Why on earth would you want to reduce the size of a hard disk? Surely we always want as much disk space as possible?
Well, in my current role one of our product streams is full disk encryption for Windows.
And one problem for testing is that large hard disks can take a long time to encrypt - many hours, even days in extreme circumstances. (Size of the disk isn't the only factor - processor power is another.) So, sometimes, we want the system under test to have the smallest hard disk possible.
Since we often need to test against specific models of laptop, rather than build our own small-disk test rigs, that can lead to frustrating delays before we get a system into the right state to get the information we want.
A simple and quick solution that one of our developers tipped us off to, is to use the hdparm command in Linux. It allows you to view and set parameters of your hard disk and one of its, perhaps lesser-used, features is the ability to force the disk to show as only having a certain number of sectors.
If you ever find yourself needing to do this, here are the simple steps which I have found to work for me. (But, of course, this should be done with caution. And it's not advised to try it on disks which already contain data you want to keep.)
You'll need to be able to boot your machine to Linux and for that I use an Ubuntu USB stick built from here.
1. Boot your system to the Ubuntu USB stick
You may need to go into the machine BIOS and select a temporary startup device. When it loads choose "Try Ubuntu without installing" to run the OS directly from the stick.
2. Find out how Ubuntu labels the HDD that you're going to change.
Typically, the main system disk of a machine will have the "logical name" /dev/sda - but do make certain you're working on the right disk before you attempt any changes. One way to check:
Press ctrl+alt+t to open the Terminal program. (You're going to need it to run hdparm anyway) and enter:
sudo -i (this gives you the necessary rights to run the commands you need)
then enter:
lshw -class disk
Look through the information returned for disks found on the system and note the "logical name" of the HDD you want to change. Check this carefully to be sure you have the right one.
Staying in the Terminal....
3. Find out how many sectors the disk has in total.
Run the following hdparm command:
hdparm - N [logical name of your disk]
You'll get a result something like this (depending on the disk size):
4. Work out how many sectors will give you the disk size you want
Assuming you know the original capacity of the hard disk, you can either:
a) Query or calculate the individual sector size, and from that work out how many sectors add up to your desired capacity
b) Simply apply a rough percentage based on the amount of disk space you want remaining after the change. Eg. If disk size is 500GB, but you want to reduce it to 50GB (10%), just divide the total number of sectors that hdparm reports by 10.
5. Set the number of visible sectors to the number you worked out at step 4
At this stage it's worth repeating that you should only do this with caution. the "-N" switch in hdparm is officially marked as "Dangerous"!
Enter the following hdparm command:
Note that "–yes-i-know-what-i-am-doing" string! This is not a joke - you really have to enter that, and it's there with good reason. Once you enter the command you won't be asked if you're sure - the change will be made.
The 'p' in front of the number of sectors indicates "permanent". It means that the change to the number of sectors will stay in effect unless and until you repeat this process and set it to something else. (Yes - you can get your original disk size back again!) If you don't include the 'p' the change you've made will be lost on a system restart.
Once you shut down Ubuntu, remove the USB, and restart your system normally the change should be in effect.
At this stage we would either install a new copy of Windows, or restore an existing image for that machine - giving us a clean OS to work on with an unusually tiny HDD to encrypt.
I haven't used this extensively so I can't vouch for it working on all types of disk and systems. My understanding is that it won't work on removable USB hard disks, for example.
And, remember, some of hdparm's functions can be dangerous to your system.
If you're at all uncertain I'd strongly recommend reading more about hdparm before use.
Well, in my current role one of our product streams is full disk encryption for Windows.
And one problem for testing is that large hard disks can take a long time to encrypt - many hours, even days in extreme circumstances. (Size of the disk isn't the only factor - processor power is another.) So, sometimes, we want the system under test to have the smallest hard disk possible.
Since we often need to test against specific models of laptop, rather than build our own small-disk test rigs, that can lead to frustrating delays before we get a system into the right state to get the information we want.
A simple and quick solution that one of our developers tipped us off to, is to use the hdparm command in Linux. It allows you to view and set parameters of your hard disk and one of its, perhaps lesser-used, features is the ability to force the disk to show as only having a certain number of sectors.
If you ever find yourself needing to do this, here are the simple steps which I have found to work for me. (But, of course, this should be done with caution. And it's not advised to try it on disks which already contain data you want to keep.)
You'll need to be able to boot your machine to Linux and for that I use an Ubuntu USB stick built from here.
1. Boot your system to the Ubuntu USB stick
You may need to go into the machine BIOS and select a temporary startup device. When it loads choose "Try Ubuntu without installing" to run the OS directly from the stick.
2. Find out how Ubuntu labels the HDD that you're going to change.
Typically, the main system disk of a machine will have the "logical name" /dev/sda - but do make certain you're working on the right disk before you attempt any changes. One way to check:
Press ctrl+alt+t to open the Terminal program. (You're going to need it to run hdparm anyway) and enter:
sudo -i (this gives you the necessary rights to run the commands you need)
then enter:
lshw -class disk
Look through the information returned for disks found on the system and note the "logical name" of the HDD you want to change. Check this carefully to be sure you have the right one.
Staying in the Terminal....
3. Find out how many sectors the disk has in total.
Run the following hdparm command:
hdparm - N [logical name of your disk]
You'll get a result something like this (depending on the disk size):
4. Work out how many sectors will give you the disk size you want
Assuming you know the original capacity of the hard disk, you can either:
a) Query or calculate the individual sector size, and from that work out how many sectors add up to your desired capacity
b) Simply apply a rough percentage based on the amount of disk space you want remaining after the change. Eg. If disk size is 500GB, but you want to reduce it to 50GB (10%), just divide the total number of sectors that hdparm reports by 10.
5. Set the number of visible sectors to the number you worked out at step 4
At this stage it's worth repeating that you should only do this with caution. the "-N" switch in hdparm is officially marked as "Dangerous"!
Enter the following hdparm command:
hdparm –N p[desired no of sectors] –yes-i-know-what-i-am-doing [logical name of your disk]
Note that "–yes-i-know-what-i-am-doing" string! This is not a joke - you really have to enter that, and it's there with good reason. Once you enter the command you won't be asked if you're sure - the change will be made.
The 'p' in front of the number of sectors indicates "permanent". It means that the change to the number of sectors will stay in effect unless and until you repeat this process and set it to something else. (Yes - you can get your original disk size back again!) If you don't include the 'p' the change you've made will be lost on a system restart.
Once you shut down Ubuntu, remove the USB, and restart your system normally the change should be in effect.
At this stage we would either install a new copy of Windows, or restore an existing image for that machine - giving us a clean OS to work on with an unusually tiny HDD to encrypt.
I haven't used this extensively so I can't vouch for it working on all types of disk and systems. My understanding is that it won't work on removable USB hard disks, for example.
And, remember, some of hdparm's functions can be dangerous to your system.
If you're at all uncertain I'd strongly recommend reading more about hdparm before use.
