Note to self: accessing LVM volumes from live CD

If you have a Linux machine with LVM volumes on it, and you need to boot it from a live CD (for example SysRescueCD) and you wonder where your volumes are, just issue (in an LVM enabled system, of course)


# vgchange -a y

and your volumes will be available under /dev/$VOLUME-GROUP-NAME or under /dev/mapper

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HOWTO: Debian and SCSI multipathing with multipath-tools

After getting iSCSI working on Debian Etch the next thing to do is to set up multipath to get redundancy in case one path from the SCSI client to the SCSI target fails.

First, let’s digg a bit more in depth about what a path is, what can go wrong and what we can do to prevent it. Usually in a simple iSCSI environment there are two network interfaces dedicated to the remote storage, each one connected to a distinct ethernet switch and each switch connected to the a distinct ethernet interface in the host SAN. Then here you have two separated controller cards (let’s call them A and B) which connect to the same logical volume (a RAID array.. so here redundancy is already covered). I repeat, this is the simplest redundant scenario, in which you can have redundancy, a good fault-tolerance and can parallelize via round-robin the requests from the initiator to the host target.
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HOWTO: the definitive guide to Debian Etch open-iscsi (take 2)

I guess the fact I’m here writing again on this topic goes for that definitive I put in the title the first time :) So obviously it was not so definitive, and here we are again with a, I hope, better and improved version.
This time we are going to use th backports repository and the Etcn’n’half kernel, cause they provide a better and far more stable support for iSCSI under Debian (Etch).

So, first of all add the backports repository:


echo "deb http://www.backports.org/debian etch-backports main contrib non-free" >> /etc/apt/sources.list

and do some basic stuff:

# aptitude update
# aptitude install debian-backports-keyring
# aptitude update

Now, let’s install the newer 2.6.24 kernel from the Debian Etch’n’half project (note: it’s present in the officila Debian repository, it doesn’t come from the backports.org one)


# aptitude install linux-image-2.6-amd64-etchnhalf # remove amd64 if you're on x86_32

now here, if you are a Broadcom NeteXtreme 2 user (lsmod|grep bnx2), be careful and remember to install these NEW package before rebooting, or you will have an unpleasant surprise


# aptitude install firmware-bnx2

This is due to a change in newer Linux versions

Then reboot, cross your fingers and then install the newer open-iscsi package:

# aptitude install -t etch-backports open-iscsi

Everything should be ok and this time you should have all the config files in the right place, a proper script to mount/unmount iSCSI target devices at boot time and so on…
Anyway, I still prefer the old-school config file, so usually I replace the Debian stock one with something like this:


node.active_cnx = 1
#node.startup = manual
node.startup = automatic
#node.session.auth.username = dima
#node.session.auth.password = aloha
#node.session.timeo.replacement_timeout = 15
node.session.timeo.recovery_timeout = 15
node.session.err_timeo.abort_timeout = 10
node.session.err_timeo.reset_timeout = 30
node.session.iscsi.InitialR2T = No
node.session.iscsi.ImmediateData = Yes
node.session.iscsi.FirstBurstLength = 262144
node.session.iscsi.MaxBurstLength = 16776192
node.session.iscsi.DefaultTime2Wait = 0
node.session.iscsi.DefaultTime2Retain = 0
node.session.iscsi.MaxConnections = 0
node.conn[0].iscsi.HeaderDigest = None
node.conn[0].iscsi.DataDigest = None
node.conn[0].iscsi.MaxRecvDataSegmentLength = 65536

I have highlighted one line because that parameter is used to choose the timeout after which an iSCSI device is considered dead, and thus that path discarded (we’ll talk about paths later).

So, time to discover new devices now:

# /etc/init.d/open-iscsi restart
# iscsiadm -m discovery -t sendtargets -p $SAN_IP_ADDRESS
# /etc/init.d/open-iscsi restart

check out your dmesg output and look for new /dev/sdX devices.
Some partitioning and formatting later, you can edit your fstab with something like this


/dev/sdb1 /mnt/files ext3 defaults,auto,_netdev 0 0

and you should be done!

*fdisk and the 1.5TB partition size limit

If you have a large volume (like a disk array or the next-generation SATA disk) and you’re trying to create a single, giant partition for whatever reason, you should know that fdisk (for DOS compatibility reasons, I suppose) cannot create partitions bigger than ~1.5TB, although it won’t throw you any error or complain So if you want to create bigger partition, use parted (or one of its frontend). The limitation applies to fdisk, cfdisk and all the *fdisk family.

EDIT: in parted you have to change the disk partition type to something like gpt or you still won’t be able to create a partition bigger than 1.5TB.
Nonetheless, once the very large partition is created, I still haven’t found a way to format it, mount it and get all my terabytes. I’m still stuck with 1.5TB. Look at this:

server:~# parted /dev/mapper/mpath1
GNU Parted 1.7.1
Using /dev/mapper/mpath1
Welcome to GNU Parted! Type 'help' to view a list of commands.
(parted) print
Disk /dev/mapper/mpath1: 6000GB
Sector size (logical/physical): 512B/512B
Partition Table: gpt
Number Start End Size File system Name Flags
1 17.4kB 6000GB 6000GB ext2
server:~# df -h |grep mpath1p1
/dev/mapper/mpath1p1 1.5T 5.1M 1.5T 1% /mnt/logs

I’m stuck with this. Any idea, dear lazyweb?

HOWTO: the definitive guide to Debian Etch open-iscsi (step-by-step)

Ok, maybe this is a little bit arrogant title :) but since I’ve experienced more than a problem/issue when installing iSCSI initiator support in Debian 4.0 Etch, I think that this howto could help people setting up their first open-iscsi in Etch (in which open-iscsi is UTTERLY broken, let me say it loud).

First of all, install open-iscsi

aptitude install open-iscsi

and remove the broken init scripts (they are going to give you lots of headaches when rebooting, if you don’t do this.)

update-rc.d -f open-iscsi remove
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