Delivery-Date: Tue Nov 11 18:25:44 2008
Message-Id: <200811120221.mAC2L3kl008620@bourbon.usc.edu>
To: cs551@merlot.usc.edu
Subject: Re: CS551: Final Project 2 : Permanent Size
Date: Tue, 11 Nov 2008 18:21:03 -0800
From: Bill Cheng <william@bourbon.usc.edu>

Someone wrote:

  > What I mean with my question about cache and permanent storage was that
  > whenever we either cache a file or permanently store a file, the operation
  > would be saving the file in a directory.
  > Now, is it the case that if we cache it then we should store it in a
  > seperate directory, say "homedir/files/Cache" and when we store it
  > permanently we should store it in "homedir/files" ?
  > or is there any alternate way to implement "cache" rather than implementing
  > it as a directory which stores all files to be "cached" ?

Please see http://merlot.usc.edu/cs551-f08/msgs/bill/420.txt.
Also, I've talked about this at the beginning of yesterday's
lecture.
--
Bill Cheng // bill.cheng@usc.edu <URL:http://merlot.usc.edu/william/usc/>


  On Tue, Nov 11, 2008 at 8:21 AM, Bill Cheng <william@bourbon.usc.edu> wrote:
  
  > 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/>