HTable.getRegionLocations() I didn't realize the KeyValue serializations/deserialization happened on a separate thread in the hbase client code.
J On Sun, Aug 12, 2012 at 3:52 PM, Gurjeet Singh <[email protected]> wrote: > Hi Mohammad, > > This is a great idea. Is there a API call to determine the start/end > key for each region ? > > Thanks, > Gurjeet > > On Sun, Aug 12, 2012 at 3:49 PM, Mohammad Tariq <[email protected]> > wrote: > > Hello experts, > > > > Would it be feasible to create a separate thread for each > region??I > > mean we can determine start and end key of each region and issue a scan > for > > each region in parallel. > > > > Regards, > > Mohammad Tariq > > > > > > > > On Mon, Aug 13, 2012 at 3:54 AM, lars hofhansl <[email protected]> > wrote: > > > >> Do you really have to retrieve all 200.000 each time? > >> Scan.setBatch(...) makes no difference?! (note that batching is > different > >> and separate from caching). > >> > >> Also note that the scanner contract is to return sorted KVs, so a single > >> scan cannot be parallelized across RegionServers (well not entirely > true, > >> it could be farmed off in parallel and then be presented to the client > in > >> the right order - but HBase is not doing that). That is why one vs 12 > RSs > >> makes no difference in this scenario. > >> > >> In the 12 node case you'll see low CPU on all but one RS, and each RS > will > >> get its turn. > >> > >> In your case this is scanning 20.000.000 KVs serially in 400s, that's > >> 50000 KVs/s, which - depending on hardware - is not too bad for HBase > (but > >> not great either). > >> > >> If you only ever expect to run a single query like this on top your > >> cluster (i.e. your concern is latency not throughput) you can do > multiple > >> RPCs in parallel for a sub portion of your key range. Together with > >> batching can start using value before all is streamed back from the > server. > >> > >> > >> -- Lars > >> > >> > >> > >> ----- Original Message ----- > >> From: Gurjeet Singh <[email protected]> > >> To: [email protected] > >> Cc: > >> Sent: Saturday, August 11, 2012 11:04 PM > >> Subject: Slow full-table scans > >> > >> Hi, > >> > >> I am trying to read all the data out of an HBase table using a scan > >> and it is extremely slow. > >> > >> Here are some characteristics of the data: > >> > >> 1. The total table size is tiny (~200MB) > >> 2. The table has ~100 rows and ~200,000 columns in a SINGLE family. > >> Thus the size of each cell is ~10bytes and the size of each row is > >> ~2MB > >> 3. Currently scanning the whole table takes ~400s (both in a > >> distributed setting with 12 nodes or so and on a single node), thus > >> 5sec/row > >> 4. The row keys are unique 8 byte crypto hashes of sequential numbers > >> 5. The scanner is set to fetch a FULL row at a time (scan.setBatch) > >> and is set to fetch 100MB of data at a time (scan.setCaching) > >> 6. Changing the caching size seems to have no effect on the total scan > >> time at all > >> 7. The column family is setup to keep a single version of the cells, > >> no compression, and no block cache. > >> > >> Am I missing something ? Is there a way to optimize this ? > >> > >> I guess a general question I have is whether HBase is good datastore > >> for storing many medium sized (~50GB), dense datasets with lots of > >> columns when a lot of the queries require full table scans ? > >> > >> Thanks! > >> Gurjeet > >> > >> >
