Systems File and Disk Layout
I/O: The Big Picture
Processor interrupts
Cache
Memory Bus
I/O Bridge
Main Memory Disk Controller
I/O Bus
Graphics Controller Graphics Network Interface
Disk
Disk
Network
Rotational Media
Track Arm Cylinder Head Platter Sector
Access time = seek time + rotational delay + transfer time
seek time = 5-15 milliseconds to move the disk arm and settle on a cylinder rotational delay = 8 milliseconds for full rotation at 7200 RPM: average delay = 4 ms transfer time = 1 millisecond for an 8KB block at 8 MB/s Bandwidth utilization is less than 50% for any noncontiguous access at a block grain.
Disks and Drivers
Disk hardware and driver software provide basic facilities for nonvolatile secondary storage (block devices). 1. OS views the block devices as a collection of volumes. A logical volume may be a partition of a single disk or a concatenation of multiple physical disks (e.g., RAID). 2. OS accesses each volume as an array of fixed-size sectors. Identify sector (or block) by unique (volumeID, sector ID). Read/write operations DMA data to/from physical memory. 3. Device interrupts OS on I/O completion.
Using Disk Storage
Typical operating systems use disks in three different ways: 1. System calls allow user programs to access a "raw" disk. Unix: special device file identifies volume directly. Any process that can open the device file can read or write any specific sector in the disk volume. 2. OS uses disk as backing storage for virtual memory. OS manages volume transparently as an "overflow area" for VM contents that do not "fit" in physical memory. 3. OS provides syscalls to create/access files residing on disk. OS file system modules virtualize physical disk storage as a collection of logical files.
Unix File Syscalls
int fd; /* file descriptor */ fd = open("/bin/sh", O_RDONLY, 0); fd = creat("/tmp/zot", 0777); unlink("/tmp/zot"); char data[bufsize]; bytes = read(fd, data, count); bytes = write(fd, data, count); lseek(fd, 50, SEEK_SET); mkdir("/tmp/dir", 0777); rmdir("/tmp/dir");
bin / etc tmp
process file descriptor table
system open file table
Nachos File Syscalls/Operations
Create("zot"); OpenFileId fd; fd = Open("zot"); Close(fd); char data[bufsize]; Write(data, count, fd); Read(data, count, fd);
FileSystem class internal methods: Create(name, size) OpenFile = Open(name) Remove(name) List()
FileSystem
BitMap Directory
Bitmap indicates whether each disk block is in-use or free.
Limitations: 1. small, fixed-size files and directories 2. single disk with a single directory 3. stream files only: no seek syscall 4. file size is specified at creation time 5. no access control, etc.
A single 10-entry directory stores names and disk locations for all currently existing files.
FileSystem data structures reside on-disk, but file system code always operates on a cached copy in memory (read/modify/write).
Preview of Issues for File Systems
1. Buffering disk data for access from the processor. block I/O (DMA) must use aligned, physically resident buffers block update is a read-modify-write 2. Creating/representing/destroying independent files. disk block allocation, file block map structures directories and symbolic naming 3. Masking the high seek/rotational latency of disk access. smart block allocation on disk block caching, read-ahead (prefetching), and writebehind
Representing a File On-Disk in Nachos
An OpenFile represents a file in active use, with a seek pointer and read/write primitives for arbitrary byte ranges.
logical block 0 logical block 1
OpenFile
OpenFile(sector) Seek(offset) Read(char* data, bytes) Write(char* data, bytes)
once upo n a time /nin a l and far far away ,/nlived t he wise and sage wizard.
FileHdr
bytes sectors
A file header describes an on-disk file as an ordered sequence of sectors with a length, mapped by a logical-to-physical block map.
logical block 2
OpenFile* ofd = filesys->Open("tale"); ofd->Read(data, 10) gives `once upon ` ofd->Read(data, 10) gives `a time/nin `
Allocate(...,filesize) length = FileLength() sector = ByteToSector(offset)
File Metadata
On disk, each file is represented by a FileHdr structure. The FileHdr object is an in-memory copy of this structure.
file attributes: may include owner, access control, time of create/modify/access, etc. The FileHdr is a file system "bookeeping" structure that supplements the file data itself: these kinds of structures are called filesystem metadata.
bytes sectors etc.
