Advise about bounce buffers.

diff --git a/doc/filesys.texi b/doc/filesys.texi
index 8b7bdb46f1d753032bd2892bcd19e35b2b2f0d3f..a359e5ea79365c436feff31e2548fa1c7175cdae 100644 (file)
--- a/doc/filesys.texi
+++ b/doc/filesys.texi
@@ -169,6 +169,18 @@ measured by the number of disk accesses.  (For example, metadata is
 generally more valuable to cache than data.)  Document your
 replacement algorithm in your design document.
 
 generally more valuable to cache than data.)  Document your
 replacement algorithm in your design document.
 

+The provided file system code uses a ``bounce buffer'' in @struct{file}
+to translate the disk's sector-by-sector interface into the system call
+interface's byte-by-byte interface.  It needs per-file buffers because,
+without them, there's no other good place to put sector
+data.@footnote{The stack is not a good place because large objects
+should not be allocated on the stack.  A 512-byte sector is pushing the
+limit there.}  As part of implementing the buffer cache, you should get
+rid of these bounce buffers.  Instead, copy data into and out of sectors
+in the buffer cache directly.  You will probably need some
+synchronization to prevent sectors from being evicted from the cache
+while you are using them.
+

 In addition to the basic file caching scheme, your implementation
 should also include the following enhancements:
 
 In addition to the basic file caching scheme, your implementation
 should also include the following enhancements: