DISTRIBUTED FILE SYSTEM Nhóm báo cáo :

1. DISTRIBUTED FILE SYSTEM • Nhóm báo cáo : • Lê Tuấn Anh • Nguyễn Hải Duy • Đặng Thanh Linh • Trần Trung Hiếu 50500892 • Nguyễn Hoàng Nam Distributed file system.

2. Content: • I. Distributed file system design. • II. Distributed file system Implementation • III. Network file system (NFS) • IV. Trends in distributed file system. Distributed file system.

3. What’s Distributed File System? Distributed File System (DFS) is a mechanism for sharing files DFS is used to make files distributed across multiple servers appear to users as if they reside in one place on the network DFS provides a mechanism to create logical views of folders and files regardless of where those files are physically located on the network Distributed file system.

4. What’s Distributed File System?(cont.) Distributed file system.

5. File Service Specify what the file system offers to its clients to manipulate on shared files ex: read,write…on files Implemented by a user/kernel process called file server A system may have one or several file servers running at the same time Distributed file system.

6. File Service (cont.) Two models for file services upload/download: files move between server and clients, few operations (read file & write file), simple, requires storage at client, good if whole file is accessed remote memory access: files stay at server, reach interface for many operations, less space at client, efficient for small accesses Distributed file system.

7. File Service (cont.) Distributed file system.

8. Directory Service Provide operations for : creating and deleting directories naming and renaming files moving files from one directory to another entering, removing, looking up files in one directory Distributed file system.

9. Naming Transparency Naming is the mapping between logical and physical objects. Ex: a user filename maps to <cylinder,sector> In a conventional file system, it's understood where the file actually resides; the system and disk are known. In a transparent DFS, the location of a file, somewhere in the network, is hidden File replication means multiple copies of a file; mapping returns a SET of locations for the replications. Distributed file system.

10. Naming Transparency(cont.) Location transparency: the path name gives no hint as to where the file (or other object) is located. ex: /server1/dir1/x specifies x is located on server1 but it does not tell where that server1 is located -> server can move the file in the network without changing the path Location independence: possible to remove one file among servers which not change the path name. Distributed file system.

11. Naming Schemes Machine + path naming, such as /machine/path Mounting remote file system onto the local file hierarchy A single name space that looks the same on all machines Distributed file system.

12. Two level naming Symbolic name (external), e.g. prog.c; binary name (internal), e.g. local i-node number as in Unix Directories provide the translation from symbolic to binary names Binary name format i-node: no cross references among servers (server, i-node): a directory in one server can refer to a file on a different server {binary_name}: binary names refer to the original file and all of its backups when looking up Distributed file system.

13. File Sharing Semantics UNIX semantics: total ordering of R/W events easy to achieve in a non-distributed system in a distributed system with one server and multiple clients with no caching at client, total ordering is also easily achieved since R and W are immediately performed at server Session semantics: writes are guaranteed to become visible only when the file is closed if two or more clients simultaneously write: one file (last one or non-deterministically) replaces the other Distributed file system.

14. File Sharing Semantics (cont.) Immutable files: create and read file operations (no write) writing a file means to create a new one and enter it into the directory replacing the previous one with the same name: atomic operations two processes try to replace the same file at the same time: last copy or nondeterministically what happens if a file is replaced while another process is busy reading it Transaction semantics: mutual exclusion on file accesses; either all file operations are completed or none is. Good for banking systems Distributed file system.

15. II.DFS Implementation • File usage • Measurements. • File Usage Pattern(Observed in a study by Satyanarayanan ). • System Structure • File-server and Directory-server Organization. • Special attention to alternative approaches. Distributed file system.

16. File usage- Measurements • - Static measurements: • * Represent a snapshot of the system at a certain instant. • * Made by examining the disk to see what is on it. • - Dynamic measurements: • * Modifying the file system to record all operations to a log for subsequent analysis Distributed file system.

17. File usage- Measurements • - Static measurements: • The distribution of files size. • The distribution of file types. • The amount of storage occupied by files of various types and size. • - Dynamic measurements: • The relative frequency of various operations • The number of files open at any moment • The amount of sharing that takes place Distributed file system.

