Chapter 12: File System Implementation
文章目錄
File-System Structure
File control block–storage structure consisting of information about a file. 文件控制塊放在硬盤上，內存中存放的有文件控制塊所包含的內容，文件存放在硬盤，硬盤上的存儲是非易失的。
File structure: Logical storage unit Collection of related information
Layered File System：
應程程序進行系統調用：open read write… 執行IO語句
邏輯文件系統：目錄 FCB 文件具體放在block（logic）上
文件組織模塊：邏輯塊映射到物理塊
基本文件系統：發出IO命令 磁盤地址
IO control：設備驅動
設備：以上都是軟件層面，只有device是硬件
A Typical File Control Block: file permission, file dates, file owner,group,ACL ,file size ,file date blocks 抓住了文件控制塊就抓住了文件
on-disk information:
boot control block,volume control block(每分區一個) directory structure(每個文件系統一個)
FCB i-node UFS (這些信息放在磁盤上，文件加載後有些信息會放在內存中)
In-Memory File System Structures：
Directory Implementation
- Linear list of file names with pointer to the data blocks ：
simple to program
time-consuming to execute
- Hash Table –linear list with hash data structure ：
decreases directory search time
collisions –situations where two file names hash to the same location
Allocation Methods
Contiguous Allocation
- Each file occupies a set of contiguous blocks on the disk.
- Simple –only starting location (block #) and length (number of blocks) are required.
- Random access.
- Wasteful of space 外部碎片
- Files cannot grow
Extent-Based Systems
Extent-based file systems allocate disk blocks in extents.
An extent is a contiguous block of disks. Extents are allocated for file allocation. A file consists of one or more extents。 類似於存儲管理中的分段segement
Linked Allocation
Each file is a linked list of disk blocks: blocks may be scattered anywhere on the disk.
- Simple –need only starting address
- Free-space management system –no waste of space
- Files can grow
- No random access
- Each block contains a pointer, wasting space
- Blocks scatter everywhere and a large number of disk seeks may be necessary
- Reliability: what if a pointer is lost or damaged 不安全不可靠
File-Allocation Table FAT
FAT表在磁盤的最開始
實現了隨機訪問，更加安全可靠
Indexed Allocation
Brings all pointers together into the index block
A file’s directory entry contains a pointer to its index. Hence, the index block of an indexed allocation plays the same role as the page table.
- **Random access **
- 無外部碎片
- 不是安全可靠的
- 文件的grow是有限的
The indexed allocation suffers from wasted space. The index block may not be fully used
The number of entries of an index table determines the size of a file. To overcome this problem, we can have ：
- multiple index blocks, chain them into a linked-list
- multiple index blocks, but make them a tree just like the indexed access method
- A combination of both
4 128 84+2^16
Free-Space Management
How do we keep track free blocks on a disk? A free-list is maintained(維護).
When a new block is requested, we search this list to find one.
- Bit Vector
- Linked List
- Linked List + Grouping
- Linked List+Address+Count
Linked Free Space List on Disk:
Link+Grouping:
The first free block contains the addresses of n other free blocks.
For each group, the first n-1 blocks are actually free and the last (i.e., n-th) block contains the addresses of the next group.
Link+Address counting:
Blocks are often allocated and freed in groups
We can store the address of the first free block and the number of the following n free blocks.
Efficiency and Performance
Efficiency dependent on:
- disk allocation and directory algorithms
- types of data kept in file’s directory entry
Performance:
- disk cache
- free-behind and read-ahead 延後釋放，提前讀取
- improve PC performance by dedicating section of memory as virtual disk, or RAM disk 提升硬件性能