Question

In the log structured file-system, is the checkpoint region marked as the most recent before or after the corresponding log is written to disk? Explain why the order matters for file-system consistency.

Answer 1

In practice, a log-structured file system writes checkpoints to disk periodically; these checkpoints describe the state of the file system at that point in time without requiring any log replay. Any changes to the file system after the checkpoint are recovered by replaying the relatively small number of log entries following the checkpoint.

A journaling file system keeps the on-disk state of the file system consistent by writing a summary of each write operation to the log, stored somewhere non-volatile like disk (or NVRAM if you have the money), before writing the changes directly to their long-term place in the file system. This summary, or log record, contains enough information to repeat the entire operation if the direct write to the file system gets interrupted mid-way through (e.g., by a system crash). This operation is called replaying the log. So, in short, every change to the file system gets written to disk twice: once to the log, and once in the permanent location.

The log-structured file system also improved write performance on spinning magnetic disks. Since changes were always appended to the end of the log, a sequence of file system changes could be buffered and written in a single disk write operation, reducing the need for time-consuming seek operations. The improvement in write performance was noticeably pronounced when writing to small files.

Of course, the log-structured approach came with trade-offs, such as additional complexity to index the log for acceptable subsequent read performance and the need to implement automatic garbage collection to reclaim unused storage space and reduce fragmentation.

Although the log-structured approach, as originally conceived, is not currently used in any production file system today, its core concept of using carefully designed data structures as a way to prevent inconsistencies and avoid the write-twice penalties of journaling was highly influential. Its influence can be seen in the “soft update” and “copy-on-write” approaches that followed.