Re: [PATCH 19/32] target: Convert to rbtree for se_dev_entry in se_node_acl

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On Mon, 2013-12-16 at 17:00 -0800, Andy Grover wrote:
> On 12/16/2013 01:40 PM, Nicholas A. Bellinger wrote:
> > On Fri, 2013-12-13 at 15:59 -0800, Andy Grover wrote:
> >> Instead of an array, use a rbtree. Less memory use on average, and
> >> can allow >255 entries. We go from O(1) to O(log n) on lookups. If this
> >> shows up on profiling (it won't) then transition to other kernel lookup
> >> methods is straightforward from here.
> >>
> >
> > Ugh.
> >
> > There is no reason to be using rbtrees in a performance critical path
> > here.
> >
> > The number of pointer lookups is what ends up hurting the most vs. a
> > flat array, so given that 256 LUNs per endpoint is currently not an
> > issue, and there is no hard limit on the number of endpoints with
> > virtual addressing, I don't see the benefit of this patch.
> >
> > NAK.
> 
> What does virtual addressing mean in this context? I'm not familiar -- 
> so instead of reporting 257+ luns per tpg, it would get split up somehow?

Virtual addressing meaning /sys/kernel/config/target/$FABRIC/$WWPN/
endpoints that are not tied to a specific hardware mapping, so any
arbitrary number of $WWPNs can be created.

Right now the only use-case where people are using more than a few dozen
LUNs per endpoint is with non NPIV qla2xxx mode, where a single physical
Fibre Channel WWPN is exporting 256-512 LUNs, with different per NodeACL
LUN mappings for different initiators.

With NPIV mode for qla2xxx (currently in development), the WWPN becomes
virtualized like iSCSI, and a unsigned short about of virtual port
addresses can be configured per physical FC port.

--nab

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