[CentOS] Re: Building software RAID mdmad adding a second disk
Aleksandar Milivojevic
alex at milivojevic.org
Thu Apr 13 14:46:04 UTC 2006
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/
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