using dm-zero to test large filesystems w/o all those pesky disks

From Documentation/device-mapper/zero.txt in the kernel:

To create a sparse device, start by creating a dm-zero device that's the desired size of the sparse device. For this example, we'll assume a 10TB sparse device.

TEN_TERABYTES=expr 10 \* 1024 \* 1024 \* 1024 \* 2 # 10 TB in sectors
echo "0 $TEN_TERABYTES zero" | dmsetup create zero1

Then create a snapshot of the zero device, using any available block-device as the COW device. The size of the COW device will determine the amount of real space available to the sparse device. For this example, we'll assume /dev/sdb1 is an available 10GB partition.

echo "0 $TEN_TERABYTES snapshot /dev/mapper/zero1 /dev/sdb1 p 128" | \
dmsetup create sparse1

This will create a 10TB sparse device called /dev/mapper/sparse1 that has 10GB of actual storage space available. If more than 10GB of data is written to this device, it will start returning I/O errors.