dufault.info

There seems to be not a lot of information on Google about this, thus, I post.

At my work, the majority of our servers have hot-swappable drive bays — however, Linux doesn’t usually automatically notice the drive is gone. Worse, sometimes it doesn’t even notice new drives hooked up.

Now, SCSI and SAS both support hot-plugging on a protocol level, and SATA II does as well. If your chassis is equipped with a hotswap drive bays, then that’s all you really need. I haven’t noticed any problems with SATA I hotswaps, but they appear to be less successful.

A common task I need to do is either:

• upgrade a drive in a server
• replace a dying drive with a new one

If the drive is in a RAID, or if it’s a separate sized drive for a disk upgrade, it’s a good idea to trigger the kernel driver to rescan information about the new disk.

In /sys/bus/scsi/devices, you’ll find a number of numbers that’ll correspond to your disk drives:
server devices # ls -1
0:0:0:0@
1:0:0:0@
2:0:0:0@

You can determine more information about the drive by cat’ing it’s model file:
server devices # cat "0:0:0:0/model"
ST3250410AS

Now, to cause the kernel to rescan the drive attached to the port, do this:
echo > "0:0:0:0/rescan"

Check dmesg now:
server devices # dmesg
---snip---
SCSI device sda: 488397168 512-byte hdwr sectors (250059 MB)
sda: Write Protect is off
sda: Mode Sense: 00 3a 00 00
SCSI device sda: drive cache: write back

Now, that’s pretty exciting, as this can be used to skip a reboot.  If you switch the drive, and trigger the rescan, it’ll update the drive information including the partition layout.  Make sure you don’t swap out your main system drive, otherwise your computer will freeze, and there will be data loss and possibly corruption.

Sometimes, this strategy fails to work, and I don’t know why (and resort to rebooting.)  I’m currently researching how to do this a bit better, I have a buggy script that I use to improve this method, but it’s not ready for public release yet. Does anyone have a better method?  Please comment!