Software Systems File Systems and Storage

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Software SystemsFile Systems and Storage

• Emery Berger and Mark Corner
• University of Massachusetts Amherst

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Files

• Associate names with data
• Usually stored on persistent media (disks)

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File Names

• Hierarchical directory structure
• Absolute, relative to current
• Windows names location dir

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File Systems

• Organized set of data types
• organize data
• point to where data is stored
• searchable database of files
• LOTS of file systems
• AFS, BFS, CFS, DFS, EFS, FFS, GFS, HFS, etc.
• Distributed, local, encrypted, different OSs

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Directories

• Directory just special file
• Contains metadata, filenames
• pointers to inodes
• Typically hierarchical tree
• odd exposure of data structure to user

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Blocks

• Storage organized as a sequence of blocks
• Unit or reading and writing
• Read, modify, write sequence
• File system tracks free and full blocks
• typically stored in a bitmap

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Inodes

• On disk data structure
• Describes where all the bits of a file (dir) are

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Storage

• Lots of forms of permanent storage
• Disk drives, flash storage, Tape, CDs, DVDs

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Storage

• Disks
• Seek latency rotational latency
• High bandwidth
• One of two moving parts in a PC

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Storage

• Flash memory
• Predictable low latency (including random)
• Lower bandwidth
• Larger erase blocks, wears out, energy
• Prediction all PC storage Flash-based in 10
  years

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Locality

• File systems use directory structure to improve
  locality
• More important for disks than Flash
• E.g., ext2 all files in same directory
  clustered in same region of disk
• Try to make all blocks of same file sequential
• Move directories apart for expansion

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Caching

• Disk blocks, inodes, directories all cached
• 1/3 to 1/2 of memory is disk cache
• Disk drive has a cache too!

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Poor Mans Database

• Because files directories are easy to use, they
  get used as de facto databases
• e.g., Internet Explorer web cache
• 1000 files in each hash subdirectory

C\Documents and Settings\Emery\Local
Settings\Temporary Internet Files\Content.IE5gtls
-ltra total 1873 -rwx------ 1 Emery None 67
Jan 10 1731 desktop.ini drwx------ 2 Emery None
0 Jan 17 2242 0NDWKTYT drwx------ 7 Emery
None 0 Feb 19 1953 . drwx------ 7 Emery
None 0 Apr 20 1445 .. drwx------ 2 Emery
None 0 May 1 2141 8HZD6WS6 drwx------ 2
Emery None 0 May 1 2154
I4F15DOK drwx------ 2 Emery None 0 May 1
2203 XM0N4Q4W -rwx------ 1 Emery None 1916928
May 3 1221 index.dat drwx------ 2 Emery None
0 May 3 1221 S0RKZRFZ C\Documents and
Settings\Emery\Local Settings\Temporary Internet
Files\Content.IE5gt
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File Systems Abstraction

• File system manages files
• Traditionally file system maps files to disk
• But files convenient abstractionuse same, easy
  interface (read, write)
• Block devices (/dev/scsi0)
• Disk drives transfer in blocks
• Character devices (/dev/tty)
• Console, printer
• Proc filesystem (/proc/mem)
• FIFO (named pipes)

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Device files

• Unix devices live in /dev,act like ordinary files

elnux14gt echo "foo" gt /dev/tty foo
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/proc filesystem

• Normal file access to kernel internals

elnux14gt ls -l /proc/30917/ total 0 dr-xr-xr-x 2
emery fac 0 May 3 1318 attr -r-------- 1 emery
fac 0 May 3 1318 auxv -r--r--r-- 1 emery fac 0
May 3 1301 cmdline lrwxrwxrwx 1 emery fac 0
May 3 1318 cwd -gt /nfs/elsrv4/users5/fac/emery -
r-------- 1 emery fac 0 May 3 1318
environ lrwxrwxrwx 1 emery fac 0 May 3 1318
exe -gt /bin/tcsh dr-x------ 2 emery fac 0 May 3
1206 fd -rw-r--r-- 1 emery fac 0 May 3 1318
loginuid -r-------- 1 emery fac 0 May 3 1318
maps -rw------- 1 emery fac 0 May 3 1318
mem -r--r--r-- 1 emery fac 0 May 3 1318
mounts lrwxrwxrwx 1 emery fac 0 May 3 1318
root -gt / -r--r--r-- 1 emery fac 0 May 3 1301
stat -r--r--r-- 1 emery fac 0 May 3 1318
statm -r--r--r-- 1 emery fac 0 May 3 1301
status dr-xr-xr-x 3 emery fac 0 May 3 1318
task -r--r--r-- 1 emery fac 0 May 3 1310 wchan
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File Metadata

• Files have a lot of associated metadata ex.
  Unix (from stat)
• Date created, last modified, last accessed
• Size (bytes)
• User group ID of files owner
• File type (not content type)
• Directory
• Regular file
• Block / character device (disk drive, screen)
• FIFO

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Untyped Files

• Unix, Windows file contents untyped
• Stream of bytes
• Type implied by convention (extensions)
• .ppt, .pdf,
• Mac file types stored in metadata

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Access Control

• Unix each file has associated bits that control
  access ( other stuff)
• Read
• Write
• Execute
• Can specify for three users
• User (file owner)
• Group (set of users)
• Other (everyone else)

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Access Control - chmod

• Can read bits via ls, set bits via chmod

elnux14gt ls -l ack.scm -rw-r----- 1 emery fac
197 Feb 25 1519 ack.scm elnux14gt chmod -r
ack.scm elnux14gt ls -l ack.scm --w------- 1
emery fac 197 Feb 25 1519 ack.scm elnux14gt cat
ack.scm cat ack.scm Permission denied
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Access Control Lists (ACLs)

• ACLs are more expressive
• Specify different rights per user or group
• Opinion one of the biggest UNIX problems

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Whats Wrong with One Disk?
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Distributed File Systems

• Numerous drawbacks of local file systems
• Inconvenient
• Administrative overhead
• Single point-of-failure
• Solution distributed file systems
• FS appears local, but data remote
• Two major implementations
• Windows (CIFS, SAMBA)
• NFS (Suns Network File System)
• Lots of manual DFSs (rsync, svn, USB keys)

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Complications

• Complexity and design tradeoffs
• Naming absolute vs. relative (to server)
• Remote access vs. caching
• Stateless or stateful server
• Single image or replication

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Naming Transparency

• Issues
• How are files named?
• Do filenames reveal location?
• Do filenames change if file moves?
• Do filenames change if user moves?

