Limitations limitations of existing file systems they

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Unformatted text preview: al seeks needed to create a file: what are they? Limitations Limitations of existing file systems They spread information around the disk • data blocks of a single large file may be together, but … • inodes stored apart from data blocks • writing small files -> less than 5% of disk bandwidth is used to access new data, rest of time is seeking Use synchronous writes to update directories and inodes • required for consistency Key Key Idea Write all modifications to disk sequentially in a log-like structure structure • Convert many small random writes into large sequential transfers transfers • Use file cache as write buffer first, then write to disk sequentially Main Main advantages Replaces many small random writes by fewer sequential writes Faster recovery after a crash • all blocks that were recently written are at the tail end of log The The Log Only structure on disk inode-map (maps inode # to its disk position) located in wellknown place on the disk Log contains modified inodes, data blocks, and directory entries Most reads will access data that are already in the cache If If not, it can get expensive to go through the log if files are fragmented Disk Disk layouts of LFS and UNIX dir 1 dir 2 Log file 1 LFS file 2 file 2 file 1 dir 1 Inode Disk dir 2 Directory Data Disk Unix FFS Inode map Segments Segments Must maintain large free disk-areas for writing new data Disk Disk is divided into large fixed-size areas called segments (512 kB in Sprite LFS) Segments are always written sequentially from one end to the other Includes summary information – what is in the segment A live block is in active use – can check inode Old segments must be cleaned before the...
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This document was uploaded on 04/02/2014.

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