Clarification on transactionality

[square]

Clarification on transactionality

[f2prateek]

fsync (or opening it in rwd mode) doesn't guarantee atomicity, it only guarantees that the data has been written to the file when it returns.

Consider the case when you write a 1MB of data into a file and call fsync, the file system will typically break it down into chunks and writes it to the file one chunk at a time. If an exceptional failure (such as a power failure, process death or system crashes) happens during this, some chunks may have not been written yet.

However, if the file system guarantees atomic segment writes, it assures that as long as you write data smaller than the size of a segment, it's all or nothing operation, even during exceptional failures.

By designing the header to be smaller than the size of a segment, QueueFile can ensure that writing the header (which is much smaller than the size of a segment) is atomic. Even during exceptional failures, the mutating change will simply be aborted since the header isn't changed at all. If the file system doesn't guarantee atomic segment writes, then the header may be written only partially and leave the QueueFile in an invalid state.

[epsstan]

Makes sense. Thanks for clarifying.