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

On Thursday, 13 January 2022 at 22:19:36 -0500, 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:
>>> 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?

We're talking about physical blocks here.  I have to guess that you're
thinking about the block remapping that SSDs do, but I've never heard
them called "logical" and "physical".

Writing all zeroes to a physical block and writing nothing to a
physical block are two very different things.  Writing nothing doesn't
change anything.  Writing zeroes to a block with content will change it.

> 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.

If I understand what you mean here, you're suggesting that SSDs may
keep a list of zeroed-out blocks and just optimize them away?  It's
possible, though I haven't heard of it.  I don't know how often blocks
are completely zeroed out, but I suspect that it wouldn't be worth
it.  If they did, it would be a good advertisement, because it would
indirectly increase the storage capacity of the SSD.

Until proof of the contrary, I'd say "no, this doesn't happen".

Greg
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