Sparse file
According to Wikipedia, in computer science, a sparse file is a type of computer file that attempts to use file system space more efficiently when blocks allocated to the file are mostly empty. This is achieved by writing brief information (metadata) representing the empty blocks to disk instead of the actual "empty" space which makes up the block, using less disk space. The full block size is written to disk as the actual size only when the block contains "real" (non-empty) data.
When reading sparse files, the file system transparently converts metadata representing empty blocks into "real" blocks filled with zero bytes at runtime. The application is unaware of this conversion.
Most modern file systems support sparse files, including most Unix variants and NTFS, but notably not Apple's HFS+. Sparse files are commonly used for disk images (not to be confused with sparse images), database snapshots, log files and in scientific applications.
The advantage of sparse files is that storage is only allocated when actually needed: disk space is saved, and large files can be created even if there is insufficient free space on the file system.
Disadvantages are that sparse files may become fragmented; file system free space reports may be misleading; filling up file systems containing sparse files can have unexpected effects; and copying a sparse file with a program that does not explicitly support them may copy the entire file, including the empty blocks which are not on explicitly stored on the disk, which wastes the benefits of the sparse property of the file.
Creating sparse files
The truncate utility can create sparse files. This command creates a 512 MiB sparse file:
$ truncate -s 512M file.img
The dd utility can also be used, for example:
$ dd if=/dev/zero of=file.img bs=1 count=0 seek=512M
Sparse files have different apparent file sizes (the maximum size to which they may expand) and actual file sizes (how much space is allocated for data on disk). To check the file's apparent size, just run:
$ du -h --apparent-size file.img
512M file.img
and, to check the actual size of the file on disk:
$ du -h file.img
0 file.img
As you can see, although the apparent size of the file is 512 MiB, its "actual" size is really zero—that's because due to the nature and beauty of sparse files, it will "expand" arbitrarily to minimize the space required to store its contents.
Making existing files sparse
The fallocate utility can make existing files sparse on supported file systems:
$ cp file.img copy.img --sparse=never $ du -h copy.img 512M copy.img
$ fallocate -d copy.img $ du -h copy.img 0 copy.img
Formatting the file with a filesystem
Now that we have created the sparse file, it is time to format it with a filesystem; for example ReiserFS:
$ mkfs.reiserfs -f -q file.img
We can now check its size to see how the filesystem has affected it:
$ du -h --apparent-size file.img
512M file.img
$ du -h file.img
33M file.img
As you may have expected, formatting it with the filesystem has increased its actual size, but left its apparent size the same. Now we can create a directory which we will use to mount our file:
# mkdir mountpoint # mount -o loop file.img mountpoint
Tada! We now have both a file and a folder into which we may store almost 512 MiB worth of information!
Mounting the file at boot
To mount the sparse image automatically at boot, add an entry to your fstab:
/path/to/file.img /path/to/mountpoint reiserfs loop,defaults 0 0
loop option, otherwise it will not mount.Copying the sparse file
Copying with cp
Normally, cp is good at detecting whether a file is sparse, so it suffices to run:
$ cp file.img new_file.img
Then new_file.img will be sparse. However, cp does have a --sparse=when option. This is especially useful if a sparse file has somehow become non sparse (i.e. the empty blocks have been written out to disk in full). Disk space can be recovered by:
$ cp --sparse=always new_file.img recovered_file.img
Archiving with tar
One day, you may decide to back up your well-loved sparse file, and choose the tar utility for that very purpose; however, you soon realize you have a problem:
$ du -h file.img
33M file.img
$ tar -cf file.tar file.img
$ du -h file.tar
513M file.tar
Apparently, even though the current size of the sparse file is only 33 MB, archiving it with tar created an archive of the ENTIRE SIZE OF THE FILE! Luckily for you, though, tar has a `--sparse' (`-S') flag, that when used in conjunction with the `--create' (`-c') operation, tests all files for sparseness while archiving. If tar finds a file to be sparse, it uses a sparse representation of the file in the archive. This is useful when archiving files, such as dbm files, likely to contain many nulls, and dramatically decreases the amount of space needed to store such an archive.
$ tar -Scf file.tar file.img
$ du -h file.tar
12K file.tar
Problem solved.
Resizing the sparse file
Before we resize the file, let us populate it with a couple small files for testing purposes:
$ for f in {1..5}; do touch folder/file${f}; done
$ ls folder/ file1 file2 file3 file4 file5
Now, let us add some content to one of the files:
$ echo "This is a test to see if it works..." >> folder/file1
$ cat folder/file1 This is a test to see if it works...
Growing the file
Should you ever need to grow the file, you may do the following:
# umount folder # dd if=/dev/zero of=file.img bs=1 count=0 seek=1G 0+0 records in 0+0 records out 0 bytes (0 B) copied, 2.2978e-05 s, 0.0 kB/s
This will increase its size to 1 Gb, and leave its information intact. Next, we need to increase the size of its filesystem:
# resize_reiserfs file.img resize_reiserfs 3.6.21 (2009 www.namesys.com) ReiserFS report: blocksize 4096 block count 262144 (131072) free blocks 253925 (122857) bitmap block count 8 (4) Syncing..done resize_reiserfs: Resizing finished successfully.
...and, remount it:
# mount -o loop file.img folder
Checking its size gives us:
# du -h --apparent-size file.img 1.0G file.img # du -h file.img 33M file.img
...and to check for consistency:
# df -h folder Filesystem Size Used Avail Use% Mounted on /tmp/file.img 1.0G 33M 992M 4% /tmp/folder
# ls folder file1 file2 file3 file4 file5 # cat folder/file1 This is a test to see if it works...