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

[Matias Bjørling]

On 01/04/2017 08:24 AM, Damien Le Moal wrote:
> 
> Slava,
> 
> On 1/4/17 11:59, Slava Dubeyko wrote:
>> What's the goal of SMR compatibility? Any unification or interface
>> abstraction has the goal to hide the peculiarities of underlying
>> hardware. But we have block device abstraction that hides all 
>> hardware's peculiarities perfectly. Also FTL (or any other
>> Translation Layer) is able to represent the device as sequence of
>> physical sectors without real knowledge on software side about 
>> sophisticated management activity on the device side. And, finally,
>> guys will be completely happy to use the regular file systems (ext4,
>> xfs) without necessity to modify software stack. But I believe that 
>> the goal of Open-channel SSD approach is completely opposite. Namely,
>> provide the opportunity for software side (file system, for example)
>> to manage the Open-channel SSD device with smarter policy.
> 
> The Zoned Block Device API is part of the block layer. So as such, it
> does abstract many aspects of the device characteristics, as so many
> other API of the block layer do (look at blkdev_issue_discard or zeroout
> implementations to see how far this can be pushed).
> 
> Regarding the use of open channel SSDs, I think you are absolutely
> correct: (1) some users may be very happy to use a regular, unmodified
> ext4 or xfs on top of an open channel SSD, as long as the FTL
> implementation does a complete abstraction of the device special
> features and presents a regular block device to upper layers. And
> conversely, (2) some file system implementations may prefer to directly
> use those special features and characteristics of open channel SSDs. No
> arguing with this.
> 
> But you are missing the parallel with SMR. For SMR, or more correctly
> zoned block devices since the ZBC or ZAC standards can equally apply to
> HDDs and SSDs, 3 models exists: drive-managed, host-aware and host-managed.
> 
> Case (1) above corresponds *exactly* to the drive managed model, with
> the difference that the abstraction of the device characteristics (SMR
> here) is in the drive FW and not in a host-level FTL implementation as
> it would be for open channel SSDs. Case (2) above corresponds to the
> host-managed model, that is, the device user has to deal with the device
> characteristics itself and use it correctly. The host-aware model lies
> in between these 2 extremes: it offers the possibility of complete
> abstraction by default, but also allows a user to optimize its operation
> for the device by allowing access to the device characteristics. So this
> would correspond to a possible third way of implementing an FTL for open
> channel SSDs.
> 
>> So, my key worry that the trying to hide under the same interface the
>> two different technologies (SMR and NAND flash) will be resulted in
>> the loss of opportunity to manage the device in more smarter way.
>> Because any unification has the goal to create a simple interface.
>> But SMR and NAND flash are significantly different technologies. And
>> if somebody creates technology-oriented file system, for example,
>> then it needs to have access to really special features of the
>> technology. Otherwise, interface will be overloaded by features of
>> both technologies and it will looks like as a mess.
> 
> I do not think so, as long as the device "model" is exposed to the user
> as the zoned block device interface does. This allows a user to adjust
> its operation depending on the device. This is true of course as long as
> each "model" has a clearly defined set of features associated. Again,
> that is the case for zoned block devices and an example of how this can
> be used is now in f2fs (which allows different operation modes for
> host-aware devices, but only one for host-managed devices). Again, I can
> see a clear parallel with open channel SSDs here.
> 
>> SMR zone and NAND flash erase block look comparable but, finally, it
>> significantly different stuff. Usually, SMR zone has 265 MB in size
>> but NAND flash erase block can vary from 512 KB to 8 MB (it will be
>> slightly larger in the future but not more than 32 MB, I suppose). It
>> is possible to group several erase blocks into aggregated entity but
>> it could be not very good policy from file system point of view.
> 
> Why not? For f2fs, the 2MB segments are grouped together into sections
> with a size matching the device zone size. That works well and can
> actually even reduce the garbage collection overhead in some cases.
> Nothing in the kernel zoned block device support limits the zone size to
> a particular minimum or maximum. The only direct implication of the zone
> size on the block I/O stack is that BIOs and requests cannot cross zone
> boundaries. In an extreme setup, a zone size of 4KB would work too and
> result in read/write commands of 4KB at most to the device.
> 
>> Another point that QLC device could have more tricky features of
>> erase blocks management. Also we should apply erase operation on NAND
>> flash erase block but it is not mandatory for the case of SMR zone.
> 
> Incorrect: host-managed devices require a zone "reset" (equivalent to
> discard/trim) to be reused after being written once. So again, the
> "tricky features" you mention will depend on the device "model",
> whatever this ends up to be for an open channel SSD.
> 
>> Because SMR zone could be simply re-written in sequential order if
>> all zone's data is invalid, for example. Also conventional zone could
>> be really tricky point. Because it is one zone only for the whole
>> device that could be updated in-place. Raw NAND flash, usually,
>> hasn't likewise conventional zone.
> 
> Conventional zones are optional in zoned block devices. There may be
> none at all and an implementation may well decide to not support a
> device without any conventional zones if some are required.
> In the case of open channel SSDs, the FTL implementation may well decide
> to expose a particular range of LBAs as "conventional zones" and have a
> lower level exposure for the remaining capacity whcih can then be
> optimally used by the file system based on the features available for
> that remaining LBA range. Again, a parallel is possible with SMR.
> 
>> Finally, if I really like to develop SMR- or NAND flash oriented file
>> system then I would like to play with peculiarities of concrete
>> technologies. And any unified interface will destroy the opportunity 
>> to create the really efficient solution. Finally, if my software
>> solution is unable to provide some fancy and efficient features then
>> guys will prefer to use the regular stack (ext4, xfs + block layer).
> 
> Not necessarily. Again think in terms of device "model" and associated
> feature set. An FS implementation may decide to support all possible
> models, with likely a resulting incredible complexity. More likely,
> similarly with what is happening with SMR, only models that make sense
> will be supported by FS implementation that can be easily modified.
> Example again here of f2fs: changes to support SMR were rather simple,
> whereas the initial effort to support SMR with ext4 was pretty much
> abandoned as it was too complex to integrate in the existing code while
> keeping the existing on-disk format.
> 
> Your argument above is actually making the same point: you want your
> implementation to use the device features directly. That is, your
> implementation wants a "host-managed" like device model. Using ext4 will
> require a "host-aware" or "drive-managed" model, which could be provided
> through a different FTL or device-mapper implementation in the case of
> open channel SSDs.
> 
> I am not trying to argue that open channel SSDs and zoned block devices
> should be supported under the exact same API. But I can definitely see
> clear parallels worth a discussion. As a first step, I would suggest
> trying to try defining open channel SSDs "models" and their feature set
> and see how these fit with the existing ZBC/ZAC defined models and at
> least estimate the implications on the block I/O stack. If adding the
> new models only results in the addition of a few top level functions or
> ioctls, it may be entirely feasible to integrate the two together.
> 

Thanks Damien. I couldn't have said it better my self.

The OCSSD 1.3 specification has been made with an eye towards the SMR
interface:

 - "Identification" - Follows the same "global" size definitions, and
also supports that each zone has its own local size.
 - "Get Report" command follows a very similar structure as SMR, such
that it can sit behind the "Report Zones" interface.
 - "Erase/Prepare Block" command follows the Reset block interface.

Those should fit right in. If the layout is planar, such that the OCSSD
only exposes a set of zones, it should be able to fit right into the
framework with minor modifications.

A couple of details are added when going towards managing multiple
parallel units, which is some of the things that require a bit of
discussion.