1、AN IMPLEMENTATION OF A LOG-STRUCTURED FILE SYSTEM FOR UNIX,Margo Seltzer, Harvard U. Keith Bostic, U. C. Berkeley Marshall Kirk McKusick, U. C. Berkeley Carl Staelin, HP Labs,Overview,Paper presents a redesign and implementation of the Sprite LFS BSD-LFS is Faster than conventional UNIX FFS (the “fa
2、st” file system of the early 80s) Not as fast as an enhanced version of FFS with read and write clustering,Historical Perspective,Early UNIX FS used small block sizes and did not try to optimize block placement The UNIX FFS Increased block sizes Added cylinder groups Incorporated rotational disk pos
3、itioning to reduce delays when accessing sequential blocks,Limitations of FFS (I),Synchronous file deletion and creation Makes file system recoverable after a crash Same result can be achieved through NVRAM hardware or logging software,Limitations of FFS (II),Seek times between I/O requests for diff
4、erent files Has most impact on performance whenever vast majority of files are small FFS does not address the problem,Log-Structured File Systems,Attempt to address both limitations of FFS Store all data in a single, continuous log Optimized for All writes Reading files written in their entirety ove
5、r a short period of time Accessing files that were created or modified at the same time,General Organization,Disk is partitioned into segments Writes are always sequential within a segment Segment cleaner maintains a pool of empty (“clean”) segments through disk compaction “Live” data existing in a
6、a set of segments are regrouped in a smaller subset of segments,Overview,LFS Data Structures,Superblock: Same function as one used by FFS I-node map: Maps i-node numbers into disk addresses Segment usage tables: Show number of live bytes in a segment and last modification time Checkpoints: Created e
7、very time system does a sync(),Limitations of Sprite LFS,Recovery does not verify the consistency of the file system directory structure LFS consumes “excessive amounts” of main memory by 1993 standards Write requests are successful even if there is insufficient disk space Segment validation is hard
8、ware dependent All file systems use a single cleaner and a single cleaning policy No measure of the cleaner overhead,Recovery (I),Two major aspects Bringing the file system to a physically consistent state Verifying the logical structure of the file system FFS achieves both goals through fsck Rebuil
9、ds the whole file system Verifies the directory structure and all block pointers,Recovery (II),Sprite LFS uses a two-step recovery process: Initializes first all the file structures from the most recent checkpoint “Roll forward” to incorporate all subsequent modifications Done by reading each segmen
10、t in time order after the last checkpoint,Recovery (III),Standard LFS recovery does not verify the directory structure Weakness to be addressed in BSD-LFS,Memory Consumption,Sprite LFS reserves “large amounts” of main memory including four half-megabyte segments and many buffers BSD-LFS: Does not us
11、e special staging buffers Does not reserve two read-only segments that can be reclaimed without any I/O Implements cleaner as a user-level process,Block Accounting,Sprite LFS maintained a count of disk blocks available for physical writing Blocks written to the cache but not written to disk do not a
12、ffect that count What if a block is “successfully” written to the cache but the disk becomes full before the blocks are actually written? BSD-LFS keeps a separate count of disk blocks that are not yet committed to any dirty block in the cache,Segment Structure (I),Sprint LFS places segment summary b
13、locks at the end of the segment Write containing the segment summary validates the whole segment Makes two incorrect assumptions Controller will not reorder write requests Disk will always write the contents of a buffer in the order presented,Segment Structure (II),BSD-LFS Does not make these assump
14、tions Segment blocks can be written in any order Segment summary is in front of each partial segment and contains a checksum of four bytes of every block in the partial segment Partial segments constitute the atomic recovery units of BDS-LFS,File System Verification,BSD-LFS offers two recovery strat
15、egies Quick roll forward from last checkpoint Complete consistency check of the file system Recovers lost or corrupted data Same functionality as FFS fsck() Takes a long time to run Can be run in the background,The Cleaner,BSD-LFS makes it possible to implement the cleaner as a user process Allows f
16、or multiple cleaning policies Makes it easier to experiment with new policies,Implementation Issues,BSD-LFS uses on-disk data structures that are nearly identical to those used by FFS Existing performance tools can continue to function with only minor modification Makes system easier to implement an
17、d maintain Two type of operations Vfs operations affect the whole file system Vnode operations affect individual files,More Implementation Issues,BSD-LFS does not implement block fragments Less needed block sizes could be smaller Still want large blocks to keep metadata to data ratio low BSD-LFS sho
18、uld (but does not yet) allocate progressively larger blocks.,The Buffer Cache (I),Had to modify the FFS buffer cache Cannot assume that cache blocks can be flushed one at a time Would destroy any performance advantage of LFS LFS may need extra memory to write modified metadata and partial segment su
19、mmary blocks,The Buffer Cache (II),Cache blocks do not have a disk address until they are written to the disk Violates assumption that all blocks have disk addresses Cannot use disk address to access indirect blocks BSD-LFS incorporates metadata block numbering (negative values),The IFILE,Sprite-LFS
20、 maintained the i-node map and segment usage table as kernel data structures written to disk at checkpoint time BSD-LFS places both data structures in a read-only file visible in the file system Allows unlimited number of i-nodes Cleaner can be migrated into user space I-node map also contains a lis
21、t of free i-nodes,Directory Operations (I),BSD-LFS does not retain synchronous behavior of directory operations (create, link, mkdir, ) Sprite-LFS maintains ordering of directory operations by maintaining a directory operation log inside the file system log Before any directory updates are written t
22、o disk, it writes a log entry describing that operation,Directory Operations (II),BSD-LFS has a unit of atomicity the partial segment It does not have a mechanism that guarantees that all i-nodes involved in a directory operation will fit into a single partial segment BSD-LFS allows operations to sp
23、an partial segments,Directory Operations (III),Introduces a new recovery restriction Cannot roll forward a partial segment that has an unfinished directory operation if the partial segment that completes the directory operation did not make it to disk (segment batching),COMPARISON,BSD-LFS was found
24、to perform Better to the 4BSD FFS in a variety of benchmarks Not significantly worse than FFS in any test EFS, a version of FFS with read and write clustering was found to provide comparable and sometimes superior performance to BSD- LFS.,EFS,Extended version of FFS Provides extent-based file system
25、 behavior Parameter maxcontig specifies how many logically sequential disk blocks should be allocated contiguously Large maxcontig is same as track allocation EFS accumulates sequential dirty buffers in the cache before writing them as a cluster,Multi-user Andrew Benchmark,We measure execution times
26、 LFS performs well in phases 1 and 2 (mostly writes) and poorly in phase 5 (random I/O),CONCLUSIONS,A LFS operates best when it can write out many dirty buffers at once Requires more buffer space in main memory Delayed allocation of BSD-LFS complicates accounting of available free space Issue was not correctly handled by Sprite-LFSCleaner might sometimes consumes more disk space than it frees Must reserve additional disk space,