A Nachos FileHdr occupies exactly one disk sector. To operate on the file (e.g., to open it), the FileHdr must be read into memory. Any changes to the attributes or block map must be written back to the disk to make them permanent.
logical-physical block map (like a translation table)
physical block pointers in the block map are sector IDs FileHdr* hdr = new FileHdr(); hdr->FetchFrom(sector) hdr->WriteBack(sector)
Representing Large Files
The Nachos FileHdr occupies exactly one disk sector, limiting the maximum file size.
sector size = 128 bytes 120 bytes of block map = 30 entries each entry maps a 128-byte sector max file size = 3840 bytes
inode direct block map
(12 entries)
In Unix, the FileHdr (called an indexnode or inode) represents large files using a hierarchical block map.
Each file system block is a clump of sectors (4KB, 8KB, 16KB). Inodes are 128 bytes, packed into blocks. Each inode has 68 bytes of attributes and 15 block map entries. suppose block size 8KB = 12 direct block map entries in the inode can map 96KB of data. One indirect block (referenced by the inode) can map 16MB of data. One double indirect block pointer in inode maps 2K indirect blocks. maximum file size is 96KB + 16MB + (2K*16MB) + ...
indirect block
double indirect block
Representing Small Files
CPS 210
Internal fragmentation in the file system blocks can waste significant space for small files. E.g., 1KB files waste 87% of disk space (and bandwidth) in a naive file system with an 8KB block size. Most files are small: one study [Irlam93] shows a median of 22KB. FFS solution: optimize small files for space efficiency.
Subdivide blocks into 2/4/8 fragments (or just frags). Free block maps contain one bit for each fragment.
To determine if a block is free, examine bits for all its fragments.
The last block of a small file is stored on fragment(s).
Basics of Directories
A directory is a set of file names, supporting lookup by symbolic name.
In Nachos, each directory is a file containing a set of mappings from name->FileHdr.
Directory(entries) sector = Find(name) Add(name, sector) Remove(name)
directory fileHdr 0 wind: 18 0 snow: 62 rain: 32 hail: 48
Each directory entry is a fixed-size slot with space for a FileNameMaxLen byte name.
Entries or slots are found by a linear scan.
A directory entry may hold a pointer to another directory, forming a hierarchical name space.
sector 32
A Nachos Filesystem On Disk
An allocation bitmap file maintains free/allocated state of each physical block; its FileHdr is always stored in sector 0. A directory maintains the name->FileHdr mappings for all existing files; its FileHdr is always stored in sector 1.
sector 0 allocation bitmap file
sector 1
11100010 00101101 10111101 once upo n a time /n in a l
directory file 0 rain: 32 hail: 48
wind: 18 0 snow: 62
10011010 00110001 00010101
00101110 00011001 01000100
and far far away , lived th
Every box in this diagram represents a disk sector.
Unix File Naming (Hard Links)
directory A directory B wind: 18 0 sleet: 48
A Unix file may have multiple names.
Each directory entry naming the file is called a hard link.
Each inode contains a reference count showing how many hard links name it.
0 rain: 32 hail: 48
inode link count = 2
inode 48
link system call link (existing name, new name) create a new name for an existing file increment inode link count
unlink system call ("remove") unlink(name) destroy directory entry decrement inode link count if count = 0 and file is not in active use free blocks (recursively) and on-disk inode
Unix Symbolic (Soft) Links
Unix files may also be named by symbolic (soft) links.
A soft link is a file containing a pathname of some other file.
symlink system call symlink (existing name, new name) allocate a new file (inode) with type symlink initialize file contents with existing name create directory entry for new file with new name
The target of the link may be removed at any time, leaving a dangling reference. How should the kernel handle recursive soft links?
directory A 0 rain: 32 hail: 48
directory B wind: 18 0 sleet: 67
inode link count = 1
inode 48 inode 67
../A/hail/0
The Problem of Disk Layout
The level of indirection in the file block maps allows flexibility in file layout. "File system design is 99% block allocation.&q...