如何在文件系统中执行条件 IO？

Question

I'm trying to implement a multi-user key-value store over the file system, such as the local Linux or Windows file system, or a network-based one (SMB or NFS). My intent is to fully avoid the need of a server because servers require some VM, deployment, upgrades, etc. And filesystems are typically readily available.

The engine returns the timestamp of when the value was set. One operation that uses the timestamp is "put if not modified since", which is similar to compare-and-swap and supports synchronization among processes. It turns out that this is quite costly to implement without a server.

It seems that no file system supports "write if not modified" or any form of conditional write semantics. At best I can lock a file, but then I need to read the date and compare inside the process, and only then write the new content and release the lock. The minimum number of IOs to implement is four: 1) lock entire file; 2) read modification date and compare locally; 3) write the new content; 4) unlock. And this ignores the IOs to open and close the file, which are pooled so they will be less frequent.

Is there any OS or filesystem facility, or algorithm that could reduce the number of IOs? Please remember that I need the solution to work over NFS or SMB...

Thanks

Answer 1

Filesystems already do read-ahead and write avoidance, so I/O calls will only block for disk when read data is not in cache or write cache is full and a flush is required. The performance problem with the "write if not modified since" is the 4 syscalls, which can get expensive. One way to fix this would be to add a conditional write kernel module. You would pass it the timestamp, file name, and data. It would do the conditional write using internal calls and callbacks, and return the status and new timestamp, reducing the overhead to a single syscall. Properly written, it should be filesystem-agnostic.