Hello everyone, Yan Zheng has been doing some major surgery to the back references and extent allocation code, tackling bottlenecks in the code that tracks extents. It scales better with many snapshots and performs better in the common case of no snapshots at all.
THE NEW CODE IS A FORWARD ROLLING DISK FORMAT CHANGE. This means it is compatible with the current btrfs disk format, but once you mount a filesystem with the new code, it WILL NO LONGER BE MOUNTABLE FROM OLD KERNELS. Old kernels spit out an error message when you try them on new format filesystems. This is a large change, and I'm hoping to have it stable in time for the 2.6.31 merge window. I've been testing it for about a week now, and haven't been able to cause major problems yet. But, testing the compatibility with old format filesystems is the hard part, and everyone that pulls the new code should backup their data first. I've setup git branches called newformat where you can pull the new code. For the kernel (based on 2.6.30-rc7): git pull git://git.kernel.org/pub/scm/linux/kernel/git/mason/btrfs-unstable.git newformat For the progs: git pull git://git.kernel.org/pub/scm/linux/kernel/git/mason/btrfs-progs-unstable.git newformat The main benefit of the new code is that backrefs on the extent allocation tree use a fuzzier format. It basically means that we search for the key in the extent allocation tree instead of providing an exact backref to the parent block. This means we can predict how many blocks will be changed when changing the extent allocation tree, and it makes enospc much less complex. It is also significantly faster. For regular subvolume trees, a similar change is made as long as there are no snapshots against a given block. This is the common case, and it makes COW less expensive overall. Yan Zheng also worked out a way to free blocks during the transaction without needing to do an explicit snapshot deletion on the old root when the transaction was done. This gets rid of some complex caching code, and fixes worst-case problems where btrfs could take a very very long time to unmount. btrfs-vol -b is faster with the new code as well, he added caching of high levels in the tree to speed things up. (Many kudos to Yan Zheng for all of this work!) -chris -- To unsubscribe from this list: send the line "unsubscribe linux-btrfs" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html