18. File Usage- Measurement Problems • - How typical the observed user population is? • Satyanarayanan's measurements were made at a university -> Also apply to industrial research lab or office automation project or banking system? • - Watching out for artifacts of the system being measured • Ex: Distribution of file names in an MS-DOS system- File names are never more than 8 characters( plus an optional three- characters extension) • -Made on more-or-less traditional UNIX systems. Whether or not they can be transferred or extrapolated to distributed systems Distributed file system.

19. File Usage- File Usage Pattern • Observed in a study by Satyanarayanan (1981) • - Most files are small (< 10K) • - Reading is much more frequent than writing • - Most R&W accesses are sequential (random access is rare) • - Most files have a short lifetime -> create the file on the client • - File sharing is unusual -> caching at client • - The average process uses only a few files Distributed file system.

20. Server System Structure • Are client and server different? • - Some system, all machines run the same basic software -> any machine can offer file-service to the public- offer names of selected directories so that other machines can access them. • - The other systems, the file server and directory server are just user programs-> run client and server software on the same machines or no Distributed file system.

21. Server System Structure • Are client and server different? • - The other extreme systems have clients and server are on different machine. Distributed file system.

22. Server System Structure • File + directory service: combined or not ? • - Combine file service and directory service into a single server that handles all the directory and file calls. • - Keep file service and directory service separate: Directory-server map symbolic name onto its binary name.File-server with the binary name to read or write the file. Distributed file system.

23. Server System Structure • Separating File + directory service • Advantage • Produce simpler software • Disadvantage • Require more communications Distributed file system.

24. Server System Structure Separating File + directory service Example: Look-up a/b/c • Client sends a symbolic name • to the directory-server • -> binary name given by file-server • Directory-hierarchy • be partitioned among multiple servers: • 1st directory on sever 1 • contain an entry a for another directory on server 2.- 2nd directory on sever 2 • contain an entry b for another directory on server 3.- 3rd directory on sever 3 • contain an entry c for a file.- File with its binary name. Distributed file system.

25. Server System Structure Separating File + directory service Example: Look-up a/b/c • Client send a message -> server 1 • Server 1 finds a and sees the binary name refers to another server -> (1) tell the client which hold b • Requires the client to know which server holds which directory -> require more messages. Distributed file system.

26. Server System Structure Separating File + directory service Example: Look-up a/b/c • Client send a message -> server 1 • Server 1 finds a and sees the binary name refers to another server -> (2) forward the remainder of the request to server 2. • Efficient • Can not use RPC (Remote Procedure Call) because the process which the client sends the message to is not one that sends the reply Distributed file system.

27. Server System Structure • Separating File + directory service • Problem • Path names look up, especially with multiple directory servers can be expensive. • Cache directory hints at client to accelerate the path name look up – directory and hints must be kept coherent Distributed file system.

28. Server System Structure • Another question • Whether or not file, directory and other servers should keep state information about clients ? • - Yes Stateful server. • - No Stateless server. Distributed file system.

29. Stateful Servers Stateless Server • shorter messages • better performance (info in memory until close) • open/close at server • file locking possible • read ahead possible • requests are self-contained • better fault tolerance • open/close at client (fewer messages) • no space reserved for tables • thus, no limit of open files • no problem if client crashes Server System Structure Stateless vs. Stateful Distributed file system.

30. Caching • Definition: A cache is a block of memory for temporary storage of data likely to be used again. Cache Memory Main memory Distributed file system.

31. Caching There are four potential places to store files, or parts of files: -The Server’s disk. -The Server’s main memory. -The Client disk. -The Client ‘s main memory. These different storage locations all have different properties . Distributed file system.

32. Caching Distributed file system.

33. Caching-Store all file in the server’s disk. • Advantages: • -Plenty of space. • -The file are accessible to all clients . • -Have one copy of each file ->no consistency problems arises. • Problem: • -Performance: the file must be transferred from the server’s disk to the server’s main memory,and then again over the network to the client’s main memory. Distributed file system.

34. Caching files in the server's main memory. • Advantages: • -Eliminates the disk transfer. • -Keep its memory and disk copies synchronized • Problems: • -Network transfer still has to be done. • -What is the unit the cache manages?(whole files or disk blocks ). • -What to do when the cache fills up and something must be evicted.(one of algorithm :LRU). Distributed file system.

