I am trying to install chrooted debian (arm64) on my android phone(snapdragon650 & 3GB RAM). I found many tutorials for it. Almost all the guides mentioned this cmd for creating img for chrooted debian.

 dd if=/dev/zero of=jessie-arm64.img bs=1M count=0 seek=5120

Here bs=1M . I don't know much about dd. But I think bs parameter is gonna affect ::

  1. read/write speed on img
  2. Space allocation to files & their file size

I am going to use chrooted environment for compiling source code for arm64 and running a bit heavy gui programs (in openbox or xfce ). I want that value for bs which would be optimum for both read/write speed and space utilisation.


2 Answers 2


The count=0 parameter is very significant here, this command is just being used to create a file of a particular size, nothing is actually being copied.

Therefore there is no optimal value for bs from the hardware point of view

The created file will have size bs * seek. The bs of 1 megabyte just makes it easy to see that you are creating a file with size 5120 megabytes.

At a system call level, this is just going to do an open, an lseek and a close call. dd will do the bs * seek calculation and give it to the lseek system call. So bs if=/dev/zero seek=5368709120 bs=1 of=jesse-arm64.img will produce exactly the same system calls.


First, some details about the command

dd if=/dev/zero of=jessie-arm64.img bs=1M count=0 seek=5120

From the man page of dd :

  • if=FILE read from FILE instead of stdin

  • of=FILE write to FILE instead of stdout

  • bs=BYTES read and write up to BYTES bytes at a time

  • count=N copy only N input blocks

  • seek=N skip N obs-sized blocks at start of output

We have count=0, so 0 input block will be copied but we have seek=5120 so 5120 blocks of size obs will be skipped. obsis the output block size, in our case it's not specified so obs=bs=1M.

If you want you can even remove the if=/dev/zero and obtain the same result.

dd of=jessie-arm64.img bs=1M count=0 seek=5120

Now the question of optimum value for bs, we observe in the source code of dd that even if count=0 blocks are copied, we have seek=5120 (seek_recordsin the source code) and the bs (obs, output_blocksizeint the source code) value is used.

  if (seek_records != 0 || seek_bytes != 0)
      size_t bytes = seek_bytes;
      uintmax_t write_records = skip (STDOUT_FILENO, output_file,
                                      seek_records, output_blocksize, &bytes);
      if (write_records != 0 || bytes != 0)
          memset (obuf, 0, write_records ? output_blocksize : bytes);
              size_t size = write_records ? output_blocksize : bytes;
              if (iwrite (STDOUT_FILENO, obuf, size) != size)
                  error (0, errno, _("writing to %s"), quoteaf (output_file));
                  quit (EXIT_FAILURE);
              if (write_records != 0)
                bytes = 0;
          while (write_records || bytes);

The optimum value of bsis dependant of your hardware, here is an article explaining how to measure it. I will not go in detailed explanations here, because this answer is becoming too long.


1) You can observe your image in hexadecimal format with :

cat jessie-arm64.img | xxd

2) This is valid for the dd command from the GNU coreutils. If you are using a BSD-style dd command such as in macOS, you have to use bs=1m instead of bs=1M.

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