[Linux-cachefs] Adventures in NFS re-exporting

[Daire Byrne]

----- On 25 Nov, 2020, at 17:14, Daire Byrne daire at dneg.com wrote:
> First off, writing direct to the originating server mount on the re-export
> server from userspace shows the ideal behaviour for all combinations:
> 
> originating server <- (vers=X,actimeo=1800,nconnect=X) <- reexport server
> writing = WRITE,WRITE .... repeating (good!)
> 
> Then re-exporting a NFSv4.2 server:
> 
> originating server <- (vers=4.2) <- reexport server - (vers=3) <- client writing
> = GETATTR,COMMIT,WRITE .... repeating
> originating server <- (vers=4.2) <- reexport server - (vers=4.2) <- client
> writing = GETATTR,WRITE .... repeating
> 
> And re-exporting a NFSv3 server:
> 
> originating server <- (vers=3) <- reexport server - (vers=4.2) <- client writing
> = WRITE,WRITE .... repeating (good!)
> originating server <- (vers=3) <- reexport server - (vers=3) <- client writing =
> WRITE,COMMIT .... repeating
>  
> So of all the combinations, a NFSv4.2 re-export of an NFSv3 server is the only
> one that matches the "ideal" case where we WRITE continuously without all the
> extra chatter.
> 
> And for completeness, taking that "good" case and making it bad with nconnect:
> 
> originating server <- (vers=3,nconnect=16) <- reexport server - (vers=4.2) <-
> client writing = WRITE,WRITE .... repeating (good!)
> originating server <- (vers=3) <- reexport server <- (vers=4.2,nconnect=16) <-
> client writing = WRITE,COMMIT,GETATTR .... randomly repeating
> 
> So using nconnect on the re-export's client causes lots more metadata ops. There
> are reasons for doing that for increasing throughput but it could be that the
> gain is offset by the extra metadata roundtrips.
> 
> Similarly, we have mostly been using a NFSv4.2 re-export of a NFSV4.2 server
> over the WAN because of reduced metadata ops for reading, but it looks like we
> incur extra metadata ops for writing.

Just a small update based on the most recent patchsets from Trond & Bruce:

https://patchwork.kernel.org/project/linux-nfs/list/?series=393567
https://patchwork.kernel.org/project/linux-nfs/list/?series=393561

For the write-through tests, the NFSv3 re-export of a NFSv4.2 server has trimmed an extra GETATTR:

Before:
originating server <- (vers=4.2) <- reexport server - (vers=3) <- client writing = WRITE,COMMIT,GETATTR .... repeating
 
After:
originating server <- (vers=4.2) <- reexport server - (vers=3) <- client writing = WRITE,COMMIT .... repeating

I'm assuming this is specifically due to the "EXPORT_OP_NOWCC" patch? All other combinations look the same as before (for write-through). An NFSv4.2 re-export of a NFSv3 server is still the best/ideal in terms of not incurring extra metadata roundtrips when writing.

It's great to see this re-export scenario becoming a better supported (and performing) topology; many thanks all.

Daire