Linux – mkfsdos: define the size of FAT16|32 file system on USB pendrive in Linux

fatlinux

Question: How does one define the size of a FAT16 / FAT 32 file system with mkdosfs in Linux

I want to create a, say 2 GB FAT16|32 filesystem on a partition in the size of 8 GB or more?

This is how far I got:

created a partition with fdisk
mkdosfs -F 16 /dev/sdb1 creates a FAT16 file system over the WHOLE partition – as long as this isn't larger tha 4 GB, I know
mkdosfs -F 32 /dev/sdb1 creates a FAT32 filesystem over the WHOLE partition. I know that this is the default and that I wouldn't neet to specify -F 32 but for the sake of completeness and style…
according to man mkdosfs the size of the file system is to be defined as the last argument
HOW?! So far all of my attempts to define the size returned error messages. All of them.

Guessing that I just ran into a massice misunderstanding and being frustrated about not being able to solve such a simple question all by myself I really wonder where the heck did I miss something in defining the size?!

Best Answer

This worked for me:

# truncate -s 8G foobar
# losetup -f --show foobar
/dev/loop0
# mkdosfs -F 32 /dev/loop0 $((2*1024*1024))
mkfs.fat 3.0.25 (2014-01-17)
Warning: block count mismatch: found 8388608 but assuming 2097152.
Loop device does not match a floppy size, using default hd params
# mount /dev/loop0 /mnt/tmp
# df -h /mnt/tmp
Filesystem      Size  Used Avail Use% Mounted on
/dev/loop0      2.0G  4.0K  2.0G   1% /mnt/tmp

An alternative approach, in case the mkfs does not have an option to limit the size, is to create a loop device with limit:

# losetup -f --show --sizelimit $((2*1024*1024*1024)) /dev/loop0
/dev/loop1
# mkdosfs -F 32 /dev/loop1
mkfs.fat 3.0.25 (2014-01-17)
Loop device does not match a floppy size, using default hd params
# mount /dev/loop0 /mnt/tmp
# df -h /mnt/tmp
Filesystem      Size  Used Avail Use% Mounted on
/dev/loop0      2.0G  4.0K  2.0G   1% /mnt/tmp

Of course, if it's a partition, you could also shrink the partition, then enlarge it again.

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Linux – Will swap file engage automatically when I write too much to /dev/shm

I just went ahead and see what happens, when you write too many files to /dev/shm.

OSError: [Errno 28] No space left on device

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "/mnt/fastssd/bot_subreddit_recom/write_user_docs.py", line 85, in <module>
    f.write(subreddits)
OSError: [Errno 28] No space left on device
inFile: RC_2018-01-02
outDir: tmp
outputToScreenOnly: 0
OSError: [Errno 28] No space left on device

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "/mnt/fastssd/bot_subreddit_recom/write_user_docs.py", line 85, in <module>
    f.write(subreddits)
OSError: [Errno 28] No space left on device

It “ran out of space” after 33 days of input files, as shown above.

>>> 36.*33/12
99.0

So an estimated 99GB -- which is very close to 100GB -- of files, is where the “no space left” error happened. This is exactly the size of the /dev/shm device which I had set to be 100G. This means swap was not really able to be used when it was needed. This has demonstrated that unfortunately, /dev/shm simply runs out, and does not let you start to use swap as virtual memory to keep going, when there’s a shortage. So now we know thanks to this experiment. You just need real physical RAM for /dev/shm to work, and virtual RAM aka swap is not helpful for /dev/shm at all.

I have posted this answer to share with others who may benefit from the information gained.

Best Answer

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