Re: [RFC PATCH v5 0/4] add simple copy support

[Matthew Wilcox]

On Sat, Feb 20, 2021 at 06:01:56PM +0000, David Laight wrote:
> From: SelvaKumar S
> > Sent: 19 February 2021 12:45
> > 
> > This patchset tries to add support for TP4065a ("Simple Copy Command"),
> > v2020.05.04 ("Ratified")
> > 
> > The Specification can be found in following link.
> > https://nvmexpress.org/wp-content/uploads/NVM-Express-1.4-Ratified-TPs-1.zip
> > 
> > Simple copy command is a copy offloading operation and is  used to copy
> > multiple contiguous ranges (source_ranges) of LBA's to a single destination
> > LBA within the device reducing traffic between host and device.
> 
> Sounds to me like the real reason is that the copy just ends up changing
> some indirect block pointers rather than having to actually copy the data.

That would be incorrect, at least for firmware that I have knowledge of.
There are checksums which involve the logical block address of the data,
and you can't just rewrite the checksum on NAND, you have to write the
entire block.

Now, firmware doesn't have to implement their checksum like this,
but there are good reasons to do it this way (eg if the command gets
corrupted in transfer and you read the wrong block, it will fail the
checksum, preventing the drive from returning Somebody Else's Data).

So let's take these people at their word.  It is to reduce traffic
between drive and host.  And that is a good enough reason to do it.