What do you mean by log-structred file system? What are the disadvantages of log structred file system?

A log-structured filesystem is a file system in which data and metadata are written sequentially to a circular buffer, called a log.[1] The design was first proposed in 1988 by John K. Ousterhout and Fred Douglis and first implemented in 1992 by John K. Ousterhout and Mendel Rosenblum.

Motivations of Log structured File system
The size of memory is increasing day after day. Although the size of memory is increasing day after day the seek time is very slow.  More is the memory high is cache allocation. When there are more cache then the number of read operations decreases automatically. As a result most of the overheads are on the write operations. To write to the disk block, seek time increases as there are more number of writes. To decrease this seek time, one of the methods is to maintain the log structured file system where a log is maintained to keep record of current data block and its corresponding inode numbers.

The design rationale for log-structured file systems assumes that most reads will be optimized away by ever-enlarging memory caches. This assumption does not always hold:
    •    On magnetic media—where seeks are relatively expensive—the log structure may actually make reads much slower, since it fragments files that conventional file systems normally keep contiguous with in-place writes.
    •    On flash memory—where seek times are usually negligible—the log structure may not confer a worthwhile performance gain because write fragmentation has much less of an impact on write throughput. However many flash based devices cannot rewrite part of a block, and they must first perform a (slow) erase cycle of each block before being able to re-write, so by putting all the writes in one block, this can help performance as opposed to writes scattered into various blocks, each one of which must be copied into a buffer, erased, and written back.


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