-@node Project 4--File Systems, Multilevel Feedback Scheduling, Project 3--Virtual Memory, Top
+@node Project 4--File Systems, References, Project 3--Virtual Memory, Top
@chapter Project 4: File Systems
In the previous two assignments, you made extensive use of a
same time. Plus, keeping VM is a great way to stress-test your
filesystem implementation.
-FIXME FIXME FIXME
-The first step is to understand the default filesystem provided by the
-base code.
+Your submission should define @code{THREAD_JOIN_IMPLEMENTED} in
+@file{constants.h} (@pxref{Conditional Compilation}).
@menu
* File System New Code::
* Problem 4-3 Subdirectories::
* Problem 4-4 Buffer Cache::
* File System Design Document Requirements::
-* File System FAQs::
+* File System FAQ::
@end menu
@node File System New Code
@item directory.h
@itemx directory.c
-Translates file names to disk file headers; the
-directory data structure is stored as a file.
+Translates file names to inodes. The directory data structure is
+stored as a file.
-@item filehdr.h
-@itemx filehdr.c
+@item inode.h
+@itemx inode.c
Manages the data structure representing the layout of a
file's data on disk.
Translates file reads and writes to disk sector reads
and writes.
-@item devices/disk.h
-@itemx devices/disk.c
-Provides access to the physical disk, abstracting away the rather
-awful IDE interface.
-
@item lib/kernel/bitmap.h
@itemx lib/kernel/bitmap.c
A bitmap data structure along with routines for reading and writing
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 (@code{struct
-filehdr}) that is a table of direct pointers to the disk blocks for
-that file. Since the header 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.
+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.
@node Problem 4-2 File Growth
@section Problem 4-2: File Growth
Implement extensible files. In the basic file system, the file size
is specified when the file is created. One advantage of this is that
-the FileHeader data structure, once created, never changes. In UNIX
-and most other file systems, a file is initially created with size 0
-and is then expanded every time a write is made off the end of the
-file. Modify the file system to allow this. As one test case, allow
-the root directory file to expand beyond its current limit of ten
-files. Make sure that concurrent accesses to the file header remain
-properly synchronized.
+the inode data structure, once created, never changes. In UNIX and
+most other file systems, a file is initially created with size 0 and
+is then expanded every time a write is made off the end of the file.
+Modify the file system to allow this. As one test case, allow the
+root directory file to expand beyond its current limit of ten files.
+Make sure that concurrent accesses to the inode remain properly
+synchronized.
@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:
+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
@end table
Also write the @command{ls} and @command{mkdir} user programs. This
-is straightforward once the above syscalls are implemented. If Unix,
+is straightforward once the above syscalls are implemented. In Unix,
these are programs rather than built-in shell commands, but
@command{cd} is a shell command. (Why?)
disk. (Otherwise, fetch the block from disk into cache, evicting an
older entry if necessary.) You are limited to a cache no greater than
64 sectors in size. Be sure to choose an intelligent cache
-replacement algorithm. Experiment to see what combination of use,
+replacement algorithm. Experiment to see what combination of accessed,
dirty, and other information results in the best performance, as
measured by the number of disk accesses. (For example, metadata is
generally more valuable to cache than data.) Document your
-replacement algoritm in your design document.
+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:
demonstrate the performance improvement.
Note that write-behind makes your filesystem more fragile in the face
-of crashes. Therefore, you should implement some manner to
+of crashes. Therefore, you should
periodically write all cached blocks to disk. If you have
-@code{timer_sleep()} from the first project working, this is an
+@func{timer_sleep} from the first project working, this is an
excellent application for it.
Likewise, read-ahead is only really useful when done asynchronously.
the process will block to wait for disk block 1, but should not block
waiting for disk block 2.
-FIXME
When you're implementing this, please make sure you have a scheme for
making any read-ahead and write-behind threads halt when Pintos is
``done'' (when the user program has completed, etc), so that Pintos
-will halt normally and print its various statistics.
+will halt normally and the disk contents will be consistent.
@node File System Design Document Requirements
@section Design Document Requirements
As always, submit a design document file summarizing your design. Be
-sure to cover the following points :
+sure to cover the following points:
@itemize @bullet
@item
How and when did you flush the cache?
