I use BitTorrent and sometimes encounter files that do not have seed(missing pieces).
At that time, we sometimes force the file transfer to end and try to open the incompleted files (for example, an image file).
If we are lucky, may be able to see the downloaded image even if some parts are lost.
I would like to artificially reproduce this situation, and here's how I tried:
1) spliting a bmp image file of about 1 megabyte into 16 kilobytes by the Linux split command,
2) and then make just one of the divided files 0 kilobytes.
3) after that, rejoin all the files with the cat command.
However, in this case, unlike the torrent's "lost pieces" situation, the file becomes completely corrupt and can not be read.
Theoretically it does not seem like anything special, but what's wrong? And how can I achieve what I want?
I would appreciate your help.
Use dd:
dd if=/dev/zero of=image.jpg bs=1 conv=notrunc seek=X count=Y
being X the offset in the file you want to erase and Y the number of bytes.
About the corruption, it depends on the type of file, the piece you are losing and the program you are using to read it.
For instance, JPG files use a variable bit-length encoding, meaning that just losing one bit may corrupt all the file from that point on. But just for that, there can be resyncronization points where the bitstream is reset, so from that point on, the file will look ok. But those resync points are optional when writing the file, and not every reader honor them in case of corruption...
And anyway, losing part of the headers will make the file totally unreadable.
I have a large file, several gig of binary data, with an ASCII header at the top. I need to make a few small changes to the ASCII header. sed does the job, but it takes a fair bit of time since the file is so large. vi/vim is slow too. Is there any linux utility that can just go into the file, make the change at the top, and then get out quickly?
An example might be a header that looks like:
Code Rev: 3.5
Platform: platform1
Run Date: 12/13/16
Data source: whatever
Restart: False
followed by a large amount of binary data ....
and then I might need to, for example, edit an error in "Data source".
Provided that you know that your header is less than X bytes, you can use dd.
(!) But it only works if both strings have the same length (!)
Lets say, that the header is less that 4096 bytes
dd if=/path/to/file bs=4096 count=1 | sed 's/XXX/YYY/' | dd of=/path/to/file conv=notrunc
You can also do it programmatically, using languages like C,Python,PHP,JAVA etc. The idea is to open the file, read the header, fix it, and write it back.
I am looking at making a write optimization for CIFS/SMB such that the writing of duplicate blocks are suppressed. For example, I read a file from the remote share and modify a portion near the end of the file. When I save the file, I only want to send write requests back to the remote side for the portions of the file that have actually changed. So basically, suppress all writes up until the point at which a non duplicate write is encountered. At that point the suppression will be disabled and the writes will be allowed as usual. The problem is I can't find any documentation MS-SMB/MS-SMB2/MS-CIFS or otherwise that indicates whether or not this is a valid thing to do. Does anyone know if this would be valid?
Dig deep into the sources of the Linux kernel, there is documentation on CIFS - both in source and text. E.g. http://www.mjmwired.net/kernel/Documentation/filesystems/cifs.txt
If you want to study the behaviour of e.g. the CIFS protocol, you may be able to test it with the unix command "dd". Mount any remote file-system via CIFS, e.g. into /media/remote. Change into this folder cd /media/remote Now create a file with some random stuff (e.g. from the kernel's random pool): dd if=/dev/urandom of=test.bin bs=4M count=5 In this example, you should see some 20MB of traffic. Then create another smaller file, somewhere on your machine, say, your home-folder: dd if=/dev/urandom of=~/test_chunk.bin bs=4M count=1 The interesting thing is what happens, if you attempt to write the chunk into a specific position of the remote test file: dd if=~/test_chunk.bin of=test.bin bs=4M count=1 seek=3 conv=notrunc Actually, this should only change block #4 out of 5 in the target file.
I guess you can adjust the block size ... I did this with 4 MB blocks. But it should help to understand what happens on the network.
The CIFS protocol does allow applications to write back specific portions of the file. This is controlled by the parameters DataOffset and DataLength in the SMB WriteAndX packet.
Documentation for the same can be found here:
http://msdn.microsoft.com/en-us/library/ee441954.aspx
The client can use these fields to write a specific length of data to specific offsets within the file.
