Device names for logical volumes

Question

Consider this:

# fdisk -l /dev/sda

Disk /dev/sda: 298.9 GB, 298999349248 bytes
255 heads, 63 sectors/track, 36351 cylinders
Units = cylinders of 16065 * 512 = 8225280 bytes

   Device Boot      Start         End      Blocks   Id  System
/dev/sda1   *           1          13      104391   83  Linux
/dev/sda2              14       36351   291884985   8e  Linux LVM

and this

# df
Filesystem           1K-blocks      Used Available Use% Mounted on
/dev/mapper/VolGroup00-LogVol00
                     276883300   3610240 258981300   2% /
/dev/sda1               101086     19371     76496  21% /boot
tmpfs                  1993292         0   1993292   0% /dev/shm

(The OS is Centos 5.5 64-bit, the HW is IBM ServeRAID M1015 using an LSI MegaRAID BIOS)

Why does df use a long filesystem name instead of /dev/sda2?

Answer 1

df shows you mounted filesystems, which reside on block devices. fdisk is showing you the partition table on your /dev/sda block device. Since you don't have a filesystem mounted directly on /dev/sda2, you won't see it appear in df output. Your root filesystem (the first entry in df) is on an LVM logical volume, which, after consulting your fdisk output, is likely in turn on an LVM physical volume on /dev/sda2.

When comparing block device names in df output with those in output from the LVM management utilities, it helps to know that the kernel uses the full device name for df (here it's /dev/mapper/VolGroup00-LogVol00). The device mapper creates convenient symlinks in /dev that correspond with your volume group names. You can correlate the two outputs by ignoring the "mapper" portion of the name in df, and replacing the hyphen with a forward slash. Running ls -al /dev/VolGroup00 will illustrate the relationship for you.

This doesn't really have anything to do with hardware raid. These utilities would give you the same information regardless of controller type.