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It's worth taking a moment to describe how mmap is implemented for zfs because it differs considerably from other Linux filesystems. However, this issue is handled the same way under OpenSolaris. The issue is that by design zfs bypasses the Linux page cache and leaves all caching up to the ARC. This has been shown to work well for the common read(2)/write(2) case. However, mmap(2) is problem because it relies on being tightly integrated with the page cache. To handle this we cache mmap'ed files twice, once in the ARC and a second time in the page cache. The code is careful to keep both copies synchronized. When a file with an mmap'ed region is written to using write(2) both the data in the ARC and existing pages in the page cache are updated. For a read(2) data will be read first from the page cache then the ARC if needed. Neither a write(2) or read(2) will will ever result in new pages being added to the page cache. New pages are added to the page cache only via .readpage() which is called when the vfs needs to read a page off disk to back the virtual memory region. These pages may be modified without notifying the ARC and will be written out periodically via .writepage(). This will occur due to either a sync or the usual page aging behavior. Note because a read(2) of a mmap'ed file will always check the page cache first even when the ARC is out of date correct data will still be returned. While this implementation ensures correct behavior it does have have some drawbacks. The most obvious of which is that it increases the required memory footprint when access mmap'ed files. It also adds additional complexity to the code keeping both caches synchronized. Longer term it may be possible to cleanly resolve this wart by mapping page cache pages directly on to the ARC buffers. The Linux address space operations are flexible enough to allow selection of which pages back a particular index. The trick would be working out the details of which subsystem is in charge, the ARC, the page cache, or both. It may also prove helpful to move the ARC buffers to a scatter-gather lists rather than a vmalloc'ed region. Additionally, zfs_write/read_common() were used in the readpage and writepage hooks because it was fairly easy. However, it would be better to update zfs_fillpage and zfs_putapage to be Linux friendly and use them instead. |
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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