Re: Is there any difference between a new-bought disk and a used disk without partitions/objects?

"Michael Yao" <[email protected]> wrote on 11/22/2006 05:15:56 AM:

> Hi,

> In user guide, I found that When a new disk is added to a system, the
disk
> shows up in EVMS as a compatibility volume. So I have to delete the
volume
> before use it.

> What is the difference between a new-bought disk and a used disk without
> objects? Any special pattern/value in a new-bought disk?

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.


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