Re: [LSF/MM TOPIC][LSF/MM ATTEND] OCSSDs - SMR, Hierarchical Interface, and Vector I/Os

[Matias Bjorling <m@bjorling.me>]

On 01/11/2017 05:07 AM, Damien Le Moal wrote:
> 
> Matias,
> 
> On 1/10/17 22:06, Matias Bjorling wrote:
>> On 01/10/2017 05:24 AM, Theodore Ts'o wrote:
>>> This may be an area where if we can create the right framework, and
>>> fund some research work, we might be able to get some researchers and
>>> their graduate students interested in doing some work in figuring out
>>> what sort of divisions of responsibilities and hints back and forth
>>> between the storage device and host have the most benefit.
>>>
>>
>> That is a good idea. There is a couple of papers at FAST with
>> Open-Channel SSDs this year.  They look into the interface and various
>> ways to reduce latency fluctuations.
>>
>> One thing I've heard a couple of times is the feature to move the GC
>> read/write process into the firmware. Enabling the host to offload GC
>> data movement, while the keeping the host in control. Would this be
>> beneficial for SMR?
> 
> Host-aware SMR drives already have GC internally implemented (for cases
> when the host does not write sequentially). Host-managed drives do not.
> As for moving an application specific GC code into the device, well,
> code injection in the storage device is not for tomorrow, and likely not
> ever.
> 
> There are however other clever ways to reduce GC related host overhead
> with basic commands. For SCSI, these may be WRITE SCATTERED, EXTENDED
> COPY, and some others can greatly improve overhead over a simple
> read+write loop. A better approach to GC offload may not be a "GC"
> command, but something more generic for moving around LBAs internally
> within the device. That is, if existing commands are not satisfactory.

Hi Damien,

You're right. I was thinking of something similar to scattered
read/write to move data from one place to another. There is no
sector-granularity mapping table maintained by the OCSSD, which leaves
the logic up to the host.

Let me know if you decide to kick of a standardized interface for code
injection. Such an interface is long overdue. ;)