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List:       evms-devel
Subject:    Re: [Evms-devel] Is there any difference between a new-bought disk
From:       "Michael Yao" <m9230 () cn ! ee ! ccu ! edu ! tw>
Date:       2006-11-30 2:23:29
Message-ID: 20061130022158.M74541 () cn ! ee ! ccu ! edu ! tw
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On Mon, 27 Nov 2006 10:21:13 -0600, Steve Dobbelstein wrote
> It depends what you mean by "used".  What is important is whether 
> the disk has been used by EVMS, not whether it is brand new or has 
> been used at all. There is no special pattern/value that EVMS 
> detects on a new bought disk. Here are the details.
> 
> When EVMS detects a disk that has no objects on it, by default it 
> creates a compatibility volume for the disk.  It does this for 
> several reasons.
> 1. It is what the user would see under native Linux.  For example, 
> if you add a third SCSI disk to your system it will show up as 
> /dev/sdc.  EVMS emulates this behavior by creating /dev/evms/sdc.
> 2. If EVMS did not do this, you would have to create a compatibility 
> volume for the disk just to be able to use it under EVMS as you 
> could under native Linux.  That's an extra step in EVMS just to be 
> able to what you could normally do in native Linux without any action.
> 3. If EVMS did not do this, then it would treat the disk as an available
> object and allow you to do things with it -- create segments,
>  regions, volumes. etc.  That could be dangerous if you already had 
> data on the disk in some format that EVMS did not recognize; EVMS 
> would allow you to blow away the data.  By requiring you to delete 
> the compatibility volume, EVMS makes you take the explicit action 
> that says it's OK to use the disk.
> 
> When you delete the compatibility volume from the disk, what EVMS 
> does under the covers is put two sectors of metadata at the end of 
> the disk that say not to make a compatibility volume for the disk. 
>  The two sectors of metadata are called "stop data" because they 
> tell EVMS to stop the discovery on the object and just leave it as 
> an available object.  When EVMS examines a sytsem, if it discovers 
> no objects on a disk and sees no stop data at the end of the disk, 
> it creates a compatibilty volume for the disk.
> 
> You can see from the above that if you delete the compatibility 
> volume from a disk and then move the disk to another machine, EVMS 
> on the other machine will detect the stop data and will not create a 
> compatibility volume for the disk.
> 
> You can also see that if you "use" the disk in any way other than deleting
> the compatibility volume, EVMS will continue to create the compatibility
> volume when it sees the disk.  It does not depend on whether the 
> disk is "new bought" or not.  It only depends on whether EVMS has 
> written stop data on the disk.
> 
> Hope this helps.
> 
> Steve D.

Hi Steve, 

Thanks for you information.
Yes, it helps a lot.

Thanks, 
Michael




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