The Unix File System. Ken Wong Washington University. Bsd Unix Kernel I/O Structure. Monolithic OS Device Drivers (1) File Systems (CSE 422S)

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1 File Systems (CSE 422S) Ken Wong Washington University usr bin ls 2 -Ken Wong, Dec 2007 The Unix File System vmunix tmp executables kenw mnt mount points arl other users Powerful, elegant file system from a small number of mechanisms Types of Files»Orinary File: An unstructure sequence of bytes» Directory: A set of files (even other irectories)»special File: An I/O evice (Normally in /ev/) ev s0g tty0 s0a other evices Socket Bs Unix Kernel I/O Structure Network Protocols Network Drivers 3 -Ken Wong, Dec 2007 NFS System-Call Interface to Kernel MFS Active File Entries VNODE Layer UFS Special Devices FFS LFS Buffer Cache Block-Device Driver Harware Cooke Disk Raw Disk/ tty tty Line Discipline VM Swap Space Character-Dev. Driver Monolithic OS Device Drivers (1) Autoconfigure an Initialize»Probes harware evices uring boot process»determines existence an evice characteristics Initialize software state Service I/O Requests (Top Half)» Hanles system calls from users»executes synchronously with top half of OS kernel an can block Service Interrupts (Bottom Half)»Hanles evice interrupts asynchronously from the top half of OS kernel n Threae kernel uses mutex locks in place of manipulating interrupt levels»does not block 4 -Ken Wong, Dec 2007

2 Monolithic OS Device Drivers (2) Function Call Function Call to Start Output System Call User Process I/O Library I/O Subsystem Write Queues Device Interface Sw splbio Rea Queue Hw splimp (cause by evice) BSD Unix I/O Buffers (1) frea(bufptr, objsz, nobj, fileptr) (2) rea(f, bufptr, nbytes) (3) memcpy(bufptr, memptr, nbytes) User Buffer I/O Library I/O Buffer (1) (2) Kernel Buffers (1,2) Device Interface (3) User Space Kernel Space 5 -Ken Wong, Dec Ken Wong, Dec 2007 Memory-Mappe File Segment text ata heap mmappe part of file stack 7 -Ken Wong, Dec 2007 Map a isk file into a memory buffer»if rea from buffer, bytes from file are rea»if write to buffer, bytes to file are written»can o I/O without using rea/write offset len File 8 -Ken Wong, Dec 2007 Unix mmap Example int fout; int nbytes; voi *src, *st; int offset; int openflags = O_RDWR O_CREAT O_TRUNC; int prot = PROT_WRITE; int mmflag = MAP_SHARED; src = some aress fout = Open( logfile, openflags, moe); set size of output file st = mmap(0, nbytes, prot, mmflag, fout, offset); memcpy(st, src, nbytes); Every call to memcpy writes to file»as bytes are written to kernel buffer, ata is automatically written to the output file

3 File Copying using Mmap File copying using mmap+memcpy is faster than rea+write»oing less work with mmap an memcpy»rea+write n kernel buffer user buffer kernel buffer»mmap+memcpy n kernel buffer kernel buffer Limitations»Can t be use between some evices (e.g. network)»size of file can t change after being mappe May simplify some algorithms Mapping Same File to Two Processes Process A: Process B: VM map memory map Physical Memory VM map memory map paging File Winows, Multics 9 -Ken Wong, Dec Ken Wong, Dec 2007 inex noe i-noe: moe owners timestamps size irect blocks 11 -Ken Wong, Dec 2007 Unix Filesystem Details single inirect ouble inirect triple inirect block count reference count flags generation number D i s k i - n o e s x MB i - l i s t w KB 1 ata 12 D a t a B l o c k s D i r e c t o r y y GB > y GB B l o c k i - n o e F i l e N a m e n u m b e r File reference count increments: ln $HOME/xxx y BSD Unix Block I/O System (1) 12 -Ken Wong, Dec 2007 write(f, buf, cnt); buf system buffers 0: 1: 2: 3: #26328 #26680 #4480 logical file logical file blocks #98034 isk blocks

4 BSD Unix Block I/O System (2) Allocate system buffer Determine location of physical block on isk using i-noe(s) Request isk controller to rea into system buffer if neee Wait for I/O transfer to complete Do memory-to-memory copy from user's I/O buffer to system buffer Start block write to isk Continue without waiting for write to complete 13 -Ken Wong, Dec Ken Wong, Dec 2007 BSD Unix Space Allocation Example: 4,096-byte blocks, 1,024-byte fragments (4096/1024) Block Map»One associate with each cyliner group»recors free fragments in the cyliner group» Free fragments: 0-3, 4-5, Bit Map Fragment Numbers Block Numbers **** ** ,000-byte file allocation: 1 full block an 2 consecutive fragments within block»may nee to split a full block to get the 2 fragments (leaves 2 free) 1 11** Ken Wong, Dec 2007 Super-block Disk rive has filesystems (has cyliner groups (has inoes an ata)) Super-Block: Describes a filesystem»critical ata Replicate in each cyliner group to protect against catastrophic loss»create at filesystem creation time (by newfs comman)»contains rea-only ata; e.g., n Location of super-block on isk n Number of blocks n Size of basic block n Size of basic fragment (1/8, 1/4, or 1/2 of a block) n Disk revolutions per secon n Tracks/sector, sectors/track, sectors/cyliner 16 -Ken Wong, Dec 2007 File System Backup Example moifie irectory moifie file Tanenbaum, Fig

