sparse-files.txt

see
	stat
	libguestfs-tools
	du
	zerofree - ext2|3|4 - https://frippery.org/uml/
	cryptsetup

	veracrypt.txt
	dd.txt
	mkfs.txt

	https://unix.stackexchange.com/questions/86442/finding-sparse-files
	https://stackoverflow.com/questions/21499451/c-linux-sparse-file-how-to-check-if-file-is-sparse-and-print-0-filled-disk-bl



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https://serverfault.com/questions/696554/creating-a-grow-on-demand-encrypted-volume-with-luks
dd of=/repository/file_container.img bs=1G count=0 seek=10  # mk sparse file
losetup /dev/loop0 /repository/file_container.img

# -y is alias for --verify-passphrase; also --iter-time, -i <number of milliseconds>, ms to spend with PBKDF2 passphrase processing :
cryptsetup -y luksFormat -i 3000 /dev/loop0

# "On a slow device it may be advisable to increase the iteration time" (entropy of key vs iter time):
cryptsetup -y luksFormat -i 11000 /dev/loop0

cryptsetup luksOpen /dev/loop0 secretfs
cryptsetup --allow-discards luksOpen /dev/loop0 secretfs

cryptsetup status secretfs
mkfs.ext4 /dev/mapper/secretfs

fallocate -l 512M /root/image   # preallocate/ deallocate file
cryptsetup luksOpen /root/image luksvolume
mkfs.ext4 -j /dev/mapper/luksvolume



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check if file is sparse: Check size, returned by du utility, and compare with the "apparent size". If you wish you may take a look on the block counting algorithm from du



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Create Truecrypt sparse file on linux (but see tcplay and I think -z option):

http://forums.truecrypt.org/viewtopic.php?t=5176

PostLink to Post: Security Concerns over Hidden Volume on Spare File
Forum: Problems - Linux	Posted: 2007JanSat 07:29
Hi,

I know it's weired - the Linux client allows me to do some interesting things like this:

1. create a spare file using dd of=file count=0 bs=XXX seek=XXX, without a TC volume of any type, then

2. create a hidden volume on top of this sparse file using truecrypt --type hidden -c file

Everything works well, and surprising quick. I guess that's due to the sparse file, the implicit quick format for hidden volume, and the needless to create a normal volume first.

The problem is when I mount the volume and copy file into it, the process is much faster than the normal volume, or hidden volume on non-spare normal volume. It is so fast that it is near raw file system copy speed.

I can't help but to think whether any actual and real encryption was actually performed by TC in this case. I grep the volume file but it seems that the contents apparently were not directly visible.

Can anyone confirm if this method of creating hidden volume is secure and safe?