Category Archives: Debugging

Virtualbox boot Ubuntu 12.04 into recovery mode

Put the Ubuntu ISO in the drive
When the VM starts up, press the ESC key to get into the install options screen
Press f6 to get the boot argument line to show (then ESC to hide the submenu)
Go down to boot from first hard disk, then type “boot=/casper single”
This should open up another menu, with the option to boot into recovery
And then you get another menu, choose to boot as root user
When you’re in the shell as root user, type: mount -o remount,rw /
Should now have read-write access to filesystem as root user

Openfiler 2.99.1 unable to create partitions from the GUI

UPDATE: Creating a new partition in /dev/sdb (second volume) works, but doesn’t use the last 5% of the disk.  I can’t seem to fix this with the Other Solution posted here; regardless of what I do, the partition shows up in the web GUI of openfiler but as unknown partition type.  I’ve tried using mkpart as described below – it gives errors when trying to use ext3; using ext2 and then converting it to ext3 with mkfs.ext3 has the same issue, as does creating the partition with no filesystem.  This is all regardless of what I do with pvcreate.  For now I’ve left it as empty space, hopefully Openfiler will fix their issue creating new volumes soon.

Experienced a lot of Google-fail before finally getting the solution to this, so I’m posting it here.

New install of Openfiler v2.99.1, trying to create a new partition in /dev/sda, clicking the ‘create’ button does nothing, just refreshes the page.

Easy Solution (but it loses some GB – for some reason, ending cylinder is reduced)
Set the ‘starting cylinder’ 80 cylinders more than the recommended number, then try and create again.  It should work.

Other Solution (works properly, hurray)
From the cli:
unit cyl
[name?] // click enter
[file system type? ext2] ext3
[start?] // here, type one cylinder up from the end of the last partition shown by the print command, and space, and then the ending cylinder (for the entire disk, enter the number shown in the print command under the Model, on the line “Disk /dev/sda : ”
print // see the new partition
pvcreate /dev/sda4

After this, Openfiler should see the new partition from the GUI.

System call tracing; strace with pid

Copied directly from ubuntu wiki, want to store here so I dont lose it

strace -Ff -tt -p <PID> 2>&1 | tee strace-<program>.log

edit: better way that creates a file per process/child process

strace -f -ff -tt -p <PID> -o strace.log

This will create files for each child process in addition to the main process, named strace.log.<PID>

Using strace to analyze apache

For times that you want to know exactly what a process is doing (for instance, if your apache is Pleskified and you’re getting weird website errors that can’t be traced back to any of your own code…) you can use strace.

service apache2 stop

strace -f -o trace.txt /etc/init.d/apache2 start  # this will run strace on all child processes of apache, so you don’t need to worry about figuring out the correct process

## do whatever to generate the error message (in my case, just accessing the site)

service apache2 stop

## use htop to verify the strace has stopped

service apache2 start

Then check trace.txt to find your issue!

And have a look here for a more thorough tutorial.