So the Ceph MDS system is a little odd. Presumably you are aware that file data is stored as chunks (default 4MB) in objects on RADOS (ie, the OSD system). Metadata is also stored on RADOS, where each directory is a single object that contains all the inodes underneath it. In principle you could construct a filesystem that simply accessed this on-disk metadata every time it changed everything. However, that would be slow for a number of reasons. To speed up metadata ops, we have the MetaData Server. Essentially, it does 3 things: 1) Cache the metadata in-memory to speed up read accesses. 2) Handle client locking of data/metadata accesses (this is the capabilities system) 3) Journal metadata write operations so that we can get streaming write latencies instead of random lookup-and-write latencies on changes. When you have multiple active MDSes, they partition the namespace so that any given directory has a single "authoritative" MDS which is responsible for handling requests. This simplifies locking and prevents the MDSes from duplicating inodes in-cache. You can add MDSes by increasing the max_mds number. (Most of the machinery is there to reduce the number of MDSes too, but it's not all tied together.) So you can see that if one daemon dies you're going to lose access to the metadata it's responsible for. :( HOWEVER, all the data it's "responsible" for resides in RADOS so you haven't actually lost any state or the ability to access it, just the authorization to do so. So we introduce standby and standby-replay MDSes. If you have more MDS daemons than max_mds, the first max_mds daemons will start up and become active and the rest will sit around as standbys. The monitor service knows they exist and are available, but they don't do anything until an active MDS dies. If an active MDS does die, the monitor assigns one of the standbys to take over for that MDS -- it becomes the same logical MDS but on a different host (or not, if you're running multiple daemons on the same host or whatever). It goes through a set of replay stages and then operates normally. If this makes you shudder in horror from previous experiences with the HDFS standby or something, fear not. :) We haven't done comprehensive tests but replay is generally pretty short -- our default timeout on an MDS is in the region of 30 seconds and I don't think I've ever seen replay take more than 45. If you want to reduce the time it takes even further you can assign certain daemons to be in standby-replay mode. In this mode they actively replay the journal of an active MDS and maintain the same cache, so if the active MDS fails they only need to go through the client reconnect stage. Also remember that during this time access to metadata which resides on other non-failed MDSes goes on uninterrupted. :) In principle we could do some whiz-bang coding so that if an MDS fails another active MDS takes over responsibility for its data and then repartitions it out to the rest of the cluster, but that's not something we're likely to do for a while given the complexity and the relatively small advantage over using standby daemons. So going through your questions specifically: 2011/6/9 Székelyi Szabolcs <szekelyi@xxxxxxx>: > If I understand things correctly, Ceph tries to have max_mds number of MDSes > active at all times. I can have more MDSes than this number, but the excess > ones will be standby MDSes, right? Right. > I can't really understand the difference between a standby and an active MDS. > Now I have two active and no standby MDSes, and the filesystem stops working if > I kill any of them. Does this mean that the system will stop working if it > can't fill up the number of MDSes to max_mds from the standby pool? Hopefully the above answered this for you a little more precisely, but the short answer is yes. > What is the reason for running standby MDSes and not setting max_mds to the > number of all MDSes? Again, hopefully you got this above. But it increases system resiliency in the case of a failure. >> 2) Create an extra MDS daemon, perhaps on your monitor node. When the >> system detects that one of your MDSes has died (a configurable >> timeout, IIRC in the neighborhood of 30-60 seconds), this extra daemon >> will take over. > > I will do this. Should this be a standby or an active MDS? Ie. should I > increase max_mds from 2 to 3 after creating the new MDS? Standby -- don't increase max_mds. -Greg -- 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
