Tetsuo Handa wrote:
Hello.
Andreas Dilger wrote:
Why would you fill the buffer with 0xff instead of 0?
In fact no such program is needed, just "dd if=/dev/zero of=/{fs}/tmp"
and then delete the file.
To avoid that the /{fs}/tmp is created as a sparse file.
Most filesystems will not create a sparse file if zero-byte filled
blocks are written. To create a sparse file you normally have to seek
beyond the file end and then write blocks, leaving a hole in-between the
positions.
The information from stat can tell you if a file has been stored
sparsely, because the blocks used count will be less than the file size
suggests.
For example:
$ dd if=/dev/zero of=small-file bs=512 count=16
16+0 records in
16+0 records out
8192 bytes (8.2 kB) copied, 0.000187744 seconds, 43.6 MB/s
phillip@dylan:/tmp$ stat small-file
File: `small-file'
Size: 8192 Blocks: 16 IO Block: 4096 regular file
Device: 801h/2049d Inode: 847831 Links: 1
The zero filled file uses 16 blocks (16 * 512 bytes = 8K), and so we
know it isn't sparsely stored.
You can get dd to seek a number of blocks into the output file before
writing, this will create a sparse file...
$ dd if=/dev/zero of=small-file bs=512 count=16 seek=16
16+0 records in
16+0 records out
8192 bytes (8.2 kB) copied, 0.000212609 seconds, 38.5 MB/s
phillip@dylan:/tmp$ stat small-file
File: `small-file'
Size: 16384 Blocks: 16 IO Block: 4096 regular file
Device: 801h/2049d Inode: 847831 Links: 1
A 16K file is only using 16 blocks, and so 8K is stored sparsely.
Phillip
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