35. Caching at client’s disk (if available): • -The disk holds more but is slower. • - If large amounts of data are being used, a client disk cache may be better. • - This method isn’t used in practice. • - In any event, most systems that do client caching do it in the client's main memory. Distributed file system.

36. Cache in the client's main memory: • There are three options to decide where to put files: • -Inside each process address space: no sharing at client,it is effective only if individual processes open and close files repeatedly • -In the kernel: kernel involvement on hits,a kernel call is needed in all cases • -In a separate user-level cache manager: flexible and efficient if paging can be controlled from user-level Distributed file system.

37. Cache in the client's main memory Distributed file system.

38. Cache Consistency. • -Two clients simultaneously read the same file and then both modify it. • -Two files are written back to the server, the one written last will overwrite the other one. • - Client caching has to be thought out fairly carefully • -There are several ways to solve the consistency problem: • - Write through; Delayed write; Write on close; • Centralized control Distributed file system.

39. Cache Consistency- Write-through algorithm • -When a cache entry (file or block) is modified, the new • value is kept in the cache, but is also sent immediately • to the server • -> high traffic, requires cache managers to check (modification time) with server before can provide cached content to any client Distributed file system.

40. Cache Consistency -Delayed write • -Delayed write: coalesces multiple writes; better performance but ambiguous semantics . • *the client just makes a note that a file has been updated. Once every 30 seconds or so, all the file updates are gathered together and sent to the server all at once. • *entire sequence happens before time to send all modified files back to the server Distributed file system.

41. Cache Consistency -Write-on-close • -Write-on-close: implements session semantics, write a file back to the server only after it has been closed. Distributed file system.

42. Cache Consistency -Central control • -Central control: file server keeps a directory of open/cached files at clients -> Unix semantics, but problems with robustness and scalability; problem also with invalidation messages because clients did not solicit them Distributed file system.

43. Replication: • -Multiple copies of selected files. • 1. To increase reliability by having independent backups of each file. • 2. To allow file access to occur even if one file server is down. A server crash should not bring the entire system down until the server can be rebooted. • 3. To split the workload over multiple .By having files replicated on two or more servers, the least heavily loaded one can be used. Distributed file system.

44. Replication transparency • Replication transparency • -explicit file replication: programmer controls replication • -lazy file replication: copies made by the server in background • -use group communication: all copies made at the same time in the foreground Distributed file system.

45. Distributed file system.

46. Replication-Update protocols: • Updating all replicas using a coordinator works but is not robust (if coordinator is down, no updates can be performed) => Voting: updates (and reads) can be performed if some specified # of servers agree. • Voting Protocol: • A version # (incremented at write) is associated with each file • To perform a read, a client has to assemble a read quorum of Nr servers; similarly, a write quorum of Nw servers for a write • If Nr + Nw > N, then any read quorum will contain at least one most recently updated file version • For reading, client contacts Nr active servers and chooses the file with largest version # • For writing, client contacts Nw active servers asking them to write. Succeeds if they all say yes. Distributed file system.

47. Replication-Update protocols: • Nr is usually small (reads are frequent), but Nw is usually close to N (want to make sure all replicas are updated). Problem with achieving a write quorum in the presence of server failures • Voting with ghosts: allows to establish a write quorum when several servers are down by temporarily creating dummy (ghost) servers (at least one must be real) • Ghost servers are not permitted in a read quorum (they don’t have any files) • When server comes back it must restore its copy first by obtaining a read quorum Distributed file system.

48. III.Network file system (NFS) Three aspects of NFS: The architecture The protocol The implementation Distributed file system.

49. NFS Architecture • Basic idea NFS: An arbitrary collection of clients and servers. • Server export one or more directory for access by remote client. • List of director is maintained /etc/exports/ Distributed file system.

50. NFS Architecture • Clients access exported directories by mounting them. • Clients diskless can mount on remote root directory and else. • To programs running on clients is no difference between a file located. • So, the basic architectural characteristic NFS is server exported directory and clients mount them remotely. Distributed file system.