[dm-devel] API for multi-segment atomic IO

[Doug Dumitru]

Mr. Assche,

Thank you for the T10 links.  I was actually looking for something at a
different level.

I have a "smart" block device that can implement multi-segment atomic
writes.  This is more than what T10 envisions in that the writes can span
multiple LBA ranges, and multiple devices in the case of an array.  Thus I
was looking at creating an OS level API for this type of operation.

My initial thought was to create two new fields in the bio struct.

u64 bi_atomic_transaction_id;
u32 bi_atomic_transaction_length;

A writer would populate multiple bio requests with the same transaction_id
and a single write length which is the length of all bio requests.  The bio
requests do not need to be linear or adjacent.  There would probably need
to be alignment rules.  There would be no requirement that all of the
requests are submitted at the same time.  There would probably need to be
size and concurrency limits to keep the engine reasonable.

My concern with just "making up an experimental API" is that, to be useful,
it needs to play nice inside of the block layer with other pieces.  For
example, if I have a device that supports this API, I would like to be able
to use LVM on top of it, or have it as a member of a software mirror set.
This implies that the block stack has a convention to handle layered
devices with these transactions embedded within them.

Again, thank you for the links.

Doug Dumitru

On Wed, Jul 8, 2015 at 8:38 AM, Bart Van Assche <bart.vanassche at sandisk.com>
wrote:

> On 07/07/2015 08:33 PM, Doug Dumitru wrote:
>
>> ... is anyone working on such an animal.  I was thinking of a "struct
>> bio" extension.  I know that Fusion had a private API for this, but I am
>> not a Fusion customer so I have not seen their concepts.
>>
>
> The following document may be helpful: "SBC-4 SPC-5 Atomic writes and
> reads" (http://www.t10.org/cgi-bin/ac.pl?t=d&f=13-064r9.pdf). Please note
> that the information about atomic writes in that document has been
> superseded by the section in the SBC-4 document about atomic writes (
> http://www.t10.org/cgi-bin/ac.pl?t=f&f=sbc4r07c.pdf).
>
> Bart.
>



-- 
Doug Dumitru
EasyCo LLC
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