Similar support exists in more recent versions of the protocol as well ...
SMB protocol have such write optimization. It works with append cifs operation. Where protocol read EOF for file and start writing new data with offset set to EOF value and length as append data bytes.
We have a smart media card with a linux install on it that we need to duplicate. We created an img with DD and then used dd to write the img back to a couple of new smart media cards. We have compared the MD5 checksum of both the original and the new copies and they are different.
Here is what we used:
dd if=/dev/sdb of=myimage.img
dd if=myimage.img of=/dev/sdb
dd if=/dev/sdb of=newimage.img
Anyone have any ideas of why these come out different?
If the cards are different sizes, dd'ing the smaller image to a larger card will not "fill it up", and zeros will remain at the end of the card. An image made from this card will be different than the original image.
It's also always possible that data was mis-written, mis-read, or otherwise corrupted in-transit.
The card capacities differ?
Running 'ls -l myimage.img newimage.img' might tell you something.
Running 'cmp -l myimage.img newimage.img' might tell you something.
If you mounted /dev/sdb in between it would be an answer. If I remember correctly ext2 and ext3 have a "mount counter".
How can I quickly create a large file on a Linux (Red Hat Linux) system?
dd will do the job, but reading from /dev/zero and writing to the drive can take a long time when you need a file several hundreds of GBs in size for testing... If you need to do that repeatedly, the time really adds up.
I don't care about the contents of the file, I just want it to be created quickly. How can this be done?
Using a sparse file won't work for this. I need the file to be allocated disk space.
dd from the other answers is a good solution, but it is slow for this purpose. In Linux (and other POSIX systems), we have fallocate, which uses the desired space without having to actually writing to it, works with most modern disk based file systems, very fast:
For example:
fallocate -l 10G gentoo_root.img
This is a common question -- especially in today's environment of virtual environments. Unfortunately, the answer is not as straight-forward as one might assume.
dd is the obvious first choice, but dd is essentially a copy and that forces you to write every block of data (thus, initializing the file contents)... And that initialization is what takes up so much I/O time. (Want to make it take even longer? Use /dev/random instead of /dev/zero! Then you'll use CPU as well as I/O time!) In the end though, dd is a poor choice (though essentially the default used by the VM "create" GUIs). E.g:
dd if=/dev/zero of=./gentoo_root.img bs=4k iflag=fullblock,count_bytes count=10G
truncate is another choice -- and is likely the fastest... But that is because it creates a "sparse file". Essentially, a sparse file is a section of disk that has a lot of the same data, and the underlying filesystem "cheats" by not really storing all of the data, but just "pretending" that it's all there. Thus, when you use truncate to create a 20 GB drive for your VM, the filesystem doesn't actually allocate 20 GB, but it cheats and says that there are 20 GB of zeros there, even though as little as one track on the disk may actually (really) be in use. E.g.:
truncate -s 10G gentoo_root.img
fallocate is the final -- and best -- choice for use with VM disk allocation, because it essentially "reserves" (or "allocates" all of the space you're seeking, but it doesn't bother to write anything. So, when you use fallocate to create a 20 GB virtual drive space, you really do get a 20 GB file (not a "sparse file", and you won't have bothered to write anything to it -- which means virtually anything could be in there -- kind of like a brand new disk!) E.g.:
fallocate -l 10G gentoo_root.img
Linux & all filesystems
xfs_mkfile 10240m 10Gigfile
Linux & and some filesystems (ext4, xfs, btrfs and ocfs2)
fallocate -l 10G 10Gigfile
OS X, Solaris, SunOS and probably other UNIXes
mkfile 10240m 10Gigfile
HP-UX
prealloc 10Gigfile 10737418240
Explanation
Try mkfile <size> myfile as an alternative of dd. With the -n option the size is noted, but disk blocks aren't allocated until data is written to them. Without the -n option, the space is zero-filled, which means writing to the disk, which means taking time.
mkfile is derived from SunOS and is not available everywhere. Most Linux systems have xfs_mkfile which works exactly the same way, and not just on XFS file systems despite the name. It's included in xfsprogs (for Debian/Ubuntu) or similar named packages.
