http://lwn.net/Articles/276025/UBIFSA number of approaches exist for making flash-based devices work well. Many devices, such as USB drives, include a software "flash translation layer" (FTL); this layer performs the necessary impedance matching to make a flash device look like an ordinary block device with small sectors. Internally, the FTL maintains a mapping between logical blocks and physical erase blocks which allows it to perform wear leveling - distributing rewrite operations across the device so that no specific erase block wears out before its time - though some observers question whether low-end flash devices bother to do that. The use of FTL layers makes life easy for the rest of the system, but it is not necessarily the way to get the best performance out of the hardware. If you can get to the device directly, without an FTL getting in the way, it is possible to create filesystems which embody an awareness of how flash works. Most of our contemporary filesystems are designed around rotating storage, with the result that they work hard to minimize time-consuming operations like head seeks. A flash-based filesystem need not worry about such issues, but it must be concerned about things like erase blocks instead. So making the best use of flash requires a filesystem written with flash in mind. The main filesystem for flash-based devices on Linux is the venerable JFFS2. This filesystem works, but it was designed for devices which are rather smaller than those available today. Since JFFS2 must do things like rebuild the entire directory tree at mount time, it can be quite slow on large devices - for relatively small values of "large" by 2008 standards. JFFS2 is widely seen as reaching the end of its time. A more contemporary alternative is LogFS, which has been discussed on these pages in the past. This work remains unfinished, though, and development has been relatively slow in recent times; LogFS has not yet been seriously considered for merging into the mainline. A more recent contender is UBIFS; this code is in a state of relative completion and its developers are asking for serious review. UBIFS depends on the UBI layer, which was merged for 2.6.22. UBI ("unsorted block images") is not, technically, an FTL, but it performs a number of the same functions. At the heart of UBI is a translation table which maps logical erase blocks (LEBs) onto physical erase blocks (PEBs). So software using UBI to access flash sees a device providing a simple set of sequential blocks which apparently do not move. In fact, when an LEB is rewritten, the new data will be placed into a different location on the physical device, but the upper layers know nothing about it. So UBI makes problems like wear leveling and bad block avoidance go away for the upper layers. UBI also takes care of running time-consuming erase operations in the background when possible so that upper layers need not wait when writing a block. One little problem with UBI is that the logical-to-physical mapping information is stored in the header of each erase block. So when the UBI layer initializes a flash device, it must read the header from every block to build the mapping table in memory; this operation clearly takes time. For 1GB flash devices, this initialization overhead is tolerable; in the future, when we'll be booting our laptops with terabyte-sized flash drives in them, the linear scan will be a problem. The UBIFS developers are aware of this issue, but believe that it can be solved at the UBI level without affecting the higher-level filesystem code. By using UBI, the UBIFS developers are able to stop worrying about some aspects of flash-based filesystem design. Other problems remain, though. For example, the large erase blocks provided by flash devices require filesystems to track data at the sub-block level and to perform occasional garbage collection: coalescing useful information into new blocks so that the remaining "dead" space can be reclaimed. Garbage collection, along with the potential for blocks to turn bad, makes space management on flash devices tricky: freeing space may require using more space first, and there is no way to know how much space will actually become available until the work has been done. In the case of UBIFS, space management is an even trickier problem for a couple of reasons. One is that, like a number of other flash filesystems, UBIFS performs transparent compression of the data. The other is that, unlike JFFS2, UBIFS provides full writeback support, allowing data to be cached in memory for some time before being written to the physical media. Writeback gives large performance improvements and reduces wear on the device, but it can lead to big trouble if the filesystem commits to writing back more data than it actually has the space to store. To deal with this problem, UBIFS includes a complex "budgeting" layer which manages outstanding writes with pessimistic assumptions on what will be possible. Like LogFS, UBIFS uses a "wandering tree" structure to percolate changes up through the filesystem in an atomic manner. UBIFS also uses a journal, though, to minimize the number of rewrites to the upper-level nodes in the tree. The latest UBIFS posting raised questions about how it compares with LogFS. The resulting discussion was ... not entirely technical, but a few clear points came out. UBIFS is in a more complete state and appears to perform quite a bit better at this time. LogFS is a lot less code, avoids the boot-time linear scan of the device, and is able to work (with some flash awareness) through an FTL. Which is better is not a question your editor is prepared to answer at this time; what does seem clear is that the growing competition between the two projects has the potential to inspire big improvements on both sides in the near future. |
- [linuxkernelnewbies] UBIFS [LWN.net] Peter Teoh
- [linuxkernelnewbies] UBIFS [LWN.net] Peter Teoh
