Category: Hack!

This is an old sysadmin trick, but a good one all the same.
Suppose that you finished installing a brand-new Debian GNU/Linux server, and for whatever reason you forgot to set aside some space for a swap partition. Or you correctly got some swap space at installation time but now you desperately need some more. Well, despair not. It’s a little known fact that you can have swap space in a file on top of the filesystem instead of using a dedicated block device.
This simple recipe will give you 2Gb of swap space. Here we go:

# mkdir /var/swap
# chown root.root /var/swap
# chmod 700 /var/swap
# dd if=/dev/zero of=/var/swap/01.swp bs=1024 count=2M
# chown root.root /var/swap/01.swp
# chmod 600 /var/swap
# mkswap /var/swap/01.swp

Now add this newly created swap space to your /etc/fstab:

/var/swap/01.swp       none    swap    sw              0 0

Teoretically, you won’t get the same performance as using a dedicated block device, and if the file actually gets fragmented it might drop rigt to the floor, so if you try this at all do it as soon as possible after installation. So if what you’re trying to do is to *increase* your available swap space instead, you may add a priority option to give preference to the block-device swap space:

/dev/hda1              none    swap    sw,pri=1              0 0
/var/swap/01.swp       none    swap    sw,pri=2              0 0

And just this time activate the swapspace with addswap (The initscripts will do it on every boot thereafter):

# swapon -av

That’s all there is to it. I particularily like how newer BOFH generations look at this with some skepticism at first, and with endless amazement later.

[tags]Debian, GNU, Linux, swap, sysadmin[/tags]

Last thursday the hard disk drive on a development machine died big time. First it started to behave erratically and dmesg showed that it has having trouble with some bad blocks. It did not survive a reboot: ReiserFS woud not mount it on boot, and reiserfsck running from an Accelerated Knoppix CD refused to bring it back to life. At first glance, the disk was beyond repair.
Of course, upon closer inspection, it turned out that the warranty expired exactly two months ago. Normally -after swearing my heart off- I would just replace the disk and make myself a nice paperweight or some other modern art piece -I’ve been looking forward to make one of those nice HDD clocks– but in the guts of that particular HDD were some uncomitted changes that I just wasn’t on the mood of rewriting. Besides, even though most of the data was expendable, the configuration hadn’t been backed up in quite a while (Yes, there is a pattern here).
So here’s the recipe I usually apply in these situations using Kurt Garloff’s dd_rescue. First get a brand-new HDD of approximately the same capacity and place both disks in a working Linux box (Depending on your necessities, booting from Knoppix might do). Let’s call the old, dying HDD /dev/hdg, and the spankin’ new disk will be /dev/hde. For the sake of simplicity, let’s assume that /dev/hdg was partitioned in /dev/hdg1 for swap and /dev/hdg2 for data.
First we’ll copy the entire data partition from /dev/hdg2 to /dev/hde2:

# dd_rescue /dev/hdg2 /dev/hde2

This will take a long, long time. dd_rescue starts with a reasonable block size, but whenever it encounters and error it retries a few times with a smaller block size before skipping the defective blocks and moving along. This is useful because it will copy all data in every readable block, instead of giving up at the first error like dd does. In my case, this took more than a day for a 248GB partition.
Once the data is in a new disk you can try to mount it directly, although it is a good idea to run reiserfsck first to make sure that the files you’ll copy are usable.

# reiserfsck /dev/hde2

Now here you might run into a small obstacle. Ideally I would buy the exact same model as the old drive for recovery purposes, because that guarantees that an exact bit-for-bit copy will work in most cases, partition maps and all. However in this case I bought a different brand, which resulted in a slightly smaller drive and a completely different geometry. When this happens, reiserfsck will complain about the different partition size, and suggests that you rebuild the superblock:

# reiserfsck --rebuild-sb /dev/hde2

Now you can do a normal reiserfsck.
When you’re done just mount the new partition and copy your data to a safe place:

# mount /dev/hde2 /mnt/tmp
# rsync -a --progress /mnt/tmp/etc /backup/dir/
# rsync -a --progress /mnt/tmp/home/arturo /another/backup/dir/

After this you can reformat the new drive for normal usage. Mine is being debbootstrapped as I write this.
This little recipe has saved quite some data and a few disks, including most of mcleod’s late Xbox hard disk. As usual your mileage may vary, but with a litle luck you just might get some of your files back.
Now about that crappy Maxtor HDD… I might just go for the wind chimes instead.
[tags]Linux, sysadmin, reiserfs, dd_rescue, data recovery[/tags]