[Lustre-devel] Role of the Metadata Server during File I/O
office at hailoo.com
office at hailoo.com
Mon Aug 10 15:31:37 PDT 2009
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
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.lustre.org/pipermail/lustre-devel-lustre.org/attachments/20090810/dd95b56f/attachment.htm>
More information about the lustre-devel
mailing list