BlockDeviceOddities

There are several things in Linux that make dealing with block devices much stranger than one would hope. Here's the ones I can think of quickly:

Depending on which subsystem is implementing a block device, there are several differences in the way block devices and their partitions are manifest:

• device node numbering

There are several schemes for numbering device nodes

• major number with assigned minors (sda is 8:0, sda1 is 8:1 , sdb is 8:16 ) This means you're limited to 15 partitions on scsi devices, and 63 for "ide" when not using libata.
• major number is in /proc/devices, minor numbers are assigned at device creation
• ioctl() calls

The kernel's internal partitioning scheme is managed with some IOCTLs, such as BLKRRPART, BLKPG/BLKPG_ADD_PARTITION, and BLKPG/BLKPG_DEL_PARTITION, which only work on some devices. They work on hd* (ide without libata), sd* (scsi), md_d* (partitionable md), but not on md (non-partitionable md) or device-mapper devices (i.e. multipath or dmraid)

• device node naming

There are several schemes for naming devices. These aren't just userland, they're also the related to the names in e.g. /sys/block/ (${driver} below is not necessarily the name of the driver, but rather an identifying tag it uses)

• (cciss, sx8, etc.):

/dev	/dev/${driver}/c${controller}d${drivenumber}[p${partnumber}]
/sys	/sys/block/${driver}!c${controller}d${drivenumber}[/${driver}!c${controller}d${drivenumber}p${partnumber}]

• (sd, hd):

/dev	/dev/${driver}${diskletter}[${partnumber}]
/sys	/sys/block/${driver}${driveletter}[/${driver}${driveletter}${partnumber}]

• md (no partitions):

/dev	/dev/md${mdnumber}
/sys	/sys/block/md${mdnumber}

• mdp (partitionable md):

/dev	/dev/md_d${mdpnumber}[p${partnumber}]
/sys	/sys/block/md_d${mdpnumber}[/md_d${mdpnumber}p${partnumber}]

• lvm (no partitions, dm device):

/dev	all of:
	/dev/dm-${dmnumber} (a real block device node}
	/dev/mapper/${volgroupname}-${logvolname} (a real block device node)
	/dev/${volgroupname}/${logvolname} (a symlink into /dev/mapper)
/sys	/sys/block/dm-${dmnumber}

• dmraid and mpath (partitioned dm device):

both the main device and the partitions are device-mapper rules; the partitions don't show up in /proc/partitions, etc.

/dev	all of:
	/dev/dm-${dmnumber} (a real block device node)
	/dev/${arbitrary name}[${arbitrary part delimiter}${arbitrary part specifier, usually an integer}] (usually a real block device node for a dm object)
/sys	/sys/block/dm-${dmnumber}

• loopback devices:

/dev	/dev/loop${loopnumber} (just the loopback device)
/dev	/dev/loop${loopnumber}${arbitrary part delimiter}${arbitrary part specifier} (partition, via device-mapper, created by e.g. kpartx)
/sys	/sys/block/loop${loopnumber}
/sys	/sys/dm-${dmnumber} (for a partition)

• sysfs wackiness

sysfs has /sys/block/$device/{holders,slaves} directories to represent which devices depend on which others. If a device uses the kernel's partitioning, these don't reflect the device<->partition relationship.

• hotplug wackiness

When a device is added, you get a bunch of hotplug events. The last one for a particular block device has SUBSYSTEM="block", and is basically the "this device is ready" event. SCSI devices (including SATA and PATA with libata) have a files /sys/block/$device/{device,model}. These don't get probed until after the hotplug event. Also, UUIDs/serial numbers/etc aren't probed at all.

• probing of md/mdp devices

two weird things here:

• to activate an md or mdp device, you have to mknod it first, then use mdadm on the created node.
• mdp's _partitions_ show up the normal ways (i.e. uevents to notify userland)
• md0 always exists (i.e. there's always a /sys/block/md0), even before you activate any devices.

I'm sure there's more here that sucks.