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Under OpenSolaris all memory reclaim is done asyncronously. Under Linux memory reclaim is done asynchronously _and_ synchronously. When a process allocates memory with GFP_KERNEL it explicitly allows the kernel to do reclaim on its behalf to satify the allocation. If that GFP_KERNEL allocation fails the kernel may take more drastic measures to reclaim the memory such as killing user space processes. This was observed to happen with ZFS because the ARC could consume a large fraction of the system memory but no synchronous reclaim could be performed on it. The result was GFP_KERNEL allocations could fail resulting in OOM events, and only moments latter the arc_reclaim thread would free unused memory from the ARC. This change leaves the arc_thread in place to manage the fundamental ARC behavior. But it adds a synchronous (direct) reclaim path for the ARC which can be called when memory is badly needed. It also adds an asynchronous (indirect) reclaim path which is called much more frequently to prune the ARC slab caches.
Native ZFS for Linux! ZFS is an advanced file system and volume manager which was originally developed for Solaris. It has been successfully ported to FreeBSD and now there is a functional Linux ZFS kernel port too. The port currently includes a fully functional and stable SPA, DMU, and ZVOL with a ZFS Posix Layer (ZPL) on the way!
$ ./configure
$ make pkg
Full documentation for building, configuring, and using ZFS can be found at: http://zfsonlinux.org