@menu
* File System New Code::
-* Problem 4-1 Large Files::
+* File System Synchronization::
+* Problem 4-1 Indexed Files::
* Problem 4-2 File Growth::
* Problem 4-3 Subdirectories::
* Problem 4-4 Buffer Cache::
* File System Design Document Requirements::
-* File System FAQ::
+* File System FAQ::
@end menu
@node File System New Code
@item filesys.h
@itemx filesys.c
-Top-level interface to the file system.
+Top-level interface to the file system. Please read the long comment
+near the top of @file{filesys.c}, which introduces some details of the
+file system code as provided.
@item directory.h
@itemx directory.c
While most of your work will be in @file{filesys}, you should be
prepared for interactions with all previous parts (as usual).
-@node Problem 4-1 Large Files
-@section Problem 4-1: Large Files
+@node File System Synchronization
+@section Synchronization
-Modify the file system to allow the maximum size of a file to be as
-large as the disk. You can assume that the disk will not be larger
-than 8 MB. In the basic file system, each file is limited to a file
-size of just under 64 kB. Each file has a header called an index node
-or @dfn{inode} (represented by @struct{inode}) that is a table of
-direct pointers to the disk blocks for that file. Since the inode is
-stored in one disk sector, the maximum size of a file is limited by
-the number of pointers that will fit in one disk sector. Increasing
-the limit to 8 MB will require you to implement doubly-indirect
-blocks.
+The provided file system requires external synchronization, that is,
+callers must ensure that only one thread can be running in the file
+system code at once. Your submission should use a finer-grained
+synchronization strategy. You will need to consider synchronization
+issues for each type of file system object. The provided code uses the
+following strategies:
+
+@itemize @bullet
+@item
+The free map and root directory are read each time they are needed for
+an operation, and if they are modified, they are written back before the
+operation completes. Thus, the free map is always consistent from an
+external viewpoint.
+
+@item
+Inodes are immutable in the provided file system, that is, their content
+never changes between creation and deletion. Furthermore, only one copy
+of an inode's data is maintained in memory at once, even if the file is
+open in multiple contexts.
+
+@item
+File data doesn't have to be consistent because it's just not part of
+the model. In Unix and many other operating systems, a read of a file
+by one process when the file is being written by another process can
+show inconsistent results: it can show that none, all, or part of the
+write has completed. (However, after the write system call returns to
+its caller, all subsequent readers must see the change.) Similarly,
+when two threads write to the same part of a file at the same time,
+their data may be interleaved. External synchronization of the provided
+file system ensures that reads and writes are fully serialized, but your
+file system doesn't have to maintain full serialization as long as it
+follows the rules above.
+@end itemize
+
+@node Problem 4-1 Indexed Files
+@section Problem 4-1: Indexed Files
+
+The basic file system allocates files as a single extent, making it
+vulnerable to external fragmentation. Eliminate this problem by
+modifying the inode structure. In practice, this probably means using
+an index structure with direct, indirect, and doubly indirect blocks.
+(You are welcome to choose a different scheme as long as you explain the
+rationale for it in your design documentation, and as long as it does
+not suffer from external fragmentation.)
+
+You can assume that the disk will not be larger than 8 MB. You must
+support files as large as the disk (minus metadata). Each inode is
+stored in one disk sector, limiting the number of block pointers that it
+can contain. Supporting 8 MB files will require you to implement
+doubly-indirect blocks.
@node Problem 4-2 File Growth
@section Problem 4-2: File Growth
Make sure that concurrent accesses to the inode remain properly
synchronized.
+The user is allowed to seek beyond the current end-of-file (EOF). The
+seek itself does not extend the file. Writing at a position past EOF
+extends the file to the position being written, and any gap between the
+previous EOF and the start of the write must be filled with zeros. A
+read past EOF returns zero bytes.
+
+Writing far beyond EOF can cause many blocks to be entirely zero. Some
+file systems allocate and write real data blocks for these implicitly
+zeroed blocks. Other file systems do not allocate these blocks at all
+until they are explicitly written. The latter file systems are said to
+support ``sparse files.'' You may adopt either allocation strategy in
+your file system.
+
@node Problem 4-3 Subdirectories
@section Problem 4-3: Subdirectories
Make sure that directories can expand beyond their original size just
as any other file can.
