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List:       evms-devel
Subject:    Re: [Evms-devel] how work the evms layer?
From:       "Don Mulvey" <dlmulvey () us ! ibm ! com>
Date:       2003-06-16 13:33:23
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>It's a newbie question.
>If I create a container and then some storage regions, for example:

>first region for /var/spool;
>second region for /var.

>Then I create the related evms volumes.
>Now I have the volume for /var/spool and contiguous the volume for /var.

Are you assuming that /var is contiguous?  LVM regions are composed of
logical extents that map to physical extents on storage.  The physical
extents should not be viewed as contiguous and often times are not
contiguous.

> /var/spool space;
> /var space.

>Now it's all right.
>Later I need some additional space for /var/spool.
>Then I expand the volume of /var/spool.
>But by now I have a situation like this in the disk:

The region is expanded by adding physical extents ... acquiring them from
the container's freespace.


>/var/spool space;
>/var space;
>/var/spool additional space.

>It's so?

No ... your thinking logical extents when your question seems to be
concerned about physical extents.  Imagine that your container was
originally created from sde1, sde2, sde3 and sde4.  You then created /var
and /var/spool.  Tell me which of the scsi partitions were used for /var
and which of them were used for /var/spool?   This question is asking how
the logical extents of the regions were mapped to physical extents inside
the scsi partitions.  You should not worry about the extent sizes,
locations and mappings.  The beauty of aix and lvm regions is that they are
so flexible ... need more room ... grab some extents from container
freespace ... running lown on freespace ... expand the freespace. Neat!


>In that manner I think I have bad performance because the /var/spool space

>isn't contiguous.
>Thank you.

Nope ... even if your storage is contiguous it doesn't mean your file
system is going to leave it that way for very long.  That is why file
systems need to defrag themselves.  Besides ... the region extent sizes are
so large that my guess is that you would not notice that the physical
extents are not actually contiguous on storage. Remember, your not buying
performance when you use a region manager ... your buying flexibility.

-Don




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