writing large files quickly

Grant Edwards grante at visi.com
Fri Jan 27 18:38:51 EST 2006

On 2006-01-27, Steven D'Aprano <steve at REMOVETHIScyber.com.au> wrote:

>> Because it isn't really writing the zeros.   You can make these
>> files all day long and not run out of disk space, because this
>> kind of file doesn't take very many blocks.   The blocks that
>> were never written are virtual blocks, inasmuch as read() at
>> that location will cause the filesystem to return a block of NULs.
> Isn't this a file system specific solution though? Won't your file system
> need to have support for "sparse files", or else it won't work?

If your fs doesn't support sparse files, then you'll end up with a
file that really does have 400MB of 0x00 bytes in it.  Which is
what the OP really needed in the first place.

> Here is another possible solution, if you are running Linux, farm the real
> work out to some C code optimised for writing blocks to the disk:
> # untested and, it goes without saying, untimed
> os.system("dd if=/dev/zero of=largefile.bin bs=64K count=16384")
> That should make a 4GB file as fast as possible. If you have lots and lots
> of memory, you could try upping the block size (bs=...).

I agree. that probably is the optimal solution for Unix boxes.
I messed around with something like that once, and block sizes
bigger than 64k didn't make much difference.

Grant Edwards                   grante             Yow!  As President I
                                  at               have to go vacuum my coin
                               visi.com            collection!

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