i get it 。3x
发自我的 iPhone 在 2011-9-13，19:20，Ted Dunning <tdunn...@maprtech.com> 写道： > > > 2011/9/13 kang hua <kanghua...@msn.com> > Hi Master: > can you explain more detail --- "The only way to avoid this is to make > the data much more cacheable and to have a viable cache coherency strategy. > Cache coherency at the meta-data level is difficult. Cache coherency at the > block level is also difficult (but not as difficult) because many blocks get > moved for balance purposes" > why "Cache coherency at the meta-data level is difficult" ? > > I said this because meta-data is updated often. Caching in the presence of > high updates requires some sort of coherency model. For meta-data, it is > difficult to detect stale information on use and use of stale information can > be disastrous. Thus, caching is difficult. > > why "Cache coherency at the block level is also difficult (but not as > difficult) because many blocks get moved for balance purposes" > > The basic problem here is update rate. Late detection of stale information > is much easier however since you can just note that the block isn't where you > thought it was and update your cache. There are still problems and the fact > that race conditions are still being found in the HDFS lease management code > is an indicator that this isn't a completely trivial problem. >
