Multimedia File Systems - Social Spaces Group：多媒体文件系统-社会空间群.ppt

MM File Management Karrie Karahlaios and Brian P. BaileySpring 2007 Announcements Physical Disk Structure Sector Track Platter Cylinder R/W head Example 16 heads x 1400 cyls x 16 sectors/track x 512 bytes/sector = 183.5MB Measures of Performance Seek time (ms) time to move disk arm to a specific track Latency (ms) time for sector to rotate under disk arm Transfer rate (Mbps) data that can be read in one time unit Zoned Bit Recording Utilize larger, outer tracks early disks could not handle varying number of sectors / track reduce density of outer sectors Each zone (set of tracks) has variable number of sectors outer part can hold more data and support higher transfer rates File System Mapped onto physical disk structure want to match user’s conceptual model Collection of files and directories file is logical storage unit directories contain information about files (names, type, location, size, protection, etc.) Basic operations create, write, read, reposition, delete sequential and random access Allocation Methods Contiguous Linked Constrained Striping … and many others Continuous Occupy contiguous set of blocks Strengths minimizes seek time supports sequential and random access Weaknesses suffers external fragmentation Linked Stored as a linked list of blocks Strengths eliminates external fragmentation supports files of arbitrary length Weaknesses random access slow, overhead of pointers susceptible to block errors Constrained Linked structure, but allocate next block based on “distance” from previous one distance = predicted seek and latency Strengths improves sequential access minimizes seek time Weaknesses increases algorithm complexity Striping (RAID-0) Stripe file across an array of N disks divide file into stripes, dive stripe into units, assign each unit to different disk Strengths reduces disk access time by N Weaknesses susceptible to failure of any one disk p(failure) = N * p(any one disk failing) MM File System Requirements Storing/retrieving