Re: [PATCHv4 0/6] scsi: use xarray for devices and targets

[Hannes Reinecke <hare@suse.de>]

On 6/3/20 2:53 PM, Christoph Hellwig wrote:
> On Tue, Jun 02, 2020 at 01:33:05PM +0200, Hannes Reinecke wrote:
>> Hi all,
>>
>> based on the ideas from Doug Gilbert here's now my take on using
>> xarrays for devices and targets.
>> It revolves around two ideas:
>>
>> - The scsi target 'channel' and 'id' numbers are never ever used
>>    to the full 32 bit range; channels are well below 10, and no
>>    driver is using more than 16 bits for the id. So we can reduce
>>    the type of 'channel' and 'id' to 16 bits, and use the 32 bit
>>    value 'channel << 16 | id' as the index into the target xarray.
>> - Nearly every target only ever uses the first two levels of the
>>    4-level SCSI LUN structure, which means that we can use the
>>    linearized SCSI LUN id as an index into the xarray.
>>    If we ever come across targets utilizing more that 2 levels of
>>    the LUN structure we'll allocate the first unused index and have
>>    to resort to a less efficient lookup instead of direct indexing.
>>
>> With these changes we can implement an efficient lookup mechanism,
>> devolving into direct lookup for most cases. It also allows us to
>> detect duplicate entries or accidental overwrites of existing elements
>> by using xa_cmpxchg().
>> And iteration over targets and devices should be as efficient as the
>> current, list-based, approach.
>>
>> As usual, comments and reviews are welcome.
> 
> I see absolutely no argument for what the point of this series.  It adds
> more code, and I don't really see any indications for it fixing bugs,
> speeding up workloads, or reducing memory usage.
> 
 From my perspective this is a proof-of-concept; using xarrays to store 
targets and LUNs has the benefit that we can directly access the 
elements, and the lookup will be more efficient for larger setups.

But it's not a clear-cut solution, merely replacing one concept with 
some issues with another concept with another set of issues.

Guess the real benefit will come only if we manage to move to explicit 
scsi target removal, and not the implicit model of making the scsi 
target dependent on the underlying scsi devices we have now.
I'll be experimenting with that and will post an update for it.

I _do_ like the xarray for targets, though; they have a fixed location 
where they can go and as such xarray are a far more natural choice.
For LUNs it's less compelling as xarrays can't use 64bits generically as 
index, but still.

Cheers,

Hannes
-- 
Dr. Hannes Reinecke            Teamlead Storage & Networking
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