As title, I wonder how fsutil in windows can create a really large file so fast. Does it really allocate real cluster for that file or it just writes down file's metadata? Consider two commands below:
fsutil file createnew testFile <1Tb>
dd if=/dev/zero of=testFile bs=1024M count=1024
So I create a file with 1Tb size, the problem is with fsutil, the file is nearly created immediately, but with dd, it took over 1 hour to complete.
Therefore, I guess that fsutil only writes metadata to the file header, the real file cluster will expand whenever needed. Do I think right?
from Microsoft documentation
createnew
Creates a file of the specified name and size, with content that consists of zeroes.
from here, you can say that the file must be all zeros ([...]with content that consists of zeroes)
but, if this is truth
[...]the file is nearly created immediately[...]
I think that you are right: probably, fsutil creates the file with bytes marked as free at the time of execution, but doesn't write those bytes
When you use dd like this
dd if=/dev/zero of=testFile bs=1024M count=1024
you are actually writing, "byte by byte", zeros in each byte of the new file
You can do this:
fsutil file createnew testFile_fsutil <1Tb> #(on Windows)
dd if=/dev/zero of=testFile_dd bs=1024M count=1024 #(on Linux)
and then, you can see the contents of testFile_fsutil on any hexeditor, and looking for non-zero bytes, or, more precisely, from Linux you can do ( 1099511627776 bytes = 1 Tebibyte ):
cmp --bytes=1099511627776 testFile_fsutil testFile_dd
or
cmp --bytes=1099511627776 testFile_fsutil /dev/zero
or even using hashes:
dd if=/dev/zero bs=1024M count=1024 | sha1sum
return
fea475204f4ad1348f21fad930a7c721a2858971
so,
dd if=testFile_fsutil bs=1024M count=1024 | sha1sum
must return the same.
Note: to prove your point much more quickly, you can use a much smaller file for test.
When I run the split command from terminal on a 1.7 million records, it only generates 1 file and it's blank.
Example command:
split -l 300000 my_file.csv
When I use the -b flag and specify Mbs it works but I don't want to use -b because it breaks the lines where the new files start.
Thanks
I want to create an iso from an external hard drive.
I used this command:
sudo dd if=/dev/sdb of=usb-image.iso
It works, however, the disk is large (700 GB), and i dont have space on my laptop to store that much.
I was thinking about creating multiple iso files (each file 5 GB for example), this way, I can manage them by storing some parts on other drives.
Any help?
Thanks
I'd use the split program to split the output from dd into different files. You can adjust the split size as you see fit (look at the 5000m argument):
dd if=/dev/sdb | split -b 5000m - /tmp/output.gz
This will yield files like /tmp/output.gz.aa, /tmp/output.gz.ab, etc.
Additionally, for further space storage, you can gzip your archive midstream, like this:
dd if=/dev/sdb | gzip -c | split -b 5000m - /tmp/output.gz
Later, when you want to restore, do this:
cat /tmp/output.gz.* | gzip -dc | dd of=/dev/sdb
I would need your help about how can I control the output when I split a large text file in pieces.
For exemple, in this moment when I run the command
split -l 2000 file newfile-
The current output is
newfile-aa
newfile-ab
etc
What I would like to have, if is possible
newfile-000
newfile-001
newfile-002
Thanks for your help.
Use
split -l 2000 -a 3 -d file newfile-
The -a 3 sets the suffix to 3 characters.
The -d uses numeric suffixes.
For testing purposes I have to generate a file of a certain size (to test an upload limit).
What is a command to create a file of a certain size on Linux?
For small files:
dd if=/dev/zero of=upload_test bs=file_size count=1
Where file_size is the size of your test file in bytes.
For big files:
dd if=/dev/zero of=upload_test bs=1M count=size_in_megabytes
Please, modern is easier, and faster. On Linux, (pick one)
truncate -s 10G foo
fallocate -l 5G bar
It needs to be stated that truncate on a file system supporting sparse files will create a sparse file and fallocate will not. A sparse file is one where the allocation units that make up the file are not actually allocated until used. The meta-data for the file will however take up some considerable space but likely no where near the actual size of the file. You should consult resources about sparse files for more information as there are advantages and disadvantages to this type of file. A non-sparse file has its blocks (allocation units) allocated ahead of time which means the space is reserved as far as the file system sees it. Also fallocate nor truncate will not set the contents of the file to a specified value like dd, instead the contents of a file allocated with fallocate or truncate may be any trash value that existed in the allocated units during creation and this behavior may or may not be desired. The dd is the slowest because it actually writes the value or chunk of data to the entire file stream as specified with it's command line options.
