Date: Fri, 14 Jan 2022 07:52:32 -0500 (EST) From: Daniel Feenberg <feenberg@nber.org> To: "Kevin P. Neal" <kpn@neutralgood.org> Cc: "Greg 'groggy' Lehey" <grog@freebsd.org>, David Christensen <dpchrist@holgerdanske.com>, freebsd-questions@freebsd.org Subject: Re: zero filling a storage device (was: dd and mbr) Message-ID: <a8c8e1b9-832d-6855-654-60cd1e5b38b@nber.org> In-Reply-To: <YeDryNdYe1S20wd2@neutralgood.org> References: <77680665-7ddb-23c5-e866-05d112339b60@holgerdanske.com> <20220114023002.GP61872@eureka.lemis.com> <YeDryNdYe1S20wd2@neutralgood.org>
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On Thu, 13 Jan 2022, Kevin P. Neal wrote: > On Fri, Jan 14, 2022 at 01:30:02PM +1100, Greg 'groggy' Lehey wrote: >> On Thursday, 13 January 2022 at 17:26:12 -0800, David Christensen wrote: >>> On 1/11/22 2:02 AM, Roderick wrote: >>> >>> >>> On 1/13/22 12:32 AM, Roderick wrote: >> >> Yes, that's fine. If the block size is larger than the maximum that >> the disk can handle, the system divides it into smaller transfers. >> >>> Suppose I have a storage device with M blocks of size B bytes each. >>> Some of the blocks contain non-zero bytes and other blocks contain only >>> zero bytes. >>> >>> Suppose I have a program with a memory buffer of N blocks filled with >>> zero bytes, where N < M. >>> >>> If the program writes the buffer to the storage device starting at >>> block-aligned location P, where P + N * B <= M * B, will the storage >>> device write zero bytes to the storage device blocks that already >>> contain only zero bytes? >> >> Yes. The storage device doesn't know the current contents of the >> disk. To do so it would first have to read them, which takes far too >> long. >> >>> Is the answer different for a HDD, an SSD, a USB flash drive, an SD >>> card, a compact flash card, etc.? >> >> No. It's also not different for other operating systems. > > Are we certain that an SSD won't at least track that there is nothing > written to a logical block and therefore it must be all zeros? I'm not > 100% that an SSD will always keep a logical block assigned to a physical > block. And I'm not 100% certain that an SSD won't notice that all zeros > are being written to a block and just optimize out the write. > > -- > Kevin P. Neal http://www.pobox.com/~kpn/ > > "What is mathematics? The age-old answer is, of course, that mathematics > is what mathematicians do." - Donald Knuth > Doesn't the filesystem code handle sparse files on its own? This man page: https://www.freebsd.org/cgi/man.cgi?du strongly implies that it does. A block of all zero bytes shouldn't occupy hardly any disk space at all. See, for instance: https://en.wikipedia.org/wiki/Sparse_file Of course, if the purpose of the write is to zero out blocks for security, then writing a sparse file won't do that. Daniel Feenberg
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