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Location naming

• Location transparency
• Use indirection!
• filename does not reveal storage location
• Normal in Unix
• Compare to Windows - C\foo\bar
• Name may still change
• if storage location changes
• transparent not independent!

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What parts are transparent?

• Windows
• Local //computer/share/./directory/file
• Remote files are explicit!
• Remote ./directory/file
• UNIX
• Local /./mountpoint/directory/file
• Remote files look like any other file
• Remote /./directory/file
• Neither reveals all of storage location
• Windows reveals machine, UNIX does not

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NFS Example
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URLs Viewed as File System

• Uniform Resource Locator names increasingly
  standard way to access dataprotocol//machine/pa
  th/to/file
• Good? Bad?
• Looks like Windows same?

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File Caching

• Cache information from file server locally
• Local disk
• Reduces access time (compared to remote)
• Safe if node fails
• Requires client to have disk ()
• Local memory
• Quick
• Works without disks
• Smaller cache size
• Not fault-tolerant

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Remote File Access Caching

• Caching issues
• Performance
• Where when to cache file blocks?
• Correctness
• When to propagate updates back to remote file?
• What happens with multiple clients sharing?

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Sharing with Others
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When do changes get written?

• User A opens a file, changes a file
• When does it write it to file server?
• If another user opens file does it see the
  changes?
• Unix/one-copy semantics
• Immediate
• keep in mind UI issues
• Session semantics
• After close
• Transaction semantics
• Defined by program
• Uncommon in FS

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How is client informed?

• Client-initiated consistency
• client contacts server and checks consistency
• every access
• at given intervals
• only upon opening a file
• Server-initiated consistency
• server detects potential conflicts, invalidates
  caches
• Server needs to know
• which clients have cached which parts of which
  files, plus
• which clients are readers which are writers

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Conflicts

• Simplest kind Read-Write Conflicts
• Two people read same thing
• The cat is red
• Both write
• The cat is brown, The cat is purple
• Which is right?
• Can this happen locally?
• Yes! Try it with an editor
• Worse with DFS, not obvious to user why

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RAID, NAS, SAN Storage

• Redundant Array of Inexpensive Disks
• Multiple disks attached to controller
• Disks each carry part of data
• Redundancy, error detection, parallel transfer
• Network Attached Storage
• Box w/network port and storage (ie. XRAID)
• Storage Area Network
• Specialized network of NAS (ie. XSAN)

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The Near-Future

• Parallel File Systems (pNFS, GFS)
• Separate meta-data and data
• Store data chunks on different machines

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The End
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Atomic Updates

• Shadowing
• Logs
• Explain!

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Named Pipes (FIFO)

• Special file acts like unnamed pipe
• E.g., cat file wc -l

elnux14gt mkfifo thePipe elnux14gt ls -ld
thePipe prw-r----- 1 emery fac 0 May 3 1400
thePipe elnux14gt cat simplesocket.h gt thePipe
1 32242 elnux14gt wc -l lt thePipe 155 1
Done cat simplesocket.h
gt thePipe elnux14gt
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Named Pipes (FIFO)

• Special file acts like unnamed pipe
• E.g., cat file wc -l

elnux14gt mkfifo thePipe elnux14gt ls -ld
thePipe prw-r----- 1 emery fac 0 May 3 1400
thePipe elnux14gt cat simplesocket.h gt thePipe
1 32242 elnux14gt wc -l lt thePipe 155 1
Done cat simplesocket.h
gt thePipe elnux14gt
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Named Pipes (FIFO)

• Special file acts like unnamed pipe
• E.g., cat file wc -l

elnux14gt mkfifo thePipe elnux14gt ls -ld
thePipe prw-r----- 1 emery fac 0 May 3 1400
thePipe elnux14gt cat simplesocket.h gt thePipe
1 32242 elnux14gt wc -l lt thePipe 155 1
Done cat simplesocket.h
gt thePipe elnux14gt
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Named Pipes (FIFO)

• Special file acts like unnamed pipe
• E.g., cat file wc l
• Useful when cannot do redirection
• Especially for compression

elnux14gt mkfifo thePipe elnux14gt ls -ld
thePipe prw-r----- 1 emery fac 0 May 3 1400
thePipe elnux14gt cat simplesocket.h gt thePipe
1 32242 elnux14gt wc -l lt thePipe 155 1
Done cat simplesocket.h
gt thePipe elnux14gt
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Named Pipes (FIFO)

• Exercise
• Program named joe outputs file joe.out
• Huge ( 3 GB)
• Compress it automagically using gzip -c named
  FIFO to joe.out.gz

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Named Pipes (FIFO)

• Exercise
• Program named joe outputs file joe.out
• Huge ( 3 GB)
• Compress it automagically using gzip -c named
  FIFO to joe.out.gz

elnux14gt mkfifo joe.out elnux14gt gzip c lt
joe.out gt joe.out.gz 1elnux14gt joe