Where is the documentation for what sda, sdb, dm-0, dm-1 mean

iostat reports statistics on block devices. Block devices include a lot of things, not just hard disks. On my desktop, iostat -p ALL -kx gives 37 lines of stats.

In your case, try the -N option, which will give you nice names for your dm devices.

The names of block devices actually come from the kernel and (maybe) udev; iostat is just dumping out the information the kernel keeps. It doesn't care about the names, really. Its the kernel and udev documentation you need to check to see what the devices are.

Here are some common ones:

• hdX — ATA hard disk, pre-libata. You'll only see this with old distros (probably based on Linux 2.4.x or older)
• sdX — "SCSI" hard disk. Also includes SATA and SAS. And IDE disks using libata (on any recent distro).
• hdXY, sdXY — Partition on the hard disk hdX or sdX.
• loopX — Loopback device, used for mounting disk images, etc.
• loopXpY — Partitions on the loopback device loopX; used when mounting an image of a complete hard drive, etc.
• scdX, srX — "SCSI" CD, using same weird definition of "SCSI". Also includes DVD, Blu-ray, etc.
• mdX — Linux MDraid
• dm-X — Device Mapper. Use -N to see what these are, or ls -l /dev/mapper. Device Mapper underlies LVM2 and dm-crypt. If you're using either LVM or encrypted volumes, you'll see dm-X devices.

Note that not all devices keep all iostat information. E.g., mdraid doesn't keep queue statistics, and those columns will always be 0.

You can look in /sys/class/block or /dev/block to see all the block devices on your system.

sda0, sda1 are the partitions of the hard drive (sda) attached to your machine.

dm-0 & dm-1 are the Logical volume managers' logical volumes you would have created while installing or configuring your machine

You can read more about it at Wiki