Thank you Stack, especially for the smart 6 round trip guess for the puzzle. :-)
1. "Yeah, we client cache's locations, not the data." -- does it mean for each client, it will cache all location information of a HBase cluster, i.e. which physical server owns which region? Supposing each region has 128M bytes, for a big cluster (P-bytes level), total data size / 128M is not a trivial number, not sure if any overhead to client? 2. A bit confused by what do you mean "not the data"? For the client cached location information, it should be the data in table METADATA, which is region / physical server mapping data. Why you say not data (do you mean real content in each region)? regards, Lin On Sun, Aug 19, 2012 at 12:40 PM, Stack <st...@duboce.net> wrote: > On Sat, Aug 18, 2012 at 2:13 AM, Lin Ma <lin...@gmail.com> wrote: > > Hello guys, > > > > I am referencing the Big Table paper about how a client locates a tablet. > > In section 5.1 Tablet location, it is mentioned that client will cache > all > > tablet locations, I think it means client will cache root tablet in > > METADATA table, and all other tablets in METADATA table (which means > client > > cache the whole METADATA table?). My question is, whether HBase > implements > > in the same or similar way? My concern or confusion is, supposing each > > tablet or region file is 128M bytes, it will be very huge space (i.e. > > memory footprint) for each client to cache all tablets or region files of > > METADATA table. Is it doable or feasible in real HBase clusters? Thanks. > > > > Yeah, we client cache's locations, not the data. > > > > BTW: another confusion from me is in the paper of Big Table section 5.1 > > Tablet location, it is mentioned that "If the client’s cache is stale, > the > > location algorithm could take up to six round-trips, because stale cache > > entries are only discovered upon misses (assuming that METADATA tablets > do > > not move very frequently).", I do not know how the 6 times round trip > time > > is calculated, if anyone could answer this puzzle, it will be great. :-) > > > > I'm not sure what the 6 is about either. Here is a guesstimate: > > 1. Go to cached location for a server for a particular user region, > but server says that it does not have a region, the client location is > stale > 2. Go back to client cached meta region that holds user region w/ row > we want, but its location is stale. > 3. Go to root location, to find new location of meta, but the root > location has moved.... what the client has is stale > 4. Find new root location and do lookup of meta region location > 5. Go to meta region location to find new user region > 6. Go to server w/ user region > > St.Ack >
