Re: zero filling a storage device (was: dd and mbr)

On Friday, 14 January 2022 at  7:52:32 -0500, Daniel Feenberg wrote:
>
> Doesn't the filesystem code handle sparse files on its own? This man page:
>
>     https://www.freebsd.org/cgi/man.cgi?du

FWIW, it's easier to access this with 'man du'.

> strongly implies that it does. A block of all zero bytes shouldn't
> occupy hardly any disk space at all.

Disk files are allocated in blocks.  No data == no blocks.  You can
also write to a file, then lseek(1) to an address beyond the end of
the file and write there.  This will create a "sparse file": the
intervening blocks will not be allocated.  It's messy, though, because
the allocation is by block, but writes don't need to adhere to block
boundaries.

Greg
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