On Thu, 21 Jul 2011, Jojy Varghese wrote: > Hi > I just started looking at the ceph code in kernel and had a question > about performance considerations for lookup operations. I noticed that > for every operation (say copying a directory), the root dentry is > "looked" up multiple times and since they all go to MDS for the actual > lookup operation, it effects the performance. I am sure consistency is > the winner here. Is there any plan to improve this, maybe by having > MDS push the capability down to the clients when the dentry is > updated. So say from CAP_EXCL to CAP_SHARED when the dentry is > modified. This was the client node can cache the lookup operation and > does not have to make a round trip to the MDS. In general, the MDS has two ways of keeping a client's cached dentry consistent: - it can issue the FILE_SHARED capability bit on the parent directory, which means the entire directory is static and the client can cache dentry. - if it can't do that, it will issue a per-dentry lease There is an additional 'complete' bit that is used to indicate on the client that it has the _entire_ directory in cache. If set, it can do negative lookups and readdir without hitting the MDS. That's currently broken, pending the addition of a d_prune dentry_operation (see linux-fsdevel email from July 8). Anyway, long story short, if you're seeing repeated lookups on a dentry that isn't changing, something is broken. Can you describe the workload in more detail? Which versions of the client and mds are you running? sage -- To unsubscribe from this list: send the line "unsubscribe ceph-devel" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
