Save hot remove SATA drives under linux

Dennis Birkholz — Sun, 09 Aug 2009 01:46:50 +0000

I bought a trayless mobile SATA rack and explored the linux SATA hot-swap capabilities. My mainboard (with a quite new AHCI compatible chipset) does support hot-swap and so I put a hdd into my mobile rack, watched dmesg to show it settle down and apear under /dev. After that, I removed the drive and noted the following dmesgoutput:

ata3.00: detaching (SCSI 2:0:0:0)
sd 2:0:0:0: [sdf] Synchronizing SCSI cache
sd 2:0:0:0: [sdf] Result: hostbyte=0x04 driverbyte=0x00
sd 2:0:0:0: [sdf] Stopping disk
sd 2:0:0:0: [sdf] START_STOP FAILED
sd 2:0:0:0: [sdf] Result: hostbyte=0x04 driverbyte=0x00
ata3: exception Emask 0x10 SAct 0x0 SErr 0x40c0000 action 0xe frozen
ata3: irq_stat 0x00000040, connection status changed
ata3: SError: { CommWake 10B8B DevExch }
ata3: hard resetting link
ata3: SATA link down (SStatus 0 SControl 300)
ata3: EH complete

That looked rather ill to me so I startet digging around to learn how to tell the system that I soon will remove the harddrive so that all cached data can be written safely to disk. I discovered how to disable SCSI devices via the /proc interface and tried it and now my dmesg looked a lot nicer to me:

sd 2:0:0:0: [sdf] Synchronizing SCSI cache
sd 2:0:0:0: [sdf] Stopping disk
ata3.00: disabled

The nasty thing is that you need to know the SCSI id of the drive to remove. So I created a small bash script that can shut down a drive by using its /dev/sdX name:

#!/bin/bash
# (c) 2009 by Dennis Birkholz (firstname DOT lastname [at] nexxes.net)
#
# This program is free software: you can redistribute it and/or modify
# it under the terms of the GNU General Public License as published by
# the Free Software Foundation, either version 2 of the License, or
# (at your option) any later version.
#
# This program is distributed in the hope that it will be useful,
# but WITHOUT ANY WARRANTY; without even the implied warranty of
# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
# GNU General Public License for more details.
#
# You can received a copy of the GNU General Public License at 
# .

function usage {
	echo "Usage $0 [device]"
	echo
	echo "Disable supplied SCSI device"
	exit
}

# Need a parameter
[ "$1" == "" ] &&
	usage

# Verify parameter exists
( [ ! -e "$1" ] || [ ! -b "$1" ] ) &&
	echo "Supplied devices does not exist or is not a block device." >/dev/stderr &&
	exit 1

# Verify SCSI disk entries exist in /sys
[ ! -d "/sys/class/scsi_disk/" ] &&
	echo "Could not find SCSI disk entries in sys, aborting." >/dev/stderr &&
	exit 2

# Get major/minor device string of device
major=$(stat --dereference --format='%t' "$1")
major=$(printf '%d\n' "0x${major}")
minor=$(stat --dereference --format='%T' "$1")
minor=$(printf '%d\n' "0x${minor}")
deviceID="${major}:${minor}"

echo "Major/Minor number for device '$1' is '${deviceID}'..."

for device in /sys/class/scsi_disk/*; do
	[ "$(< ${device}/device/block*/dev)" != "${deviceID}" ] && continue
	
	scsiID=$(basename "${device}")
	echo "Found SCSI ID '${scsiID}' for device '${1}'..."
	
	echo 1 > ${device}/device/delete
	echo "SCSI device removed."
	exit 0
done

echo "Could not identify device as SCSI device, aborting." >/dev/stderr
exit 4

Put it under /usr/local/sbin and name it like hotremove and you can remove your devices by calling hotremove /dev/sdX without the pain of knowing SCSI ids.

(Warning: don’t forget to unmount first! Maybe later I will add auto unmounting also …)

Syslog-ng and /dev/null

Dennis Birkholz — Sat, 11 Jul 2009 01:11:29 +0000

Beware to try to use /dev/null as a drop target for syslog-ng by using something like the following in your syslog-ng.conf:

destination d_drop {
	file("/dev/null");
};

This will chmod your /dev/null file to 0600 (also known as crw-------) at every startup and cause many failures on your system/server. Use an empty target to drop your log messages:

destination d_drop {
};

nexxes open source blog » System Administration

Save hot remove SATA drives under linux

Syslog-ng and /dev/null