Which version of HBase? Are there concurrent inserts? If so, do you see splits in the log files happening while you do the scanning?
I am pretty sure this has nothing to do with concurrent scans. ________________________________ From: Bryan Keller <[email protected]> To: Bryan Keller <[email protected]> Cc: [email protected] Sent: Sunday, October 9, 2011 11:03 AM Subject: Re: Using Scans in parallel On further thought, it seems this might be a serious issue, as two unrelated processes within an application may be scanning the same table at the same time. On Oct 9, 2011, at 10:59 AM, Bryan Keller wrote: > I was not able to get consistent results using multiple scanners in parallel > on a table. I implemented a counter test that used 8 scanners in parallel on > a table with 2m rows with 2k+ columns each, and the results were not > consistent. There were no errors thrown, but the count was off by as much as > 2%. Using a single thread gave the same (correct) result every run. > > I tried various approaches, such as creating an HTable and opening a > connection per thread, but I was not able to get stable results. I would do > some testing before using parallel scanners as described here. > > > On Oct 5, 2011, at 10:11 PM, lars hofhansl wrote: > >> That's part of it, the other part is to get the region demarcations. >> You can also just get the smallest and largest key of the table and pick >> other demarcations for your scans. Then your individual scans will likely >> cover multiple regions and regionservers. >> >> >> Your threading model depends on your needs. If you interested in lowest >> latency you want to keep your regionservers busy for each query. >> What exactly that means depends on your setup. Maybe you split up the >> overall scan so that no more than N scans are active at any regionserver. >> >> If you're more interested in overall predictability, you might not want >> parallelize each scan too much. >> >> >> >> ----- Original Message ----- >> From: Sam Seigal <[email protected]> >> To: [email protected]; lars hofhansl <[email protected]> >> Cc: "[email protected]" <[email protected]> >> Sent: Wednesday, October 5, 2011 6:18 PM >> Subject: Re: Using Scans in parallel >> >> So the whole point of getting the region locations is to ensure that >> there is one thread per region server ? >> >> >> On Wed, Oct 5, 2011 at 4:42 PM, lars hofhansl <[email protected]> wrote: >>> Hi Sam, >>> >>> >>> There were some attempts to build this in. In the end I think the exact >>> patterns are different based on what one is trying to achieve. >>> Currently what you can do is getting all the region locations >>> (HTable.getRegionLocations). From the HRegionInfos you can >>> get the regions start and end keys. >>> Now you can issue parallel scan for as many regions as you want (by create >>> a Scan object with start and row set to the region's >>> start and end key). >>> You probably want to group the regions by regionserver and have one thread >>> per region server, or something. >>> >>> >>> -- Lars >>> ________________________________ >>> From: Sam Seigal <[email protected]> >>> To: [email protected] >>> Sent: Wednesday, October 5, 2011 4:29 PM >>> Subject: Using Scans in parallel >>> >>> Hi , >>> >>> Is there a known way to be able to do Scan's in parallel (in different >>> threads even) and then sort/combine the output ? >>> >>> For a row key like: >>> >>> prefix-event_type-event_id >>> prefix-event_type-event_id >>> >>> I want to declare two scan objects (for say event_id_type foo) >>> >>> Scan 1 => 0-foo >>> Scan 2 => 1-foo >>> >>> execute the scans in parallel (maybe even in different threads) and >>> then merge the results ? >>> >>> Thank you, >>> >>> Sam >>> >> >
