UNIX File and Directory Caching How UNIX Optimizes File System Performance and Presents Data to User Processes Using a Virtual File System

V-NODE Layer V-node in-memory interface to the disk consist of: File system independent functions dealing with: –Hierarchical naming; –Locking; –Quotas; –Attribute management and protection. Object (file) creation and deletion, read and write, changes in space allocation: –These functions refer to file-store internals specific to the file system: –Physical organization of data on device; –For local data files, these functions refer to v-node refers to UNIX-specific structure called i-node (index node) that has all necessary information to access the actual data store.

Actual File I/O CPU cannot access the file data directly. Must be first brought to the main memory. –How to do this efficiently? Read/Write mapping using in-memory system file/directory buffer cache. Memory mapped files – INODE lists, Directories, Regular files. Then fed from memory to data pipeline in CPU.

Virtual INODES (In Memory)

Relation between logical and physical data views

Program I/O - Virtual to Real

File Read/Write Memory Mapping File data is made available to applications via a pre-allocated main memory region - the buffer cache. The file systems transfers data between the buffer cache and disk in granularity of disk blocks. The data is explicitly copied from/to buffer cache to/from the user application address space (process). A file (or a portion thereof) is mapped into a contiguous region of the process virtual memory. Mapping operation is very efficient: just marking a block. The access to file is governed by the virtual memory subsystem. Advantages: –reduce copying –no need for a pre-allocated buffer cache in the main memory Disadvantages: –less or no control over the actual disk writing: the file data becomes volatile –A mapped area must fit the virtual address space

Read/Write Mapping

Reading data (Disk block=1K)

Writing data (Disk block=1K)

Buffer Cache management All disk I/O goes through the buffer cache. Both data and metadata (e.g., i-node, directories) are cached using LRU replacement Dirty (modified) marker to indicate whether write-back is needed for data blocks. Advantages: - Hiding disk access the user program. Block size, memory alignment, memory allocation in multiples of the block size, etc… - Disk blocks are cached - Block aggregation for small transfers (locality) - Block re-use across processes - Transient data might be never written to disk Disadvantages: - Extra copying: Disk->buffer cache->user space - Vulnerability to failures - Does not care about the user data blocks - Control data blocks (metadata) are the real problem INODES, pointer blocks, directories can be in cache when a failure occurs As a result the file system internal state might be corrupted fsck required, resulting in long (re-)boot times

File System Reliability and Recovery File system data consists of file control data (metadata), user data Failures can cause data loss and corruption for cached metadata or user data Power failure during the sector write may corrupt physically the data stored in the sector Lost or corruption of the metadata might lead to a more massive user data loss. –File systems must care about the metadata more than about the user data –The Operating System cares about the file system data (e.g. metadata) –Users must care about their data themselves (e.g., backups) Caching affects the WRITE process reliability. –Is it guaranteed that the requested data is indeed written on disk? –What if cache blocks are the metadata blocks versus user data? Solutions: –write-through: writes bypass cache –write-back: dirty blocks are written asynchronously [bracket processes]

Data Reliability in UNIX User data writes based on write-back policy: –User data is written back to disk periodically –Program commands like sync and fsync are used for forced write of the dirty blocks. Metadata writes are based on write-through policy. Updates are written to disk immediately bypassing cache. Problem: - Some data is not written in-place. Can go back to the last consistent version - Some data is replicated like UNIX superblock. - File system goes through consistency check/repair cycle at the boot time as specified in /etc/fstab options (see manpage on fsck, fstab). - Write-through negatively affects performance Solution: maintain a sequential log of metadata updates, a Journal: e.g. IBM’s Journal File System (JFS) in AIX

Journal File System (JFS) Metadata operations logged (journaled): –create,link,mkdir,truncate,allocating, write, … –each operation may involve several metadata updates (transaction) Once operation is logged it returns, write ahead logging The disk writes are performed asynchronously. Block aggregation possible. A cursor (pointer) is maintained. The cursor is advanced once the updated blocks associated with the transaction are written to disk (hardened). Hardened transaction records can be deleted from the journal. Upon recovery: Re-do all the operations starting from the last cursor position. Advantages: –Asynchronous metadata write –Fast recovery: depends on the Journal size and not on the file-system size Disadvantages –extra write –space wasted by journal (insignificant)