[CentOS] Re: raid1 disk format?

William L. Maltby CentOS4Bill at triad.rr.com
Thu Jun 12 21:39:27 UTC 2008

On Thu, 2008-06-12 at 15:14 -0400, Ross S. W. Walker wrote:
> Les Mikesell wrote:
> > You can't beat dd for getting everything exactly the same regardless of 
> > what you changed - or just splitting the mirrors and letting each sync 
> > to new partners but then you have to reinstall grub.  I prefer 
> > clonezilla for non-raid configurations but most of the machines I care 
> > about are configured with raid1.
> Well, actually dd isn't so good in this area. dd will do the whole disk
> no matter how much data is actually stored on it and for a 500GB disk
> that can take a lot of time. It also doesn't take into consideration
> any disk geometry differences.

True. But with a small amount of scripting (assuming some experience
like I had at the time I did this professionally), you can quickly
produce a fairly flexible, automated, fast and reliable process that
accomplishes the task.

I'll elaborate a little below.

> <snip>

> -Ross
> <snip sig stuff>

First, as to speed. Using a blksize= parameter (I used a cyl size as my
"standard" unit of transfer), the number of system calls is reduced and
the speed of the hardware becomes the limiting factor. Back when I
tested this (old slower low-single-digit GB drives, circa 2000-2002),
very large speedups were seen. I don't recall the percentages.

Second, "copies the whole disk ...". Here is where a small amount of
scripting becomes useful. You can copy only specific partitions. If the
whole disk is a single partition, some stats gathered by various
utilities can determine used counts, and a combination of shrinking the
file system (didn't have a shrink ability back when) and (if desired)
shrinking the partition can be used to "compact" the source.

In conjunction with sfdisk to both gather configuration information and
generate new configuration (via scripts), you can reduce the source
copied to very close to just that needed.

In the final step, this is engendered via the "count=", "skip=" and
"seek=" parameters to dd.

This was implemented in a NAS product as part of the RAS process that
could not predict the specifications of HDs that might be replaced in
the field.

Lastly, as to geometry differences, again sfdisk is your friend. What I
can not address is how to integrate this with raid - no experience
there. I do presume that one knowledgeable in that area could also
automate that.

If this sounds like a lot of work, it's not really. This part of my
effort was minimal. The large part was creation of the boot CD,
interfacing with the custom hardware timeout facility for auto-reboot to
a fallback device and implementing and testing the software install and
rejoin with the cluster.

Oh...  and the constantly changing specs (list of requirements kept
changing as they saw opportunities I brought to the table - their *IX
experience was quite limited).


More information about the CentOS mailing list