How does partprobe work?

Question

I was looking for a way to refresh the scsi bus (or any other bus, for that matter) that would allow my kernel ( 2.6.18-194-el5 on CentOS 5.5) to know about the partitions on a drive, and I couldn't find a way. partprobe did it instantly. How?

Since it's important, here's what I was doing:

I wanted to get some practice in partition-specific things like resizing, backing up the MBR and restoring it, and so on. I had created a 20GB partition on /dev/sdb, backed up the MBR:

dd if=/dev/sdb of=sdb.mbr bs=512 count=1

then I went into fdisk, deleted the partition, wrote it, and exited.

ls /dev/sdb*

showed that there was no partition, and

fdisk -l /dev/sdb'

matched, so I figured I was good.

I then reversed the dd:

dd if=sdb.mbr of=/dev/sdb bs=512 count=1

Of course, I didn't do anything at that point, so

ls /dev/sdb*

didn't list any partitions, but

fdisk -l

showed the partition, presumably because it reads the first 512 bytes on the disk and doesn't rely on the kernel. I knew that I'd have to refresh the bus, so I went into into /sys/class/scsi_host/host1, and did

echo "- - -" > scan

and doing

ls /dev/sdb

didn't show anything new, so then I went to /sys/bus/scsi/devices and for each of the listed devices, I did

echo 1 > rescan

and that didn't work.

I then did more research on the problem, and came across 'partprobe', which comes with parted. I ran it, and it worked instantly.

If I don't get a likely answer here, I'm probably going to just go get the source and look it up, but I figure there are wizards here beyond me, so I thought I'd appeal to you all.

Solution

Partprobe calls the BLKRRPART ioctl, which is documented in, err, include/linux/fs.h, and beyond that the kernel source (the meat is in rescan_partitions()):

#define BLKRRPART  _IO(0x12,95) /* re-read partition table */

The easiest way to find this out is to run strace -e raw=ioctl -e open,ioctl partprobe /dev/sdb.

I think what you tried with /sys/*/*scan tells the kernel to check if there's been a change of drive. That doesn't help if the drive hasn't changed (or has been hotswapped in a way that the kernel doesn't detect?) but the partition structure on it has changed.