I too thought there are multiple meta regions where as just one ROOT. May be I am mixing b/w Big Table and Hbase.
Thanks, Abhishek -----Original Message----- From: Lin Ma [mailto:[email protected]] Sent: Thursday, August 23, 2012 9:41 AM To: [email protected]; [email protected] Cc: [email protected] Subject: Re: how client location a region/tablet? Thanks, Harsh! - "HBase currently keeps a single META region (Doesn't split it). " -- does it mean there is only one row in ROOT table, which points the only one META region? - In Big Table, it seems they have multiple META regions (tablets), is it an advantage over HBase? :-) regards, Lin On Thu, Aug 23, 2012 at 11:48 PM, Harsh J <[email protected]> wrote: > HBase currently keeps a single META region (Doesn't split it). ROOT > holds META region location, and META has a few rows in it, a few of > them for each table. See also the class MetaScanner. > > On Thu, Aug 23, 2012 at 9:00 PM, Lin Ma <[email protected]> wrote: > > Dong, > > > > Some more thoughts, after reading data structure for HRegionInfo => > > http://hbase.apache.org/apidocs/org/apache/hadoop/hbase/HRegionInfo. > > html > , > > start key and end key looks informative which we could leverage, > > > > - I am not sure if we could leverage this information (stored as > > part of value in table ROOT) to find which META region may contains > > region server information for row-key 123 of data table ABC; > > - But I think unfortunately the information is stored in value of > > table ROOT, other than key field of table ROOT, so that we have to > > iterate each row in ROOT table one by one to figure out which META > > region server to access. > > > > Not sure if I get the points. Please feel free to correct me. > > > > regards, > > Lin > > > > On Thu, Aug 23, 2012 at 11:15 PM, Lin Ma <[email protected]> wrote: > > > >> Doug, very informative document. Thanks a lot! > >> > >> I read through it and have some thoughts, > >> > >> - Supposing at the beginning, client side cache for region > >> information > is > >> empty, and the client wants to GET row-key 123 from table ABC; > >> - The client will read from ROOT table at first. But unfortunately, > >> ROOT table only contains region information for META table (please > >> correct > me if > >> I am wrong), but not region information for real data table (e.g. > >> table ABC); > >> - Does the client have to call each META region server one by one, > >> in order to find which META region contains information for region > >> owner of row-key 123 of data table ABC? > >> > >> BTW: I think if there is a way to expose information about what > >> range of table/region each META region contains from .META. region > >> key, it will > be > >> better to save time to iterate META region server one by one. > >> Please > feel > >> free to correct me if I am wrong. > >> > >> regards, > >> Lin > >> > >> > >> On Thu, Aug 23, 2012 at 8:21 PM, Doug Meil < > [email protected]>wrote: > >> > >>> > >>> For further information about the catalog tables and > region-regionserver > >>> assignment, see thisŠ > >>> > >>> http://hbase.apache.org/book.html#arch.catalog > >>> > >>> > >>> > >>> > >>> > >>> > >>> On 8/19/12 7:36 AM, "Lin Ma" <[email protected]> wrote: > >>> > >>> >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 <[email protected]> wrote: > >>> > > >>> >> On Sat, Aug 18, 2012 at 2:13 AM, Lin Ma <[email protected]> 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 client¹s 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 > >>> >> > >>> > >>> > >>> > >> > > > > -- > Harsh J >
