[Lustre-devel] Role of the Metadata Server during File I/O

[office at hailoo.com <office@hailoo.com>]

One of Lustre's key innovations seems to be the separation of metadata from 
file data.  According to Sun, "Lustre file operations bypass the MetaData 
server completely and fully utilize the parallel data paths to all OSSs in 
the cluster."  (See 
http://www.sun.com/software/products/lustre/features.xml) 

However, can this really be the case? 

In POSIX-compliant file I/O, a call to write() that starts at an offset 
which is greater than the file size must write zeroes to disk to make up for 
the missing space.  So if you have a file size of 0 bytes, and then you 
write a single byte at offset 10, bytes 0 through 9 of the file will contain 
zeros. 

But if a file on a Lustre system is striped across multiple OSTs, how does 
Lustre avoid communicating with the Metadata Server at every write 
operation?  Consider the following scenario: 

You have 4 OSTs and you create a new file and stripe it across all 4 OSTs, 
and you set the stripe size to 4 bytes.  (I know that is too small but I'm 
just keeping this simple.) 

Now, suppose you call write() and write 1 byte to the file at offset 5.  
Lustre must now write 4 zero bytes on the first OST, and 1 non-zero byte on 
the second OST.  But in order to know that it is necessary to write zeroes 
to the first OST, the client would need access to global information about 
the total size of the file.  Therefore, wouldn't it need to check with the 
Metadata Server to determine the total file size before every call to 
write()? 

Any information anyone can provide me on the implementation 
details/strategies used here would be greatly appreciated. 

 -Charles Salvia
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