Title: More%20on%20File%20Management
1More on File Management
2File Management
- provide file abstraction for data storage
- guarantee, to the extend possible, that data in
the file is valid - performance throughput and response time
- minimize the potential for lost or destroyed
data reliability - provide protection
- API create, delete, read, write files
3File Naming
- files must be referable by unique names
- external names symbolic
- in a hierarchical file system (UNIX) external
names are given as pathnames (path from the root
to the file) - internal names i-node in UNIX (an index into an
array of file descriptors/headers for a volume) - directory translation from external to internal
names (more than one external name for an
internal name is allowed) - information about file is split between the
directory and the file descriptor (in UNIX all of
it is stored in the file descriptor) size,
location on disk, owner, permissions, date
created, date last modified, date last access,
link count (in UNIX)
4Protection Mechanisms
- files are OS objects unique names and a finite
set of operations that processes can perform on
them - protection domain is a set of object,rights
where right is the permission to perform one of
the operations - at every instant in time, each process runs in
some protection domain - in Unix, a protection domain is uid, gid
- protection domain in Unix is switched when
running a program with SETUID/SETGID set or when
the process enters the kernel mode by issuing a
system call - how to store all the protection domains ?
5Protection Mechanisms (contd)
- Access Control List (ACL) associate with each
object a list of all the protection domains that
may access the object and how - in Unix ACL is reduced to three protection
domains owner, group and others - Capability List (C-list) associate with each
process a list of objects that may be accessed
along with the operations - C-list implementation issues where/how to store
them (hardware, kernel, encrypted in user space)
and how to revoke them
6Secondary Storage Management
- Space must be allocated to files
- Must keep track of the space available for
allocation
7Preallocation
- Need the maximum size for the file at the time of
creation - Difficult to reliably estimate the maximum
potential size of the file - Tend to overestimated file size so as not to run
out of space
8Methods of File Allocation
- Contiguous allocation
- Single set of blocks is allocated to a file at
the time of creation - Only a single entry in the file allocation table
- Starting block and length of the file
- External fragmentation will occur
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11Methods of File Allocation
- Chained allocation
- Allocation on basis of individual block
- Each block contains a pointer to the next block
in the chain - Only single entry in the file allocation table
- Starting block and length of file
- No external fragmentation
- Best for sequential files
- No accommodation of the principle of locality
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14Methods of File Allocation
- Indexed allocation
- File allocation table contains a separate
one-level index for each file - The index has one entry for each portion
allocated to the file - The file allocation table contains block number
for the index
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17File Allocation
- contiguous a contiguous set of blocks is
allocated to a file at the time of file creation
- good for sequential files
- file size must be known at the time of file
creation - external fragmentation
- chained allocation each block contains a pointer
to the next one in the chain
- consolidation to improve locality
- indexed allocation good both for sequential and
direct access (UNIX)
18Free Space Management
- bitmap one bit for each block on the disk
- good to find a contiguous group of free blocks
- small enough to be kept in memory
- chained free portions pointer to the next one,
length - index treats free space as a file
19UNIX File System
- Naming
- External/Internal names, Directories
- Lookup
- File blocks ? Disk blocks
- Protection
- Free Space Management
20File Naming
- External names (used by the application)
- Pathname /usr/users/file1
- Internal names (used by the OS kernel)
- I-node file number/index on disk
File system on disk
0
1
superblock
I-node area ( one I-node per file)
File-block area
21Directories
- Files which store translation tables (external
names to internal names)
usr
usr
users
usr 23
users 41
file1 87
Root directory (always I-node 2)
/usr/users/file1 corresponds to I-node 87
22File Content Lookup
- address table used to translate logical file
blocks into disk blocks - address table stored in the I-node
0
1
2
45
File with i-node 87
65
Address Table
85
File System disk
45
65
85
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24File Protection
- ACL with three protection domains (file owner,
file owner group, others) - Access rights read/write/execute
- Stored in the I-node
25Free Space Management
- Free I-nodes
- Marked as free on disk
- An array of 50 free I-nodes stored in the
superblock - Free file blocks
- Stored as a list of 50- free block arrays
- First array stored in the superblock
26In-Kernel File System Data Structures
fdopen(pathname,mode) / fd index in
Per-Proc OFT / for (..) read(fd,buf,size) close(
fd)
Application
PCBs
Per-process Open File Table
OS Kernel
I-node cache
Per-OS Open File Table (offset in file, ptr to
I-node)
Buffer cache
File system on disk
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1
27File System Consistency
- a file system uses the buffer cache for
performance reasons - two copies of a disk block (buffer cache, disk)
-gt consistency problem if the system crashes
before all the modified blocks are written back
to disk - the problem is critical especially for the blocks
that contain control information (meta-data)
directory blocks, i-node, free-list - Solution
- write through meta-data blocks (expensive) or
order of write-back is important - ordinary file data blocks written back
periodically (sync) - utility programs for checking block and directory
consistency after crash
28More on File System Consistency
- Example 1 create a new file
- Two updates (1) allocate a free I-node (2)
create an entry in the directory - (1) and (2) must be write-through (expensive) or
(1) must be written-back before (2) - If (2) is written back first and a crash occurs
before (1) is written back the directory
structure is inconsistent and cannot be recovered - Example 2 write a new block to a file
- Two updates (1) allocate a free block (2)
update the address table of the I-node - (1) and (2) must be write-through or (1) must be
written-back before (2) - If (2) is written back first and a crash occurs
before (1) is written back the I-node structure
is inconsistent and cannot be recovered
29Log-Structured File System (LFS)
- as memory gets larger, buffer cache size
increases -gt increase the fraction of read
requests which can be satisfied from the buffer
cache with no disk access - conclusion in the future most disk accesses will
be writes - but writes are usually done in small chunks in
most file systems (meta data for instance) which
makes the file system highly inefficient - LFS idea (Berkeley) to structure the entire disk
as a log - periodically, or when required, all the pending
writes (data and metadata together) being
buffered in memory are collected and written as a
single contiguous segment at the end of the log
30LFS segment
- contain i-nodes, directory blocks and data
blocks, all mixed together - each segment starts with a segment summary
- segment size 512 KB - 1MB
- two key issues
- how to retrieve information from the log
- how to manage the free space on disk
31File location in LFS
- the i-node contains the disk addresses of the
file block as in the standard UNIX - but there is no fixed location for the i-node
- an i-node map is used to maintain the current
location of each i-node - i-node map blocks can also be scattered but a
fixed checkpoint region on the disk identifies
the location of all the i-node map blocks - usually i-node map blocks are cached in main
memory most of the time, thus disk accesses for
them are rare
32Segment cleaning in LFS
- LFS disk is divided in segments which are written
sequentially - live data must be copied out of a segment before
the segment can be re-written - the process of copying data out of a segment
cleaning - a separate cleaner thread moves along the log,
removes old segments from the end and puts live
data into memory for rewriting in the next
segment - as a result a LFS disk appears like a big
circular buffer with the writer thread adding new
segments to the front and the cleaner thread
removing old segments from the end - book-keeping is not trivial i-node must be
updated when blocks are moved to the current
segment