File Protection and File System Implementation in CMSC 412 - Prof. Jeffrey K. Hollingswort, Study notes of Operating Systems

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Announcements

z^ Reading Chapter 12 z^ Please see class web site for info on projectsubmission format

File Protection Example (AFS)

z^ Each Directory has an ACL^ – protection information applies to all files in a directory^ – file access types are:

• read, write, lookup, delete, insert, lock (k), administer – an ACL may be for a user or a group – ACL may contain negative rights • everyone but Joe Smith may read this file

z^ Groups^ – are collections of users^ – each user can create up to a fixed number of groups

• users can administrate their own groups

z^ Cells^ – collections of computers (e.g. wam)

Filesystems

z^ Raw Disks can be viewed as:^ – a linear array of fixed sized units of allocation, called blocks

• assume that blocks are error free (for now) • typical block size is 512 to 4096 bytes – can update a block in place, but must write the entire block – can access any block in any desired order • blocks must be read as a unit • for performance reasons may care about “near” vs. “far”blocks (but that is covered in a future lecture)

z^ A Filesystem:^ – provides a hierarchical namespace via directories^ – permits files of variable size to be stored^ – provides disk protection by restricting access to files basedon permissions

File System Implementation

Application Programs^ Logical file system:

Knows about directories, application view of file names

File Organization Module:

Can translate logical block addresses to physical block addresses

Basic File System:

Issues physical block read/write commands

Low Level I/O Control^ Interfaces to hardware

Linked Allocation

z^ Each file is a linked list of disk blocks, blocks can belocated anywhere^ – Directory contains a pointer to the first and last block of a file^ – Each block contains a pointer to the next block^ – This is essentially a linked-list data structure z^ Problems:^ – Best for sequential access data structures

• requires sequential access whether you want to or not! – Reliability - one bad sector and all portions of your filedownstream are lost

z^ Useful fix:^ – Maintain a separate data structure just to keep track oflinked lists^ – Data-structure includes pointers to actual blocks

Indexed Allocation

z^ Bring all pointers together in an

index block

-^ Each file has its own index block -

i th entry of index block points to

i th block making up the file z How large to make an index block? – unless one only wants to support fixed size files, index blockscheme needs to be extensible z Linked scheme: – maintain a linked list of indexed blocks z Multilevel index: – Index block can point to other index blocks (which point to indexblocks ....), which point to files z Hybrid multi-level index – first n blocks are from a fixed index – next m blocks from an indirect index – next o blocks from a double indirect index