5 17 -Ken Wong, Dec 2007 File System Backup (1) Typical Strategy»Perioically, perform full ump of volatile file systems n Stable file system (binaries) may nee umping less often than user irectories (e.g., /usr/home)»perform aily incremental umps to capture changes since last full ump n Saves on space an time n Unix ump has ump levels (0: full, 1-9: incremental) n Unix restore can restore from full or incremental ump Issues»Restore file F in irectory D but D no longer exists»free blocks when restoring to a new file system»symbolic links to a irectory» Files with holes (e.g., core umps) File System Backup (2) Unix Dump Implementation»Dump algorithm maintains bitmap inexe by i-noe number»bitmap entry = 1 for all i-noes that shoul be umpe n all moifie regular files n all irectories leaing to moifie regular files»dump irectories first n ensures that file system skeleton can be create in a complete restore» Dump regular files last Complete Unix Restore»Straightforwar»Rea ump tape file sequentially restoring irectories first an then regular files 18 -Ken Wong, Dec 2007 NFS (Network File System) Disks Disks Network Transparent remote access to share file systems Protocol esign is inepenent of Machine, OS, Network Architecture, Transport Protocol»Achieve through the use of RPC (Remote Proceure Call) on top of XDR (external Data Representation) 19 -Ken Wong, Dec 2007 File-System Interface VFS Interface Local File System Local File System Remote File System Motivation for Log-Structure FS Efficiency of small write is less than 1%»10,000 RPM 3 msec average rotation time»100 sectors/track 0.06 msec transfer time»average ranom seek time = 20 msec»efficiency = Transfer Time / (Total Time) 20 -Ken Wong, Dec 2007 n = 0.06/23.06 = or 0.3% n Even if seek time = 0, efficiency is only 2% Other factors» Seek an rotational elays are not ecreasing much» Main memories are growing exponentially»disks are getting bigger an cheaper»most file reas can be hanle from kernel buffers Nee to substantially increase write efficiency

6 Log-structure File System (1) Motivation»File system cache contains a large fraction of requeste rea blocks n Memory sizes increasing exponentially in size n Disks are getting bigger an cheaper but NOT faster»disk writes usually one in small chunks 21 -Ken Wong, Dec 2007 n Inefficient because seek + rotational latency is often 15 msec n Small writes come from i-noe upates Neee to maintain file system consistency Iea»Structure entire isk as a log n Perioically collect all pening writes an write out as 1 segment Segment can mix i-noes, irectory blocks an ata blocks»get almost full banwith of isk with 1 MB segment Log-structure File System (2) Example 1: ls l I/O operations» Rea i-noe of current irectory (metaata)» Rea contents of irectory (ata) Example 2: ls l > xxx I/O operations A. Write i-noe of current irectory (metaata) B. Write irectory contents (ata) C. Write i-noe for file xxx (metaata) D. Write contents of file xxx (ata) LFS» Do operations A-D in one segment write to en of log What if we elete file xxx?» Nee to reclaim i-noe an ata blocks 22 -Ken Wong, Dec 2007 i-noe: 128 bytes Log-structure File System (3) LFS cleaner threa»disk looks like one big circular buffer 23 -Ken Wong, Dec 2007 n Cleaner threa removes ol segments from back n Writer threa as new segments to front»non-trivial bookkeeping Example: Delete file xxx i-noe»directory entry: (i-noe#, filename)»nee i-noe map to fin i-noe to be elete» Rea segment containing targete i-noe»put targete i-noe an associate ata blocks on their free lists»cleaner reas segment 0 n write all i-noes an ata that are still in use to en of log file n move beginning of log file to next segment xxx Log-structure File System (4) File System Inconsistency»Metaata (ata about user ata) oesn t accurrately escribe ata n Example: system crashes after eleting i-noe but not its relate ata blocks»occurs in non-journale file system when system goes own without completing all writes n Fixe by running fsck comman»minimize in log-structure file systems n Recors are written to a isk log immeiately (before changes are mae) 10X write spee an 1X rea spee»linux ext3 filesystem has several journalling options 24 -Ken Wong, Dec 2007

7 25 -Ken Wong, Dec 2007 Making Journaling Work Example: Delete file xxx A. Delete xxx entry from irectory B. Return xxx s i-noe to free list C. Return all xxx s isk blocks to free list What if system crashes after step A?» No irectory contains xxx s i-noe» xxx s ata blocks are not free but aren t part of any file Journaling FS» First: write log entry (operations A, B, C) to isk» Secon: treat A-C as a transaction n Erase log entry only after A-C have been one» If system crashes n Reo incomplete transactions same result after repeate operations» Transaction operations nee to be iempotent