[Lustre-devel] Reducing amount of glimpses

Nikita Danilov Nikita.Danilov at Sun.COM
Sat Feb 2 03:22:21 PST 2008

Oleg Drokin writes:
 > Hello!
 >     Doing some large scale testing at ORNL, interesting pattern came up.
 >     Suppose we are doing large-scale IOR testing on a shared file.
 >     Some unlucky client does its writing at highest offset (or, at the  
 > beginning,
 >     was unlucky enough to grab whole-object PW lock).
 >     As other clients do their writes, they would do glimpses first to  
 > find out file
 >     size. Now those glimpses turn into thousands of glimpse requests  
 > to that poor
 >     client. And many of them actually coming in parallel.

I think there is a way to further reduce the number of glimpses sent in
the read/write paths. In a situation when client is doing read and the
region being read ends in a certain stripe, it's often enough to send a
glimpse request only to the OST where this stripe is located. Indeed, if
stripe is not a hole, then size of this stripe alone is sufficient to
determine whether we are in a short read situation or not. Of course if
stripe happens to end in a hole, client needs to send more glimpses, but
in the most common case of non-sparse file, 1 RPC seems to be enough.

 > Bye,
 >      Oleg


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