lwn.net has an overview of the history of the btrfs filesystem (wikipedia link). He also touches on the difference and similarities between btrf and ZFS, Sun’s Zetabyte File System, something that I was most intersted in:
“People often ask about the relationship between btrfs and ZFS. From one point of view, the two file systems are very similar: they are copy-on-write checksummed file systems with multi-device support and writable snapshots. From other points of view, they are wildly different: file system architecture, development model, maturity, license, and host operating system, among other things. Rather than answer individual questions, I’ll give a short history of ZFS development and compare and contrast btrfs and ZFS on a few key items.
When ZFS first got started, the outlook for file systems in Solaris was rather dim as well. Logging UFS was already nearing the end of its rope in terms of file system size and performance. UFS was so far behind that many Solaris customers paid substantial sums of money to Veritas to run VxFS instead. Solaris needed a new file system, and it needed it soon.
Jeff Bonwick decided to solve the problem and started the ZFS project inside Sun. His organizing metaphor was that of the virtual memory subsystem – why can’t disk be as easy to administer and use as memory? The central on-disk data structure was the slab – a chunk of disk divided up into the same size blocks, like that in the SLAB kernel memory allocator, which he also created. Instead of extents, ZFS would use one block pointer per block, but each object would use a different block size – e.g., 512 bytes, or 128KB – depending on the size of the object. Block addresses would be translated through a virtual-memory-like mechanism, so that blocks could be relocated without the knowledge of upper layers. All file system data and metadata would be kept in objects. And all changes to the file system would be described in terms of changes to objects, which would be written in a copy-on-write fashion.
In summary, btrfs organizes everything on disk into a btree of extents containing items and data. ZFS organizes everything on disk into a tree of block pointers, with different block sizes depending on the object size. btrfs checksums and reference-counts extents, ZFS checksums and reference-counts variable-sized blocks. Both file systems write out changes to disk using copy-on-write – extents or blocks in use are never overwritten in place, they are always copied somewhere else first.
So, while the feature list of the two file systems looks quite similar, the implementations are completely different. It’s a bit like convergent evolution between marsupials and placental mammals – a marsupial mouse and a placental mouse look nearly identical on the outside, but their internal implementations are quite a bit different!
In my opinion, the basic architecture of btrfs is more suitable to storage than that of ZFS. One of the major problems with the ZFS approach – “slabs” of blocks of a particular size – is fragmentation. Each object can contain blocks of only one size, and each slab can only contain blocks of one size. You can easily end up with, for example, a file of 64K blocks that needs to grow one more block, but no 64K blocks are available, even if the file system is full off nearly empty slabs of 512 byte blocks, 4K blocks, 128K blocks, etc. To solve this problem, we (the ZFS developers) invented ways to create big blocks out of little blocks (“gang blocks”) and other unpleasant workarounds. In our defense, at the time btrees and extents seemed fundamentally incompatible with copy-on-write, and the virtual memory metaphor served us well in many other respects.
In contrast, the items-in-a-btree approach is extremely space efficient and flexible. Defragmentation is an ongoing process – repacking the items efficiently is part of the normal code path preparing extents to be written to disk. Doing checksums, reference counting, and other assorted metadata busy-work on a per-extent basis reduces overhead and makes new features (such as fast reverse mapping from an extent to everything that references it) possible.
Now for some personal predictions (based purely on public information – I don’t have any insider knowledge). Btrfs will be the default file system on Linux within two years. Btrfs as a project won’t (and can’t, at this point) be canceled by Oracle. If all the intellectual property issues are worked out (a big if), ZFS will be ported to Linux, but it will have less than a few percent of the installed base of btrfs. Check back in two years and see if I got any of these predictions right!”
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As governments around the globe feel the economic pinch, many are realizing the benefits of storage virtualisation – risk avoidance, productivity increases, cost containment, investment protection, and more.
