Supporting transactions in btrfs

Posted Nov 20, 2009 9:23 UTC (Fri) by forthy (guest, #1525) [Link]

The interface reminds me on one of the proposed syslet asynchronous IO API, where you send a list of syscalls to the kernel (to be processed asynchronously in the background). I thought this was a cool idea (not new, active message passing is decades old, but not well understood by most developers ;-), and it was a pity that it didn't make it (due to being not well understood). For transactions, I don't think it's the right way.

Chris Mason works at Oracle. He should ask some peers how to implement transactions properly. Locking is not a proper way to implement transactions. What you do is: you fork the file system (i.e. create a writable snapshot and bind the calling process to that snapshot), an when you are done, you try to merge it. If the merge is successful, continue, if not, abort (and tell the caller, which can try again). If the system crashes during the transaction, file system repair will purge that snapshot. Unlike a git merge, a transaction should abort when a file that was written to during the transaction had other writers outside. The atomic part here is only the merge window, and this merge has complete information available, and especially only has to update metadata.

This doesn't need a special syslet-like ioctrl, the only thing you need to add is the btrfs_merge() call - creating writable snapshots is already there. I'd still like to have syslets - but please with the complete set of kernel calls and for asynchronous IO and such.