@end itemize
-@node File System FAQs
+@node File System FAQ
@section FAQ
@enumerate 1
@b{What exec modes for running Pintos do I absolutely need to
support?}
-FIXME FIXME
-The most standard mode is to run your Pintos with all the command
-flags on one command line, like this: @samp{pintos -f -cp shell
-shell -ex "shell"}. However, you also need to support these flags
-individually---especially since that's how the grader tests your
-program. Thus, you should be able to run the above instead as:
+You also need to support the @option{-f}, @option{-ci}, @option{-co},
+and @option{-ex} flags individually, and you need to handle them when
+they're combined, like this: @samp{pintos -f -ci shell 12345 -ex
+"shell"}. Thus, you should be able to treat the above as equivalent to:
-FIXME
@example
pintos -f
-pintos -cp shell shell
+pintos -ci shell 12345
pintos -ex "shell"
@end example
-Note that this also provides a way for you to validate that your disks
-are really persistent. This is a common problem with a write behind
-cache: if you don't shut down properly it will leave the disk in an
-inconsistent state.
+If you don't change the filesystem interface, then this should already
+be implemented properly in @file{threads/init.c} and
+@file{filesys/fsutil.c}.
+
+You must also implement the @option{-q} option and make sure that data
+gets flushed out to disk properly when it is used.
@item
@b{Will you test our file system with a different @code{DISK_SECTOR_SIZE}?}
-No, @code{DISK_SECTOR_SIZE} will not change.
+No, @code{DISK_SECTOR_SIZE} is fixed at 512. This is a fixed property
+of IDE disk hardware.
@item
-@b{Will the @code{struct filehdr} take up space on the disk too?}
+@b{Will the @struct{inode} take up space on the disk too?}
-Yes. Anything stored in @code{struct filehdr} takes up space on disk,
+Yes. Anything stored in @struct{inode} takes up space on disk,
so you must include this in your calculation of how many entires will
fit in a single disk sector.
@item
-File Growth FAQs
+@b{What's the directory separator character?}
+
+Forward slash (@samp{/}).
+@end enumerate
+
+@menu
+* Problem 4-2 File Growth FAQ::
+* Problem 4-3 Subdirectory FAQ::
+* Problem 4-4 Buffer Cache FAQ::
+@end menu
+
+@node Problem 4-2 File Growth FAQ
+@subsection Problem 4-2: File Growth FAQ
@enumerate 1
@item
The disk we create will be 8 MB or smaller. However, individual files
will have to be smaller than the disk to accommodate the metadata.
-You'll need to consider this when deciding your @code{struct filehdr}
-(inode) organization.
+You'll need to consider this when deciding your @struct{inode}
+organization.
@end enumerate
-@item
-Subdirectory FAQs
+@node Problem 4-3 Subdirectory FAQ
+@subsection Problem 4-3: Subdirectory FAQ
@enumerate 1
@item
Yes. Implementing @file{.} and @file{..} is extra credit, though.
@item
-@b{Should @code{remove()} also be able to remove directories?}
+@b{Should @func{remove} also be able to remove directories?}
Yes. The @code{remove} system call should handle removal of both
regular files and directories. You may assume that directories can
only be deleted if they are empty, as in Unix.
@end enumerate
-@item
-Buffer Cache FAQs
+@node Problem 4-4 Buffer Cache FAQ
+@subsection Problem 4-4: Buffer Cache FAQ
@enumerate 1
@item
@b{We're limited to a 64-block cache, but can we also keep a copy of
-each @code{struct filehdr} for an open file inside @code{struct file},
+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 @code{struct filehdr} in its constructor
+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 @code{struct file}.
+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 @code{struct file} has its own copy of the inode.
+for yourself if each @struct{file} has its own copy of the inode.
-Note that you can store pointers to inodes in @code{struct file} if
+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.
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 @code{struct file}? We
+@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 @code{struct filehdr}s has to do with
+The issue regarding storing @struct{inode}s has to do with
implementation of the buffer cache. Basically, you can't store a
-@code{struct filehdr *} in @code{struct filehdr}. Each time you need
-to read a @code{struct filehdr}, you'll have to get it either from the
+@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 @code{file_read_at()}, it uses @code{hdr} directly
+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 @code{hdr} from the storage hierarchy before using it.
-@end enumerate
+reads the inode from the storage hierarchy before using it.
@end enumerate
projects / pintos-anon / blobdiff