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List:       evms-devel
Subject:    Re: [Evms-devel] Obscure problem with drive linking (possible bug?)
From:       Kevin Corry <corryk () us ! ibm ! com>
Date:       2003-03-18 15:28:41
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Hi Stuart,

On Monday 10 March 2003 13:24, Stuart Livings wrote:
> I'm new to EVMS so please don't hesitate to correct me if I misunderstand
> things.

Sorry for the late response. Email has been stacking up lately.

> At the moment I am experimenting with EVMS prior to actually converting an
> existing RAID to run under EVMS.  The primary drive behind doing this is
> to be able to expand the RAID without destroying the filesystem.
>
> Before creating the real RAID across many disks I played with creating the
> RAID on partitions on a disk I had lying around.  Each partition was 15Gb
> and I created a dozen of them for testing.  I created these partitions
> inside evmsn using the 'segment manager' interface.
>
> First I tried to create a RAID5 region of three of these partitions and
> successfully created a volume from this region.  This volume works
> perfectly.  Yay, we say.
>
> I then tried to expand this volume by adding further partitions to the
> RAID5 array but couldn't find any mechanism to do so.  This makes some
> sense and although this would be the ideal solution I can see why this is
> omitted from the features list of EVMS and attempted to achieve my
> desires through another mechanism.

Currently, EVMS does not support "expanding" raid devices. Raid-1 and raid-5 
can have additional spare disks added to them, but directly expanding the 
raid devices is not yet allowed.

However, many people have asked about this, and we will be working on this 
functionality later this year.

> After rereading the Howto I came to the conclusion that the way to achieve
> it is to use the drive linking feature to add a layer of abstraction
> between the RAID5 region and the volume itself.  This is where I came into
> trouble.
>
> I was intending to create further RAID5 regions and add them to the drive
> link, expanding the volume.
>
> With the volume created directly from the region the device node in
> /dev/evms is created properly and behaves as expected.

Did you create this volume as an EVMS volume or a compatibility volume?

> If I create a drive link from the RAID5 region and create a volume from
> the drive link the device node is NOT created and then all file-system
> functions fail on that volume (unsurprisingly).
>
> If I create the volume from the region and then add drive linking the
> device node is created properly and operates correctly.  Until I reboot.
> Then the EVMS engine seems to get very very confused, evms_devnode_fixup
> (which is run on boot) deletes the device node and evmsn doesn't recognise
> that the device node has gone.

Instead of using DriveLinking on top of the raid-5, you could also use LVM on 
top of the raid-5. Using evms[n|gui], create an LVM container with the raid-5 
region. Then you can create LVM regions from the container, and make each of 
them into volumes. These volumes will be easily resizeable. Then, the next 
time you create a raid-5, you can add it to the existing container, 
increasing the available freespace, which can be used to expand the LVM 
regions.

Try out this scenario and see if you have better luck.

> I'm using EVMS 1.2.0 on Kernel 2.4.19 on x86.  I tried running EVMS on
> 2.4.19 on usparc but that didn't work at all.  I'm using Debian Linux.

You might want to upgrade to 1.2.1, or try out the new 1.9.2 beta, which will 
work quite well for you in this situation.

> For further testing I will hand build a 2.4.20 kernel with 1.2.1 but I
> don't see anything in the 1.2.1 changelog that references this "bug" so
> here I am hoping that one of you gurus can help me out.
>
> As I say I am something of a newbie to EVMS so please don't hesitate to
> suggest a better way of creating a volume with RAID5-like resilience and
> is still expandable.
>
> Many thanks in advance for your help,
>
> Stuart Livings
>
-- 
Kevin Corry
corryk@us.ibm.com
http://evms.sourceforge.net/


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