Date: Sun, 16 Jan 2022 11:08:24 +1100 From: Greg 'groggy' Lehey <grog@FreeBSD.org> To: Daniel Feenberg <feenberg@nber.org> Cc: "Kevin P. Neal" <kpn@neutralgood.org>, David Christensen <dpchrist@holgerdanske.com>, freebsd-questions@freebsd.org Subject: Re: zero filling a storage device (was: dd and mbr) Message-ID: <20220116000824.GT61872@eureka.lemis.com> In-Reply-To: <a8c8e1b9-832d-6855-654-60cd1e5b38b@nber.org> References: <77680665-7ddb-23c5-e866-05d112339b60@holgerdanske.com> <20220114023002.GP61872@eureka.lemis.com> <YeDryNdYe1S20wd2@neutralgood.org> <a8c8e1b9-832d-6855-654-60cd1e5b38b@nber.org>
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--9r3HF47jptiQlX4s Content-Type: text/plain; charset=us-ascii Content-Disposition: inline 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 -- Sent from my desktop computer. See complete headers for address and phone numbers. This message is digitally signed. If your Microsoft mail program reports problems, please read http://lemis.com/broken-MUA.php --9r3HF47jptiQlX4s Content-Type: application/pgp-signature; name="signature.asc" -----BEGIN PGP SIGNATURE----- iEYEARECAAYFAmHjYfgACgkQIubykFB6QiNwZQCfQrX7qq1a13eJPRJgi+t8kK+F I7kAnj7Ojnee8/Qr+UpMeljtRy4LykcB =dh2W -----END PGP SIGNATURE----- --9r3HF47jptiQlX4s--
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