Re: [gentoo-user] md-device too small: where's my mistake?

[Heiko Wundram <heiko@xencon.net>]

Am Wednesday 22 October 2008 15:58:44 schrieb Stefan G. Weichinger:
> md2 is the one that gives me headaches. AFAI understand it should be
> about 3TB in size, but it is only 774 GB ....
>
> # fdisk -l /dev/md2
>
> Platte /dev/md2: 774.0 GByte, 774044975104 Byte
> 2 Köpfe, 4 Sektoren/Spuren, 188975824 Zylinder
> Einheiten = Zylinder von 8 × 512 = 4096 Bytes
> Disk identifier: 0x00000000
>
>
> Why?

You cannot manage disks >= ~2TB with fdisk (i.e., DOS partition tables), as 
they (or rather the on-disk-structure of DOS partition tables) have an 
inherent limitation in the maximum number of LBA48-blocks they can address.

I'd presume that because of this inherent limitation, fdisk is reporting the 
wrong total size (2TB+774G+epsilon ~ 3TB; sounds like somewhere someone is 
doing a modulo operation, possibly), and completely "off" values for 
heads/sectors.

Anyway, md-devices cannot be partitioned anyway (of course you can write a 
partition table on them, but the kernel won't use that to create md2-1,-2, 
etc.), so using fdisk is wrong.

If you want to check the "real" size of the device, don't use fdisk, but 
rather use

blockdev --getsize64 /dev/md2

which shows you the byte-count of the corresponding volume, and which I think 
will be 3TB, as you want it to be.

If you want to subpartition large devices, use lvm(2), which does not have the 
2TB limitation on size.

Hope this helps!

-- 
Heiko Wundram
hackerkey://v4sw7CHJLSUY$hw5ln5pr7FOP$ck2ma9u7FL$w3DVWXm0l7GL$i65e6t3EMRSXb7ADORen5a26s5MSr2p-6.62/-6.56g5AORZ