This behavior could potentially be different - depending on file system used and conformance of that file system to any standard or specification. Therefore it is advised that proper research is done to ensure that the appropriate method is used.
Just to follow up Tom's post, you can use dd to create sparse files as well:
dd if=/dev/zero of=the_file bs=1 count=0 seek=12345
This will create a file with a "hole" in it on most unixes - the data won't actually be written to disk, or take up any space until something other than zero is written into it.
Use this command:
dd if=$INPUT-FILE of=$OUTPUT-FILE bs=$BLOCK-SIZE count=$NUM-BLOCKS
To create a big (empty) file, set $INPUT-FILE=/dev/zero.
Total size of the file will be $BLOCK-SIZE * $NUM-BLOCKS.
New file created will be $OUTPUT-FILE.
On OSX (and Solaris, apparently), the mkfile command is available as well:
mkfile 10g big_file
This makes a 10 GB file named "big_file". Found this approach here.
You can do it programmatically:
#include <unistd.h>
#include <sys/types.h>
#include <sys/stat.h>
#include <fcntl.h>
#include <stdlib.h>
int main() {
int fd = creat("/tmp/foo.txt", 0644);
ftruncate(fd, SIZE_IN_BYTES);
close(fd);
return 0;
}
This approach is especially useful to subsequently mmap the file into memory.
use the following command to check that the file has the correct size:
# du -B1 --apparent-size /tmp/foo.txt
Be careful:
# du /tmp/foo.txt
will probably print 0 because it is allocated as Sparse file if supported by your filesystem.
see also: man 2 open and man 2 truncate
Some of these answers have you using /dev/zero for the source of your data. If your testing network upload speeds, this may not be the best idea if your application is doing any compression, a file full of zeros compresses really well. Using this command to generate the file
dd if=/dev/zero of=upload_test bs=10000 count=1
I could compress upload_test down to about 200 bytes. So you could put yourself in a situation where you think your uploading a 10KB file but it would actually be much less.
What I suggest is using /dev/urandom instead of /dev/zero. I couldn't compress the output of /dev/urandom very much at all.
you could do:
[dsm#localhost:~]$ perl -e 'print "\0" x 100' > filename.ext
Where you replace 100 with the number of bytes you want written.
dd if=/dev/zero of=my_file.txt count=12345
Use fallocate if you don't want to wait for disk.
Example:
fallocate -l 100G BigFile
Usage:
Usage:
fallocate [options] <filename>
Preallocate space to, or deallocate space from a file.
Options:
-c, --collapse-range remove a range from the file
-d, --dig-holes detect zeroes and replace with holes
-i, --insert-range insert a hole at range, shifting existing data
-l, --length <num> length for range operations, in bytes
-n, --keep-size maintain the apparent size of the file
-o, --offset <num> offset for range operations, in bytes
-p, --punch-hole replace a range with a hole (implies -n)
-z, --zero-range zero and ensure allocation of a range
-x, --posix use posix_fallocate(3) instead of fallocate(2)
-v, --verbose verbose mode
-h, --help display this help
-V, --version display version
This will generate 4 MB text file with random characters in current directory and its name "4mb.txt"
You can change parameters to generate different sizes and names.
base64 /dev/urandom | head -c 4000000 > 4mb.txt
There are lots of answers, but none explained nicely what else can be done. Looking into man pages for dd, it is possible to better specify the size of a file.
This is going to create /tmp/zero_big_data_file.bin filled with zeros, that has size of 20 megabytes :
dd if=/dev/zero of=/tmp/zero_big_data_file.bin bs=1M count=20
This is going to create /tmp/zero_1000bytes_data_file.bin filled with zeros, that has size of 1000 bytes :
dd if=/dev/zero of=/tmp/zero_1000bytes_data_file.bin bs=1kB count=1
or
dd if=/dev/zero of=/tmp/zero_1000bytes_data_file.bin bs=1000 count=1
In all examples, bs is block size, and count is number of blocks
BLOCKS and BYTES may be followed by the following multiplicative suffixes: c =1, w =2, b =512, kB =1000, K =1024, MB =1000*1000, M =1024*1024, xM =M GB =1000*1000*1000, G =1024*1024*1024, and so on for T, P, E, Z, Y.
As shell command:
< /dev/zero head -c 1048576 > output
Kindly run below command for quickly creating larger file with certain size
in linux
for i in {1..10};do fallocate -l 2G filename$i;done
explanation:-Above command will create 10 files with 10GB size in just few seconds.