block layer API for file system creation - when to use multidisk mode

[Ric Wheeler <ricwheeler@gmail.com>]

On 11/30/18 7:55 AM, Dave Chinner wrote:
> On Thu, Nov 29, 2018 at 06:53:14PM -0500, Ric Wheeler wrote:
>> On 11/29/18 4:48 PM, Dave Chinner wrote:
>>> On Thu, Nov 29, 2018 at 08:53:39AM -0500, Ric Wheeler wrote:
>>>> On 10/6/18 8:14 PM, Eric Sandeen wrote:
>>>>> On 10/6/18 6:20 PM, Dave Chinner wrote:
>>>>>>> Can you give an example of a use case that would be negatively affected
>>>>>>> if this heuristic was switched from "sunit" to "sunit < swidth"?
>>>>>> Any time you only know a single alignment characteristic of the
>>>>>> underlying multi-disk storage. e.g. hardware RAID0/5/6 that sets
>>>>>> iomin = ioopt, multi-level RAID constructs where only the largest
>>>>>> alignment requirement is exposed, RAID1 devices exposing their chunk
>>>>>> size, remote replication chunk alignment (because remote rep. is
>>>>>> slow and so we need more concurrency to keep the pipeline full),
>>>>>> etc.
>>>>> So the tl;dr here is "given any iomin > 512, we should infer low seek
>>>>> latency and parallelism and adjust geometry accordingly?"
>>>>>
>>>>> -Eric
>>>> Chiming in late here, but I do think that every decade or two (no
>>>> disrespect to xfs!), it is worth having a second look at how the
>>>> storage has changed under us.
>>>>
>>>> The workload that has lots of file systems pounding on a shared
>>>> device for example is one way to lay out container storage.
>>> The problem is that defaults can't cater for every use case.
>>> And in this case, we've got nothing to tell us that this is
>>> aggregated/shared storage rather than "the fileystem owns the
>>> entire device".
>>>
>>>> No argument about documenting how to fix this with command line
>>>> tweaks for now, but maybe this would be a good topic for the next
>>>> LSF/MM shared track of file & storage people to debate?
>>> Doubt it - this is really only an XFS problem at this point.
>>>
>>> i.e. if we can't infer what the user wants from existing
>>> information, then I don't see how the storage is going to be able to
>>> tell us anything different, either.  i.e. somewhere in the stack the
>>> user is going to have to tell the block device that this is
>>> aggregated storage.
>>>
>>> But even then, if it's aggregated solid state storage, we still want
>>> to make use of the concurency on increased AG count because there is
>>> no seek penalty like spinning drives end up with. Or if the
>>> aggregated storage is thinly provisioned, the AG count of filesystem
>>> just doesn't matter because the IO is going to be massively
>>> randomised (i.e take random seek penalties) by the thinp layout.
>>>
>>> So there's really no good way of "guessing" whether aggregated
>>> storage should or shouldn't use elevated AG counts even if the
>>> storage says "this is aggregated storage". The user still has to
>>> give us some kind of explict hint about how the filesystem should
>>> be configured.
>>>
>>> What we need is for a solid, reliable detection hueristic to be
>>> suggested by the people that need this functionality before there's
>>> anything we can talk about.
>> I think that is exactly the kind of discussion that the shared
>> file/storage track is good for.
> Yes, but why on earth do we need to wait 6 months to have that
> conversation. Start it now...


Sure, that is definitely a good idea - added in some of the storage lists to 
this reply. No perfect all encompassing block layer list that I know of.


>
>> Other file systems also need to
>> accommodate/probe behind the fictitious visible storage device
>> layer... Specifically, is there something we can add per block
>> device to help here? Number of independent devices
> That's how mkfs.xfs used to do stripe unit/stripe width calculations
> automatically on MD devices back in the 2000s. We got rid of that
> for more generaly applicable configuration information such as
> minimum/optimal IO sizes so we could expose equivalent alignment
> information from lots of different types of storage device....
>
>> or a map of
>> those regions?
> Not sure what this means or how we'd use it.
>
> Cheers,
>
> Dave.

What I was thinking of was a way of giving up a good outline of how many 
independent regions that are behind one "virtual" block device like a ceph rbd 
or device mapper device. My assumption is that we are trying to lay down (at 
least one) allocation group per region.

What we need to optimize for includes:

     * how many independent regions are there?

     * what are the boundaries of those regions?

     * optimal IO size/alignment/etc

Some of that we have, but the current assumptions don't work well for all device 
types.

Regards,

Ric



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