create: Volume, snapshot_objects, Name=snap
> 
> Please tell me if you have the answer.
> Any help is appreciated, thanks.
> At last, sorry for my poor English.
> 
> Thanks, 
> Michael

Hi, list

Sorry I did not made a good survey on the list.
The answer is in 
http://marc.theaimsgroup.com/?l=evms-devel&m=112699668724855&w=2

I tried to modify SNAPSHOT_PAGES in drivers/md/dm-snapshot.c from 256 to 64.
Then I can successfully take more than 30 snapshots in kernel 2.6.15 now.
I have 83 snapshots now and all successfully mounted.

My question is:
What is the Effect of changing SNAPSHOT_PAGES?
i.e. Changing "Each snapshot reserves this many pages for io" from 256 to 64.

Thanks, 
Michael




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