Review How do we actually access files All information about a file contained in its file header UNIX calls this an inode CS162 Operating Systems and Systems Programming Lecture 20 Inodes are global resources identified by index inumber Once you load the header structure all the other blocks of the file are locatable Naming The process by which a system translates from user visible names to system resources In the case of files need to translate from strings textual names or icons to inumbers inodes Distributed Systems Name Resolution The process of converting a logical name into a physical resource like a file Traverse succession of directories until reach target file Global file system May be spread across the network November 9 2005 Prof John Kubiatowicz http inst eecs berkeley edu cs162 Directory a relation used for naming Just a table of file name inumber pairs Directories often stored in files Reuse of existing mechanism Directory named by inode inumber like other files 11 09 05 Goals for Today Kubiatowicz CS162 UCB Fall 2005 Lec 20 2 File System Caching Key Idea Exploit locality by caching data in memory File Caching Data Durability Distributed Systems Name translations Mapping from paths inodes Disk blocks Mapping from block address disk content Buffer Cache Memory used to cache kernel resources including disk blocks and name translations Can contain dirty blocks blocks yet on disk Replacement policy LRU Can afford overhead of timestamps for each disk block Advantages Works very well for name translation Works well in general as long as memory is big enough to accommodate a host s working set of files Disadvantages Fails when some application scans through file system thereby flushing the cache with data used only once Example find exec grep foo Note Some slides and or pictures in the following are adapted from slides 2005 Silberschatz Galvin and Gagne 11 09 05 Kubiatowicz CS162 UCB Fall 2005 Lec 20 3 Other Replacement Policies Some systems allow applications to request other policies Example Use Once 11 09 05 File system can discard blocks as soon as they are used Kubiatowicz CS162 UCB Fall 2005 Lec 20 4 File System Caching con t Cache Size How much memory should the OS allocate to the buffer cache vs virtual memory Too much memory to the file system cache won t be able to run many applications at once Too little memory to file system cache many applications may run slowly disk caching not effective Solution adjust boundary dynamically so that the disk access rates for paging and file access are balanced Read Ahead Prefetching fetch sequential blocks early Key Idea exploit fact that most common file access is sequential by prefetching subsequent disk blocks ahead of current read request if they are not already in memory Elevator algorithm can efficiently interleave groups of prefetches from concurrent applications How much to prefetch Too many imposes delays on requests by other applications Too few causes many seeks and rotational delays among concurrent file requests 11 09 05 Kubiatowicz CS162 UCB Fall 2005 Lec 20 5 Important ilities Availability the probability that the system can accept and process requests Often measured in nines of probability So a 99 9 probability is considered 3 nines of availability Key idea here is independence of failures Durability the ability of a system to recover data despite faults File System Caching con t Delayed Writes Writes to files not immediately sent out to disk Instead write copies data from user space buffer to kernel buffer in cache Enabled by presence of buffer cache can leave written file blocks in cache for a while If some other application tries to read data before written to disk file system will read from cache Flushed to disk periodically e g in UNIX every 30 sec Advantages Disk scheduler can efficiently order lots of requests Disk allocation algorithm can be run with correct size value for a file Some files need never get written to disk e g temporary scratch files written tmp often don t exist for 30 sec Disadvantages What if system crashes before file has been written out Worse yet what if system crashes before a directory file has been written out lose pointer to inode 11 09 05 Kubiatowicz CS162 UCB Fall 2005 How to make file system durable Disk blocks contain Reed Solomon error correcting codes ECC to deal with small defects in disk drive Can allow recovery of data from small media defects Make sure writes survive in short term Either abandon delayed writes or use special battery backed RAM called non volatile RAM or NVRAM for dirty blocks in buffer cache This idea is fault tolerance applied to data Doesn t necessarily imply availability information on pyramids was very durable but could not be accessed until discovery of Rosetta Stone Make sure that data survives in long term Usually stronger than simply availability means that the system is not only up but also working correctly Includes availability security fault tolerance durability Must make sure data survives system crashes disk crashes other problems RAID Redundant Arrays of Inexpensive Disks Reliability the ability of a system or component to perform its required functions under stated conditions for a specified period of time IEEE definition 11 09 05 Kubiatowicz CS162 UCB Fall 2005 Lec 20 7 Lec 20 6 Need to replicate More than one copy of data Important element independence of failure Could put copies on one disk but if disk head fails Could put copies on different disks but if server fails Could put copies on different servers but if building is struck by lightning Could put copies on servers in different continents Data stored on multiple disks redundancy Either in software or hardware In hardware case done by disk controller file system may not even know that there is more than one disk in use 11 09 05 Kubiatowicz CS162 UCB Fall 2005 Lec 20 8 Hardware RAID Subsystem Organization CPU host adapter single board disk controller array controller manages interface to host DMA control buffering parity logic physical device control Some systems duplicate all hardware namely controllers busses etc 11 09 05 RAID 1 Disk Mirroring Shadowing recovery group single board disk controller Each disk is fully duplicated onto its shadow single board disk controller Bandwidth sacrificed on write single board disk controller Reads may be optimized For high I O rate high availability environments Most expensive solution 100 capacity overhead Logical write two physical
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