Advanced Operating Systems: Lecture 30 - Mr. Farhan Zaidi

Advanced Operating Systems - Lecture 30: Indexed allocation. This lecture will cover the following: need index table; random access; file buffer cache; caching writes; read ahead; creating synonyms - hard and soft links;...

CS703 ­ Advanced  Operating Systems By Mr Farhan Zaidi Lecture No.  30 Consistency problem?  The Big File System Promise: persistence it will hold your data until you explicitly delete it  (and sometimes even beyond that: backup/restore) What’s hard about this? Crashes  If your data is in main memory, a crash destroys it  Performance tension: need to cache everything But if so, then crash = lose everything  More fundamental: interesting ops = multiple block modifications, but can only atomically modify disk a sector at a time   Indexed Allocation   Need index table Random access Indexed Allocation:  Brings all pointers together into the  index block index table UNIX: All disks are divided into five parts …  Boot block   Superblock   contains descriptors (i-nodes) for each file on the disk; all i-nodes are the same size; head of freelist is in the superblock File contents area   specifies boundaries of next areas, and contains head of freelists of i-nodes and file blocks i-node area   can boot the system by loading from this block fixed-size blocks; head of freelist is in the superblock Swap area  holds processes that have been swapped out of memory The “block list” portion of the i­node  (Unix Version 7)  Each i-node contains 13 block pointers  first 10 are “direct pointers” (pointers to 512B blocks of file data)  then, single, double, and triple indirect pointers … … … … 10 11 12 … … …  A later version of Bell Labs Unix utilized 12 direct pointers rather than 10  Berkeley Unix went to 1KB block sizes   What’s the effect on the maximum file size?  256x256x256x1K = 17 GB Suppose you went 4KB blocks?  1Kx1Kx1Kx4K = 4TB i­node Format         User number Group number Protection bits Times (file last read, file last written, inode last written) File code: specifies if the i-node represents a directory, an ordinary user file, or a “special file” (typically an I/O device) Size: length of file in bytes Block list: locates contents of file (in the file contents area) Link count: number of directories referencing this i-node Creating synonyms: Hard and soft links  More than one dir entry can refer to a given file  to make: “ln foo bar” creates a synonym (‘bar’) for ‘foo’ Soft links:   Unix stores count of pointers (“hard links”) to inode  foo bar ref = 2 also point to a file (or dir), but object can be deleted from underneath it (or never even exist)  ... of next areas, and contains head of freelists of i-nodes and file blocks i-node area   can boot the system by loading from this block fixed-size blocks; head of freelist is in the superblock... UNIX: All disks are divided into five parts …  Boot block   Superblock   contains descriptors (i-nodes) for each file on the disk; all i-nodes are the same size; head of freelist is in the superblock File contents.. .Lecture? ?No.  30 Consistency problem?  The Big File System Promise: persistence it will hold your data