U NIVERSITY OF M ASSACHUSETTS A MHERST Department of Computer Science Software Systems File Systems...

UUNIVERSITY OF NIVERSITY OF MMASSACHUSETTS ASSACHUSETTS AAMHERST • MHERST • Department of Computer Science Department of Computer Science

Software SystemsFile Systems and Storage

Emery Berger and Mark CornerUniversity of Massachusetts

Amherst

UUNIVERSITY OF NIVERSITY OF MMASSACHUSETTS ASSACHUSETTS AAMHERST • MHERST • Department of Computer Science Department of Computer Science 2

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


File Names Hierarchical directory structure

– Absolute, relative to current Windows names = location + dir


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


Directories Directory – just special file

– Contains metadata, filenames • pointers to inodes

Typically hierarchical tree– odd exposure of data structure to user


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


Inodes On disk data structure

– Describes where all the bits of a file (dir) are


Storage Lots of forms of permanent storage

– Disk drives, flash storage, Tape, CDs, DVDs


Storage Disks

– Seek latency & rotational latency– High bandwidth– One of two moving parts in a PC


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


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


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


C:\Documents and Settings\Emery\Local Settings\Temporary Internet Files\Content.IE5>ls -ltratotal 1873-rwx------+ 1 Emery None 67 Jan 10 17:31 desktop.inidrwx------+ 2 Emery None 0 Jan 17 22:42 0NDWKTYTdrwx------+ 7 Emery None 0 Feb 19 19:53 .drwx------+ 7 Emery None 0 Apr 20 14:45 ..drwx------+ 2 Emery None 0 May 1 21:41 8HZD6WS6drwx------+ 2 Emery None 0 May 1 21:54 I4F15DOKdrwx------+ 2 Emery None 0 May 1 22:03 XM0N4Q4W-rwx------+ 1 Emery None 1916928 May 3 12:21 index.datdrwx------+ 2 Emery None 0 May 3 12:21 S0RKZRFZ

C:\Documents and Settings\Emery\Local Settings\Temporary Internet Files\Content.IE5>

Poor Man’s 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


File Systems Abstraction File system manages files

– Traditionally: file system maps files to disk But: files convenient abstraction

use 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)


elnux14> echo "foo" > /dev/ttyfoo

Device files Unix devices live in /dev,

act like ordinary files


elnux14> ls -l /proc/30917/total 0dr-xr-xr-x 2 emery fac 0 May 3 13:18 attr-r-------- 1 emery fac 0 May 3 13:18 auxv-r--r--r-- 1 emery fac 0 May 3 13:01 cmdlinelrwxrwxrwx 1 emery fac 0 May 3 13:18 cwd -> /nfs/elsrv4/users5/fac/emery-r-------- 1 emery fac 0 May 3 13:18 environlrwxrwxrwx 1 emery fac 0 May 3 13:18 exe -> /bin/tcshdr-x------ 2 emery fac 0 May 3 12:06 fd-rw-r--r-- 1 emery fac 0 May 3 13:18 loginuid-r-------- 1 emery fac 0 May 3 13:18 maps-rw------- 1 emery fac 0 May 3 13:18 mem-r--r--r-- 1 emery fac 0 May 3 13:18 mountslrwxrwxrwx 1 emery fac 0 May 3 13:18 root -> /-r--r--r-- 1 emery fac 0 May 3 13:01 stat-r--r--r-- 1 emery fac 0 May 3 13:18 statm-r--r--r-- 1 emery fac 0 May 3 13:01 statusdr-xr-xr-x 3 emery fac 0 May 3 13:18 task-r--r--r-- 1 emery fac 0 May 3 13:10 wchan

/proc filesystem Normal file access to kernel internals


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 file’s owner– File type (not content type)

• Directory• Regular file• Block / character device (disk drive, screen)• FIFO


Untyped Files Unix, Windows – file contents untyped

– Stream of bytes– Type implied by convention (extensions)

• .ppt, .pdf, …

Mac: file types stored in metadata


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)


elnux14> ls -l ack.scm-rw-r----- 1 emery fac 197 Feb 25 15:19 ack.scmelnux14> chmod -r ack.scmelnux14> ls -l ack.scm--w------- 1 emery fac 197 Feb 25 15:19 ack.scmelnux14> cat ack.scmcat: ack.scm: Permission denied

Access Control - chmod Can read bits via ls, set bits via chmod


Access Control Lists (ACLs) ACLs are more expressive

– Specify different rights per user or group– Opinion: one of the biggest UNIX problems


What’s Wrong with One Disk?

22


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 (Sun’s Network File System)

Lots of manual DFSs (rsync, svn, USB keys)


Complications Complexity and design tradeoffs

– Naming – absolute vs. relative (to server)– Remote access vs. caching– Stateless or stateful server– Single image or replication


Naming & Transparency Issues

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


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!


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


NFS Example


URLs Viewed as File System Uniform Resource Locator names

increasingly standard way to access data

protocol://machine/path/to/file

Good? Bad? Looks like Windows… same?


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


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?


Sharing with Others


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


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


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


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)


The “Near”-Future Parallel File Systems (pNFS, GFS) Separate meta-data and data Store data chunks on different machines


The End


Atomic Updates Shadowing Logs

Explain!


elnux14> mkfifo thePipeelnux14> ls -ld thePipeprw-r----- 1 emery fac 0 May 3 14:00 thePipeelnux14> cat simplesocket.h > thePipe &[1] 32242elnux14> wc -l < thePipe155[1] Done cat simplesocket.h > thePipeelnux14>

Named Pipes (FIFO) Special file: acts like unnamed pipe

– E.g., cat file | wc -l



E.g., cat file | wc -l

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E.g., cat file | wc -l

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E.g., cat file | wc –l

Useful when cannot do redirection Especially for compression

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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”

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elnux14> mkfifo joe.outelnux14> gzip –c < joe.out > joe.out.gz &[1]elnux14> joe

U NIVERSITY OF M ASSACHUSETTS A MHERST Department of Computer Science Software Systems File Systems...

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