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Distributed File Systems Recall: A Little Queuing Theory: Some Results • Assumptions: CS162 – System in equilibrium; No limit to the queue Operating Systems and – Time between successive arrivals is random and memoryless Systems Programming Queue Server Lecture 19 Arrival Rate Service Rate μ=1/Tser • Parameters that describe our system: File Systems (Con’t), – : mean number of arriving customers/second –Tser: mean time to service a customer (“m1”) MMAP, Buffer Cache –C: squared coefficient of variance = 2/m12 – μ: service rate = 1/Tser –u: server utilization (0u1): u = /μ = Tser April 7th, 2020 • Parameters we wish to compute: –T: Time spent in queue Prof. John Kubiatowicz q –Lq: Length of queue = Tq (by Little’s law) http://cs162.eecs.Berkeley.edu • Results: –Memoryless service distribution (C = 1): (an “M/M/1 queue”): »Tq = Tser x u/(1 – u) –General service distribution (no restrictions), 1 server (an “M/G/1 queue”): »Tq = Tser x ½(1+C) x u/(1 – u) 4/7/20 Kubiatowicz CS162 © UCB Spring 2020 Lec 19.2 Recall: A Little Queuing Theory: Some Results Components of a File System • Assumptions: – System in equilibrium; No limit to theWhy queue does response/queueing File path – Time between successive arrivals isdelay random grow and unboundedly memoryless even One file system block though the utilization is < 1 ? Directory File Index Queue Server usually = multiple sectors Arrival Rate Service300 RateResponse Structure Structure Time (ms) File number Ex: 512 sector, 4K block μ=1/Tser • Parameters that describe our system: “inumber” 200 – : mean number of arriving customers/second Data blocks –Tser: mean time to service a customer (“m1”) –C: squared coefficient of variance100 = 2/m12 – μ: service rate = 1/Tser –u: server utilization (0u1): u = /μ = Tser 0 100% • Parameters we wish to compute: 0% “inode” … –Tq: Time spent in queue Throughput (Utilization) Note: we have an unfortunate conflicting –Lq: Length of queue = Tq (by Little’s law) (% total BW) terminology for FLASH: • Results: • Block 256KB group of pages –Memoryless service distribution (C = 1): (an “M/M/1 queue”): • Page 4KB chunk inside block »Tq = Tser x u/(1 – u) A FLASH page corresponds to a disk –General service distribution (no restrictions), 1 server (an “M/G/1 queue”): sector (minimum addressable unit) and »Tq = Tser x ½(1+C) x u/(1 – u) block (file system unit), i.e. sector=block 4/7/20 Kubiatowicz CS162 © UCB Spring 2020 Lec 19.3 4/7/20 Kubiatowicz CS162 © UCB Spring 2020 Lec 19.4 Components of a file system Directories file name file number Index Storage block offset directory offset structure • Open performs Name Resolution – Translates pathname into a “file number” » Used as an “index” to locate the blocks – Creates a file descriptor in PCB within kernel – Returns a “handle” (another integer) to user process • Read, Write, Seek, and Sync operate on handle – Mapped to file descriptor and to blocks 4/7/20 Kubiatowicz CS162 © UCB Spring 2020 Lec 19.5 4/7/20 Kubiatowicz CS162 © UCB Spring 2020 Lec 19.6 Directory Directory Structure • Basically a hierarchical structure • How many disk accesses to resolve “/my/book/count”? – Read in file header for root (fixed spot on disk) • Each directory entry is a collection of – Read in first data block for root – Files » Table of file name/index pairs. Search linearly – ok since – Directories directories typically very small » A link to another entries – Read in file header for “my” – Read in first data block for “my”; search for “book” • Each has a name and attributes – Read in file header for “book” – Files have data – Read in first data block for “book”; search for “count” – Read in file header for “count” • Links (hard links) make it a DAG, not just a tree • Current working directory: Per-address-space pointer to – Softlinks (aliases) are another name for an entry a directory (inode) used for resolving file names – Allows user to specify relative filename instead of absolute path (say CWD=“/my/book” can resolve “count”) 4/7/20 Kubiatowicz CS162 © UCB Spring 2020 Lec 19.7 4/7/20 Kubiatowicz CS162 © UCB Spring 2020 Lec 19.8 File In-Memory File System Structures • Named permanent storage Data blocks • Contains –Data » Blocks on disk somewhere – Metadata (Attributes) … » Owner, size, last opened, … » Access rights •R, W, X • Owner, Group, Other (in Unix systems) • Open system call: • Access control list in Windows system – Resolves file name, finds file control block (inode) – Makes entries in per-process and system-wide tables – Returns index (called “file handle”) in open-file table 4/7/20 Kubiatowicz CS162 © UCB Spring 2020 Lec 19.9 4/7/20 Kubiatowicz CS162 © UCB Spring 2020 Lec 19.10 In-Memory File System Structures Our first filesystem: FAT (File Allocation Table) • The most commonly used filesystem in the world! • Assume (for now) we have a FAT Disk Blocks way to translate a path to 0: 0: a “file number” File number – i.e., a directory structure 31: File 31, Block 0 • Disk Storage is a collection of Blocks File 31, Block 1 – Just hold file data (offset o = < B, x >) • Example: file_read 31, < 2, x > – Index into FAT with file number – Follow linked list to block • Read/write system calls: – Read the block from disk into memory File 31, Block 2 –Use file handle to locate inode –Perform appropriate reads or writes N-1: N-1: memory 4/7/20 Kubiatowicz CS162 © UCB Spring 2020 Lec 19.11 4/7/20 Kubiatowicz CS162 © UCB Spring 2020 Lec 19.12 FAT Properties FAT Properties • File is collection of disk blocks • File is collection of disk blocks • FAT is linked list 1-1 with blocks FAT Disk Blocks • FAT is linked list 1-1 with blocks FAT Disk Blocks • File Number is index of root 0: 0: • File Number is index of root 0: 0: of block list for the file File number of block list for the file File number • File offset (o = < B, x >) 31: File 31, Block 0 • File offset (o = < B, x > ) 31: File 31, Block 0 File 31, Block 1 File 31, Block 1 • Follow list to get block # • Follow list to get block # • Unused blocks Marked free (no • Unused blocks Marked free (no ordering, must scan to find) ordering, must scan to find) • Ex: file_write(31, < 3, y >) File 31, Block 3 free free – Grab free block File 31, Block 2 – Linking them into file File 31, Block 2 N-1: N-1: N-1: N-1: memory memory 4/7/20 Kubiatowicz CS162 © UCB Spring 2020 Lec 19.13 4/7/20 Kubiatowicz CS162 © UCB Spring 2020 Lec 19.14 FAT Properties FAT Assessment • File is collection of disk blocks • FAT32 (32 instead of 12 bits) used in Windows, USB drives, • FAT is linked list 1-1 with blocks FAT Disk Blocks SD cards, … FAT Disk Blocks • File Number is index of root 0: 0: • Where is FAT stored? 0: 0: File 1 number File 1 number of block list for the file – On Disk, on boot cache in memory, • Grow file by allocating free blocks 31: File 31, Block 0 second (backup) copy on disk 31: File 31, Block 0 File 31, Block 1 File 31, Block 1 and linking them in free • What happens when you format a disk? File 63, Block 1 File 63, Block 1 • Ex: Create file, write, write – Zero the blocks, Mark FAT entries “free” • What happens when you File 31, Block 3 quick format a disk? File 31, Block 3 File 63, Block 0 File 63, Block 0 63: – Mark all entries in FAT as free 63: File 2 number File 31, Block 2 File 2 number File 31, Block 2 • Simple – Can implement in N-1: N-1: N-1: N-1: device firmware memory memory 4/7/20 Kubiatowicz CS162 © UCB Spring 2020 Lec 19.15 4/7/20 Kubiatowicz CS162 © UCB Spring 2020 Lec 19.16 FAT Assessment – Issues What about FAT directories? • Time to find block (large files) ?? FAT Disk Blocks • Block layout for file ??? 0: 0: File #1 • Sequential Access ??? 31: File 31, Block 0 • Directory is a file containing <file_name: file_number> File 31, Block 1 mappings • Random Access ??? File 63, Block 1 – Free space for new/deleted entries • Fragmentation ??? – In FAT: file attributes are kept in directory (!!!) File 31, Block 3 – MSDOS defrag tool – Each directory is a linked list of entries File 63, Block 0 63: • Where do you find root directory ( “/” )? • Small files ??? File #2 File 31, Block 2 – At well-defined place on disk – For FAT, this is at block 2 (there are no blocks 0 or 1) • Big files ??? N-1: N-1: – Remaining directories are accessed via their file_number 4/7/20 Kubiatowicz CS162 © UCB Spring 2020 Lec 19.17 4/7/20 Kubiatowicz CS162 © UCB Spring 2020 Lec 19.18 Many Huge FAT Security Holes! Characteristics of Files • FAT has no access rights – No way, even in principle, to track ownership of data • FAT has no header in the file blocks – No way to enforce control over data, since all processes have access of FAT table – Just follow pointer to disk blocks • Just gives an index into the FAT to read data – (file number = block number) – Could start in middle of file or access deleted data 4/7/20 Kubiatowicz CS162 © UCB Spring 2020 Lec 19.19 4/7/20 Kubiatowicz CS162 © UCB Spring 2020 Lec 19.20 Characteristics of Files Characteristics of Files • Most files are small, growing numbers of files • Most of the space is occupied by the rare big ones over time 4/7/20 Kubiatowicz CS162 © UCB Spring 2020 Lec 19.21 4/7/20 Kubiatowicz CS162 © UCB Spring 2020 Lec 19.22 Unix File System (1/2) Unix File System (2/2) • Original inode format appeared in BSD 4.1 • Metadata associated with the file – Berkeley Standard Distribution Unix – Rather than in the directory that points to it – Part of your heritage! • UNIX Fast File System (FFS) BSD 4.2 Locality Heuristics: – Similar structure for Linux Ext2/3 – Block group placement • File Number is index into inode arrays – Reserve space • Multi-level index structure
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