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List:       evms-devel
Subject:    [Evms-devel] suggestion for the "move mapping" feature
From:       Jure =?ISO-8859-2?Q?Pe=E8ar?= <pegasus () nerv ! eu ! org>
Date:       2005-09-21 21:06:32
Message-ID: 20050921230632.3c0b1642.pegasus () nerv ! eu ! org
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Hi all,

First, my home setup:
3ware 6800 with four hw mirrors of 80gb, presenting sd[abcd]. On top of
these is lvm2, in which 17 regions with volumes are created, some on single
disks, some stripped across all four, all with reiserfs. Debian 2.6.11
kernel, latest evms in debian-testing.

What I needed to do:
replace a pair of these 80gb disks with 200gb, thus enlarging the whole
capacity.

What I did:
I pulled one of the 80gb drives from a mirror (sdc), connected it as a
standalone disk on another ide cntrlr and copied 80gb of data over. Now I
could shrink appropriate volumes by that size, creating freespace, to which
I could move mappings off the remaining disk in sdc. Once sdc was free, I
removed it from lvm2, turned off the machine, replaced that disk with two
new ones, created a mirror on them and bought the machine back online.
Whole operation took only a couple of hours and completed without a
problem. The only unexpected kernel msg was a few tens of "bio too big
device dm-0 (256 > 255)". EVMS rocks! :)

The only step in the whole process that could be made more admin-friendly
is the new "move mapping feature". As I said, I have some volumes that are
stripped across all four mirrors. "Move mapping" only allows you to move
one stripe at a time. I didn't study the underlying lvm2 capabilities, but
why exactly is that so? Next, whole "move mapping" UI is a bit complicated.
One needs to closely examine the "display details" of a region to figure
out the index of the logical mapping to move and the stripe within that
mapping. Could this be done a bit more user friendlier? For example, if
there was a device name on which the mapping is next to the logical mapping
number in the drop down menu, the whole thing would be much more simple.

Also, what I did here (move data off one disk, pull it out, replace it with
a bigger one) almost seems like a common operation in the volume
management. Maybe less common than "just add more disks", but very likely
when you run out of drive bays. :) So, could it be possible to construct a
function that would move everything on particular drive off to free space
on other drives with the goal of freeing it and enabling its removal?


-- 

Jure Pečar
http://jure.pecar.org/


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