Most Linux systems also have fallocate, which only works on certain file systems (such as btrfs, ext4, ocfs2, and xfs), but is the fastest, as it allocates all the file space (creates non-holey files) but does not initialize any of it.
truncate -s 10M output.file
will create a 10 M file instantaneously (M stands for 10241024 bytes, MB stands for 10001000 - same with K, KB, G, GB...)
EDIT: as many have pointed out, this will not physically allocate the file on your device. With this you could actually create an arbitrary large file, regardless of the available space on the device, as it creates a "sparse" file.
For e.g. notice no HDD space is consumed with this command:
### BEFORE
$ df -h | grep lvm
/dev/mapper/lvm--raid0-lvm0
7.2T 6.6T 232G 97% /export/lvm-raid0
$ truncate -s 500M 500MB.file
### AFTER
$ df -h | grep lvm
/dev/mapper/lvm--raid0-lvm0
7.2T 6.6T 232G 97% /export/lvm-raid0
So, when doing this, you will be deferring physical allocation until the file is accessed. If you're mapping this file to memory, you may not have the expected performance.
But this is still a useful command to know. For e.g. when benchmarking transfers using files, the specified size of the file will still get moved.
$ rsync -aHAxvP --numeric-ids --delete --info=progress2 \
root#mulder.bub.lan:/export/lvm-raid0/500MB.file \
/export/raid1/
receiving incremental file list
500MB.file
524,288,000 100% 41.40MB/s 0:00:12 (xfr#1, to-chk=0/1)
sent 30 bytes received 524,352,082 bytes 38,840,897.19 bytes/sec
total size is 524,288,000 speedup is 1.00
Where seek is the size of the file you want in bytes - 1.
dd if=/dev/zero of=filename bs=1 count=1 seek=1048575
Examples where seek is the size of the file you want in bytes
#kilobytes
dd if=/dev/zero of=filename bs=1 count=0 seek=200K
#megabytes
dd if=/dev/zero of=filename bs=1 count=0 seek=200M
#gigabytes
dd if=/dev/zero of=filename bs=1 count=0 seek=200G
#terabytes
dd if=/dev/zero of=filename bs=1 count=0 seek=200T
From the dd manpage:
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, GB =1000*1000*1000, G=1024*1024*1024, and so on for T, P, E, Z, Y.
To make a 1 GB file:
dd if=/dev/zero of=filename bs=1G count=1
I don't know a whole lot about Linux, but here's the C Code I wrote to fake huge files on DC Share many years ago.
#include < stdio.h >
#include < stdlib.h >
int main() {
int i;
FILE *fp;
fp=fopen("bigfakefile.txt","w");
for(i=0;i<(1024*1024);i++) {
fseek(fp,(1024*1024),SEEK_CUR);
fprintf(fp,"C");
}
}
You can use "yes" command also. The syntax is fairly simple:
#yes >> myfile
Press "Ctrl + C" to stop this, else it will eat up all your space available.
To clean this file run:
#>myfile
will clean this file.
I don't think you're going to get much faster than dd. The bottleneck is the disk; writing hundreds of GB of data to it is going to take a long time no matter how you do it.
But here's a possibility that might work for your application. If you don't care about the contents of the file, how about creating a "virtual" file whose contents are the dynamic output of a program? Instead of open()ing the file, use popen() to open a pipe to an external program. The external program generates data whenever it's needed. Once the pipe is open, it acts just like a regular file in that the program that opened the pipe can fseek(), rewind(), etc. You'll need to use pclose() instead of close() when you're done with the pipe.
If your application needs the file to be a certain size, it will be up to the external program to keep track of where in the "file" it is and send an eof when the "end" has been reached.
One approach: if you can guarantee unrelated applications won't use the files in a conflicting manner, just create a pool of files of varying sizes in a specific directory, then create links to them when needed.
For example, have a pool of files called:
/home/bigfiles/512M-A
/home/bigfiles/512M-B
/home/bigfiles/1024M-A
/home/bigfiles/1024M-B
Then, if you have an application that needs a 1G file called /home/oracle/logfile, execute a "ln /home/bigfiles/1024M-A /home/oracle/logfile".
