[CentOS] Re: Building software RAID mdmad adding a second disk

Thu Apr 13 23:15:07 UTC 2006
Scott Silva <ssilva at sgvwater.com>

Aleksandar Milivojevic spake the following on 4/13/2006 7:46 AM:
> Quoting israel.garcia at cimex.com.cu:
>> Thanks Alexandar for your soon answer, BUT, is there another easier way
>> to do this?
> No, not really.  As far as I know, there isn't any GUI interface that
> will do the steps behind the scenes.  Anyhow, it isn't that complicated.
> One thing I forgot to mention is to use fdisk to tag partitions as
> "Linux raid autodetect" (fd) when you are done.
> The most troublesome part is to find out how much you need to shrink
> file systems before building mirrors.  According to md man page, md
> superblock (or metadata) is 4KB long and its start position is 64KB
> aligned.  That means you will loose between 64 and 128KB of partition
> space when building mirrors.
> The simple way is to just assume worst case and shrink your file systems
> to be 128KB smaller than partition size.  Assuming example below
> (partition size 265041KB), you would shrink it to 265041 - 128 =
> 264913KB.  Something like "resize2fs -p /dev/sda1 264913K".  You can
> always use resize2fs second time after mirrors are created to get any
> possibly unused space at the end of metadevice (as if anybody will care
> about couple of KB, but if it makes you happy go for it).  Just invoke
> resizefs like before, but this time do not specify size ("resize2fs -p
> /dev/sda1").
> Or if you are really into it, you can calculate exactly by how much you
> need to shrink file system.
> [root at wis165 ~]# fdisk /dev/sda
> Command (m for help): p
> Disk /dev/sda: 80.0 GB, 80026361856 bytes
> 255 heads, 63 sectors/track, 9729 cylinders
> Units = cylinders of 16065 * 512 = 8225280 bytes
>    Device Boot      Start         End      Blocks   Id  System
> /dev/sda1   *           1          33      265041   83  Linux
> So the sda1 is 265041KB in size.  This gives 265041 / 64 = 4141.265625
> 64KB blocks.  Round it *down* to 4141.  Never round up.  You need to
> substract 1 for MD superblock, which gives 4140 usable 64KB blocks, so
> in this case file system needs to be resized to 4140 * 64 = 264960KB (in
> this case the file system will be shrinked by 81KB).  So you would
> simply do "resize2fs -p /dev/sda1 264960K".
> Hmmm...  Maybe I should publish a new HOWTO ;-)
> --See Ya' later, alligator!
> http://www.8-P.ca/
Or just set up identical sized partitions on second drive, set up raid arrays
with "missing" parameter, rsync data from one partition to the other, edit
fstab and grub.conf.
Reboot from the second drive. Should be a complete system, abeit slower
because of the "failed" raid arrays. Then you can use sfdisk to copy partition
data from second drive to first, and add the partitions from first drive to
 It is safer, as you never change the old drive until you are sure the second
is working, and you can always fall back or redo something up to the point
that you use sfdisk. I did this on a RedHat 9 system a couple of years ago,
and did most of it while the system was "live", rsyncing data along the way
until I had everything set up the way I wanted. I then booted from a rescue
disk to finish up and change the boot to the second drive, and then it was all
about rebuilding the arrays.


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