optimum value for bs in 'dd' for chrooted linux?

Question

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 ::

read/write speed on img
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.

This dd command zeros the image. It seems likely that you will only be doing this once. Speed should not matter. — Stephen Rauch, Jan 14 '17 at 08:03
What about the file size like a file having actual file size 300KB but taking space 1MB on img ? — jonny789, Jan 14 '17 at 08:12
Possible duplicate of Is there a way to determine the optimal value for the bs parameter to dd? — Gilles 'SO- stop being evil', Jan 15 '17 at 21:09
@Gilles not a duplicate in this case, as the bs is only used by the human and not the hardware in this case. — icarus, Jan 15 '17 at 22:38

icarus · Answer 1 · 2017-01-15T22:39:51.583

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.

Ortomala Lokni · Answer 2 · 2017-01-14T19:12:27.240

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);
          do
            {
              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)
                write_records--;
              else
                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.

Remarks:

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.

optimum value for bs in 'dd' for chrooted linux?

2 Answers2