Delivery-Date: Tue Nov 11 08:26:10 2008
Message-Id: <200811111621.mABGLMGK000743@bourbon.usc.edu>
To: cs551@merlot.usc.edu
Subject: Re: CS551: Final Project 2 : Permanent Size
Date: Tue, 11 Nov 2008 08:21:22 -0800
From: Bill Cheng <william@bourbon.usc.edu>

Someone wrote:

  > When storing the file, we have to check if the permanent storage has space
  > sufficient enough to store the file or not, but the PermSize key is
  > obsolete. So how do we get the size of permanent storage ?

It's difficult to do, especially when disk quota is
enforced.

  > Do we have to use start on our HomeDir to get this size ? or is there any
  > other way we need to handle this ?

It's actually quite easy.  When you write to the filesystem
and when you close the file you were writing to, you must
check the return code!  If write() or close() is not
successful, you need to take appropriate action.  If you are
interacting with the user, you need to tell the user.  You
should probably also log the condition into the logfile so
you know what has failed.

  > Also, can you provide some details as to what does cache mean over here ?

If you decide to keep a file in HomeDir/files, if it's not
in the permanent space, then it's "cached".  If this doesn't
make sense, please ask me a more specific question.

  > caching will also be in a way saving the file in the directory then how do
  > we differ the permanent storage and cache ?

If a file is not stored in the perm space, then it goes into
the cache space.  Slide 16 of lecture 18 says:

    If a node *initiates* a GET or a STORE, the file goes
    into its permanent space.

So, if you decide to keep a file for another other reason than
the one stated above, you should keep it in the cache space.
--
Bill Cheng // bill.cheng@usc.edu <URL:http://merlot.usc.edu/william/usc/>