[Lustre-devel] Large readdir RPCs project

Fan Yong yong.fan at whamcloud.com
Wed Sep 29 18:56:54 PDT 2010

  On 9/30/10 3:01 AM, Jeremy Filizetti wrote:
>     On the other hand, Large readdir RPCs is basic of another metadata
>     read performance improvement features - "readdir+", which is quite
>     useful for "ls -l" operation (for large directory), and reduce
>     lookup/getattr RPC as much as possible. In such feature, MDS will
>     pack more dir-item's attribute (not only name/ino as does
>     currently by readdir, but also mode/owner, and etc) information
>     back to client-side in "readdir+" RPC. That means the dir-item
>     count in one "readdir+" page is less than in the traditional
>     readdir page, then more pages to be sent back to client. If
>     without "Large readdir RPCs", the advantage of "readdir+" will be
>     discounted.
> I'd be interested in working this as well but probably as a separate 
> effort since SOM isn't in 1.8 and that's my main focus.  In our 
> testing, SOM had significant benefits over the WAN and I'd expect even 
> better from readdir+.  I have tried Oleg's patch for 
> asynchronous ll_glimpse_size but oddly I've seen someone erradic 
> performance where at times it was worse then statahead and synchronous 
> ll_glimpse_size.
> Jeremy
Yes, SOM is another important feature for metadata reading performance 
improvement by bypassing the glimpse RPC between client and OSS. 
Engineers from Lustre Group worked for that for some time, hope can be 
released soon.

As for the asynchronous ll_glimpse_size maybe cause bad performance 
occasionally, one possible reason is that: glimpse RPC maybe not obtain 
extent lock(s) if some others in using such lock(s), so the file size 
information obtained by asynchronous glimpse is invalid when it is 
really used later, means the caller ("stat") has to send synchronous 
glimpse again. Anyway, I did not study such patch, so it is just a guess.

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