-To take advantage of hierarchical name spaces in user programs,
-provide the following syscalls:
+Each process has its own current directory. When one process starts
+another with the @code{exec} system call, the child process inherits its
+parent's current directory. After that, the two processes' current
+directories are independent, so that either changing its own current
+directory has no effect on the other.
+
+Update the existing system calls so that, anywhere a file name is
+provided by the caller, an absolute or relative path name may used.
+Also, implement the following new system calls:
@table @code
@item SYS_chdir
of crashes. Therefore, you should
periodically write all cached blocks to disk. If you have
@func{timer_sleep} from the first project working, this is an
-excellent application for it.
+excellent application for it. (If you're still using the base
+implementation of @func{timer_sleep}, be aware that it busy-waits, which
+is not an acceptable solution.) If @func{timer_sleep}'s delays seem too
+short or too long, reread the explanation of the @option{-r} option to
+@command{pintos} (@pxref{Debugging versus Testing}).
Likewise, read-ahead is only really useful when done asynchronously.
That is, if a process wants disk block 1 from the file, it needs to
sure to cover the following points:
@itemize @bullet
+@item
+How did you choose to synchronize file system operations?
+
@item
How did you structure your inodes? How many blocks did you access
directly, via single-indirection, and/or via double-indirection? Why?
@enumerate 1
@item
-@b{We're limited to a 64-block cache, but can we also keep a copy of
-each @struct{inode} for an open file inside @struct{file},
-the way the stub code does?}
-
-No, you shouldn't keep any disk sectors stored anywhere outside the
-cache. That means you'll have to change the way the file
-implementation accesses its corresponding inode right now, since it
-currently just creates a new @struct{inode} in its constructor
-and reads the corresponding sector in from disk when it's created.
-
-There are two reasons for not storing inodes in @struct{file}.
-First, keeping extra copies of inodes would be cheating the 64-block
-limitation that we place on your cache. Second, if two processes have
-the same file open, you will create a huge synchronization headache
-for yourself if each @struct{file} has its own copy of the inode.
-
-Note that you can store pointers to inodes in @struct{file} if
-you want, and you can store some other small amount of information to
-help you find the inode when you need it.
-
-Similarly, if you want to store one block of data plus some small
-amount of metadata for each of your 64 cache entries, that's fine.
-
-@item
-@b{But why can't we store copies of inodes in @struct{file}? We
-don't understand the answer to the previous question.}
-
-The issue regarding storing @struct{inode}s has to do with
-implementation of the buffer cache. Basically, you can't store a
-@code{struct inode *} in @struct{inode}. Each time you need
-to read a @struct{inode}, you'll have to get it either from the
-buffer cache or from disk.
-
-If you look at @func{file_read_at}, it uses the inode directly
-without having first read in that sector from wherever it was in the
-storage hierarchy. You are no longer allowed to do this. You will
-need to change @code{file_read_at} (and similar functions) so that it
-reads the inode from the storage hierarchy before using it.
+@b{We're limited to a 64-block cache, but can we also keep an
+@struct{inode_disk} inside @struct{inode}, the way the provided code
+does?}
+
+The goal of the 64-block limit is to bound the amount of cached file
+system data. If you keep a block of disk data---whether file data or
+metadata---anywhere in kernel memory then you have to count it against
+the 64-block limit. The same rule applies to anything that's
+``similar'' to a block of disk data, such as a @struct{inode_disk}
+without the @code{length} or @code{sector_cnt} members.
+
+You can keep a cached copy of the free map in memory permanently if you
+like. It doesn't have to count against the cache size.
+
+That means you'll have to change the way the inode implementation
+accesses its corresponding on-disk inode right now, since it currently
+just embeds a @struct{inode_disk} in @struct{inode} and reads the
+corresponding sector in from disk when it's created. Keeping extra
+copies of inodes would be cheating the 64-block limitation that we place
+on your cache.
+
+You can store pointers to inode data in @struct{inode}, if you want, and
+you can store some other small amount of information to help you find
+the inode when you need it. Similarly, if you want to store one block
+of data plus some small amount of metadata for each of your 64 cache
+entries, that's fine.
+
+If you look at @func{inode_byte_to_sector}, it uses the
+@struct{inode_disk} directly without having first read in that sector
+from wherever it was in the storage hierarchy. This will no longer
+work. You will need to change @func{inode_byte_to_sector} so that it
+reads the @struct{inode_disk} from the storage hierarchy before using
+it.
@end enumerate
projects / pintos-anon / blobdiff