File System Reliability & Virtual File in Operating System
- 1. TOPIC : FILE SYSTEM RELIABILITY & VIRTAUL FILE SYSTEM PRESENTED BY: MEGHAJ KUMAR MALLICK (MCA/25017/18) 1ST YEAR ,2ND SEMESTER
- 3. What can happen if disk loses power or machine software crashes? ◦ Some operations in progress may complete ◦ Some operations in progress may be lost ◦ Overwrite of a block may only partially complete File system wants durability (as a minimum!) ◦ Data previously stored can be retrieved (maybe after some recovery step), regardless of failure
- 4. Single logical file operation can involve updates to multiple physical disk blocks ◦ inode, indirect block, data block, bitmap, … ◦ With remapping, single update to physical disk block can require multiple (even lower level) updates At a physical level, operations complete one at a time ◦ Want concurrent operations for performance How do we guarantee consistency regardless of when crash occurs
- 5. Volatile storage – information stored here does not survive system crashes ◦ Example: main memory, cache Nonvolatile storage – Information usually survives crashes ◦ Example: disk and tape Stable storage – Information never lost ◦ Not actually possible, so approximated via replication or RAID to devices with independent failure modes
- 6. Sequence operations in a specific order ◦ Careful design to allow sequence to be interrupted safely Post-crash recovery ◦ Read data structures to see if there were any operations in progress ◦ Clean up/finish as needed Approach taken in FAT, FFS (fsck), and many app-level recovery schemes (e.g., Word)
- 8. A virtual file system (VFS) is programming that forms an interface between an operating system's kernel and a more concrete file system. The VFS serves as an abstraction layer that gives applications access to different types of file systems and local and network storage devices. For that reason, a VFS may also be known as a virtual file system switch. It also manages the data storage and retrieval between the operating system and the storage sub-system. The VFS maintains a cache of directory lookups to enable easy location of frequently accessed directories.
- 9. One of the first virtual file system mechanisms on Unix-like systems was introduced by Sun Microsystemsin SunOS 2.0 in 1985 In Microsoft Windows, virtual file systems can also be implemented through userland Shell namespace extensions.
- 10. Sometimes Virtual File System refers to a file or a group of files (not necessarily inside a concrete file system) that acts as a manageable container which should provide the functionality of a concrete file system through the usage of software.
- 11. Examples of such containers are CBFS Storage or a single-file virtual file system in an emulator like PCTask or so-called WinUAE, Oracle's VirtualBox, Microsoft's Virtual PC, VMware. The primary benefit for this type of file system is that it is centralized and easy to remove
- 12. Direct examples of single-file virtual file systems include emulators, such as PCTask and WinUAE, which encapsulate not only the file system data but also emulated disk layout. This makes it easy to treat an OS installation like any other piece of software— transferring it with removable media or over the network.