Reread partition table without rebooting?

Question

Sometimes, when resizing or otherwise mucking about with partitions on a disk, cfdisk will say:

Wrote partition table, but re-read table failed. Reboot to update table.

(This also happens with other partitioning tools, so I'm thinking this is a Linux issue rather than a cfdisk issue.) Why is this, and why does it only happens sometimes, and what can I do to avoid it?

Note: Please assume that none of the partitions I am actually editing are opened, mounted or otherwise in use.

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Update:

cfdisk uses ioctl(fd, BLKRRPART, NULL) to tell Linux to reread the partition table. Two of the other tools recommended so far (hdparm -z DEVICE, sfdisk -R DEVICE) does exactly the same thing. The partprobe DEVICE command, on the other hand, seems to use a new ioctl called BLKPG, which might be better; I don't know. (It also falls back on BLKRRPART if BLKPG fails.)

BLKPG seems to be a "this partition has changed; here is the new size" operation, and it looked like partprobe called it individually on all the partitions on the device passed, so it should work if the individual partitions are unused. However, I have not had the opportunity to try it.

Answer 1

IMHO the most reliable/best answer is

partprobe /dev/sdX

Answer 2

"Note: Please assume that none of the partitions I am actually editing are opened, mounted or otherwise in use"

Given that assumption, the partition table can be successfully rescanned, and the issue won't arise. If you're getting that error, it's because the partition table is currently in use, and hence can't be re-scanned without creating inconsistencies.

Answer 3

Rereading partition table information doesn't always work, but try

hdparm -z /dev/sda

or

sfdisk -R /dev/sda

If it works the values in /proc/partitions will change.

Answer 4

It is not based on partition that you are editing.

Suppose you have only one harddisk (/dev/sda) and two partitions (/dev/sda1, /dev/sda2) and you have mounted only one partition (/dev/sda1). If you delete or change anything about other partition which is not even mounted (/dev/sda2) you will get the error that re-reading of partition table failed and kernel will use old table.

But if you have two harddisks (/dev/sda, /dev/sdb) and none of the partitions of (/dev/sdb) are in use. Then you can add / delete / resize /edit partitions of /dev/sdb and they will be re-read without any problem. But even if one partition of /dev/sdb was mounted during change. Then kernel will keep using old table.

Answer 5

You can also try:

echo 1 > /sys/block/sdX/device/rescan