If it's on a separate filesystem, you will have to use a symbolic link.
The A/B/etc files can be used to ensure there's no conflicting use between unrelated applications.
The link operation is about as fast as you can get.
The GPL mkfile is just a (ba)sh script wrapper around dd; BSD's mkfile just memsets a buffer with non-zero and writes it repeatedly. I would not expect the former to out-perform dd. The latter might edge out dd if=/dev/zero slightly since it omits the reads, but anything that does significantly better is probably just creating a sparse file.
Absent a system call that actually allocates space for a file without writing data (and Linux and BSD lack this, probably Solaris as well) you might get a small improvement in performance by using ftrunc(2)/truncate(1) to extend the file to the desired size, mmap the file into memory, then write non-zero data to the first bytes of every disk block (use fgetconf to find the disk block size).
This is the fastest I could do (which is not fast) with the following constraints:
The goal of the large file is to fill a disk, so can't be compressible.
Using ext3 filesystem. (fallocate not available)
This is the gist of it...
// include stdlib.h, stdio.h, and stdint.h
int32_t buf[256]; // Block size.
for (int i = 0; i < 256; ++i)
{
buf[i] = rand(); // random to be non-compressible.
}
FILE* file = fopen("/file/on/your/system", "wb");
int blocksToWrite = 1024 * 1024; // 1 GB
for (int i = 0; i < blocksToWrite; ++i)
{
fwrite(buf, sizeof(int32_t), 256, file);
}
In our case this is for an embedded linux system and this works well enough, but would prefer something faster.
FYI the command dd if=/dev/urandom of=outputfile bs=1024 count = XX was so slow as to be unusable.
Shameless plug: OTFFS provides a file system providing arbitrarily large (well, almost. Exabytes is the current limit) files of generated content. It is Linux-only, plain C, and in early alpha.
See https://github.com/s5k6/otffs.
So I wanted to create a large file with repeated ascii strings. "Why?" you may ask. Because I need to use it for some NFS troubleshooting I'm doing. I need the file to be compressible because I'm sharing a tcpdump of a file copy with the vendor of our NAS. I had originally created a 1g file filled with random data from /dev/urandom, but of course since it's random, it means it won't compress at all and I need to send the full 1g of data to the vendor, which is difficult.
So I created a file with all the printable ascii characters, repeated over and over, to a limit of 1g in size. I was worried it would take a long time. It actually went amazingly quickly, IMHO:
cd /dev/shm
date
time yes $(for ((i=32;i<127;i++)) do printf "\\$(printf %03o "$i")"; done) | head -c 1073741824 > ascii1g_file.txt
date
Wed Apr 20 12:30:13 CDT 2022
real 0m0.773s
user 0m0.060s
sys 0m1.195s
Wed Apr 20 12:30:14 CDT 2022
Copying it from an nfs partition to /dev/shm took just as long as with the random file (which one would expect, I know, but I wanted to be sure):
cp ascii1gfile.txt /home/greygnome/
uptime; free -m; sync; echo 1 > /proc/sys/vm/drop_caches; free -m; date; dd if=/home/greygnome/ascii1gfile.txt of=/dev/shm/outfile bs=16384 2>&1; date; rm -f /dev/shm/outfile
But while doing that I ran a simultaneous tcpdump:
tcpdump -i em1 -w /dev/shm/dump.pcap
I was able to compress the pcap file down to 12M in size! Awesomesauce!
Edit: Before you ding me because the OP said, "I don't care about the contents," know that I posted this answer because it's one of the first replies to "how to create a large file linux" in a Google search. And sometimes, disregarding the contents of a file can have unforeseen side effects.
Edit 2: And fallocate seems to be unavailable on a number of filesystems, and creating a 1GB compressible file in 1.2s seems pretty decent to me (aka, "quickly").
You could use https://github.com/flew-software/trash-dump
you can create file that is any size and with random data
heres a command you can run after installing trash-dump (creates a 1GB file)
$ trash-dump --filename="huge" --seed=1232 --noBytes=1000000000
BTW I created it