You have n different scans and you then have to put the rows in sort order from each scan in to a single result set.
While in each scan, the RS is in sort order, the overall set of RS needs to be merged in to one RS and that’s where you start to have issues. Again YMMV… And again… depending on what you want, in the general case, you need the RS in sort order. On May 19, 2014, at 1:24 PM, Mike Axiak <[email protected]> wrote: > 1) You can still query in sorted order, in which case N scans is > beneficial. (In our tests: ~25% faster for N=2, going up to about ~50% > faster for N=16.) > > 2) Many times you would issue a scan without necessarily caring about > individual record order. (e.g.: let me perform some operation on all > events in this hour), while still requiring the ordering can work in > the general case. > > Best, > Mike > > On Mon, May 19, 2014 at 3:24 AM, Michael Segel > <[email protected]> wrote: >>> This is even better when you don't necessary care about the >>> order of every row, but want every row in a given range (then you can >>> just get whatever row is available from a buffer in the client). >> >> You do realize that in the general case you want to return the result set in >> sort order. >> So you will have to put the resulting range scans in sort order. >> >> If you’re saying that you don’t care about the order of the row sets… then >> why are you using a sequential row key which causes hot spotting in the >> first place? >> >> >> On May 18, 2014, at 9:19 PM, Mike Axiak <[email protected]> wrote: >> >>> In our measurements, scanning is improved by performing against n >>> range scans rather than 1 (since you are effectively striping the >>> reads). This is even better when you don't necessary care about the >>> order of every row, but want every row in a given range (then you can >>> just get whatever row is available from a buffer in the client). >>> >>> -Mike >>> >>> On Sun, May 18, 2014 at 1:07 PM, Michael Segel >>> <[email protected]> wrote: >>>> No, you’re missing the point. >>>> Its not a good idea or design. >>>> >>>> Is your data mutable or static? >>>> >>>> To your point. Everytime you want to do a simple get() you have to open up >>>> n get() statements. On your range scans you will have to do n range scans, >>>> then join and sort the result sets. The fact that each result set is in >>>> sort order will help a little, but still not that clean. >>>> >>>> >>>> >>>> On May 18, 2014, at 4:58 PM, Software Dev <[email protected]> >>>> wrote: >>>> >>>>> You may be missing the point. The primary reason for the salt prefix >>>>> pattern is to avoid hotspotting when inserting time series data AND at >>>>> the same time provide a way to perform range scans. >>>>> http://blog.sematext.com/2012/04/09/hbasewd-avoid-regionserver-hotspotting-despite-writing-records-with-sequential-keys/ >>>>> >>>>>> NOTE: Many people worry about hot spotting when they really don’t have >>>>>> to do so. Hot spotting that occurs on a the initial load of a table is >>>>>> .OK. Its when you have a sequential row key that you run in to problems >>>>>> with hot spotting and regions being only half filled. >>>>> >>>>> The data being inserted will be a constant stream of time ordered data >>>>> so yes, hotspotting will be an issue >>>>> >>>>>> Adding a random value to give you a bit of randomness now means that you >>>>>> can’t do a range scan.. >>>>> >>>>> That's not accurate. To perform a range scan you would just need to >>>>> open up N scanners where N is the size of the buckets/random prefixes >>>>> used. >>>>> >>>>>> Don’t take the modulo, just truncate to the first byte. Taking the >>>>>> modulo is again a dumb idea, but not as dumb as using a salt. >>>>> >>>>> Well the only reason why I would think using a salt would be >>>>> beneficial is to limit the number of scanners when performing a range >>>>> scan. See above comment. And yes, performing a range scan will be our >>>>> primary read pattern. >>>>> >>>>> On Sun, May 18, 2014 at 2:36 AM, Michael Segel >>>>> <[email protected]> wrote: >>>>>> I think I should dust off my schema design talk… clearly the talks given >>>>>> by some of the vendors don’t really explain things … >>>>>> (Hmmm. Strata London?) >>>>>> >>>>>> See my reply below…. Note I used SHA-1. MD-5 should also give you >>>>>> roughly the same results. >>>>>> >>>>>> On May 18, 2014, at 4:28 AM, Software Dev <[email protected]> >>>>>> wrote: >>>>>> >>>>>>> I recently came across the pattern of adding a salting prefix to the >>>>>>> row keys to prevent hotspotting. Still trying to wrap my head around >>>>>>> it and I have a few questions. >>>>>>> >>>>>> >>>>>> If you add a salt, you’re prepending a random number to a row in order >>>>>> to avoid hot spotting. It amazes me that Sematext never went back and >>>>>> either removed the blog or fixed it and now the bad idea is getting >>>>>> propagated. Adding a random value to give you a bit of randomness now >>>>>> means that you can’t do a range scan, or fetch the specific row with a >>>>>> single get() so you’re going to end up boiling the ocean to get your >>>>>> data. You’re better off using hive/spark/shark than hbase. >>>>>> >>>>>> As James tries to point out, you take the hash of the row so that you >>>>>> can easily retrieve the value. But rather than prepend a 160 bit hash, >>>>>> you can easily achieve the same thing by just truncating the hash to the >>>>>> first byte in order to get enough randomness to avoid hot spotting. Of >>>>>> course, the one question you should ask is why don’t you just take the >>>>>> hash as the row key and then have a 160 bit row key (40 bytes in >>>>>> length)? Then store the actual key as a column in the table. >>>>>> >>>>>> And then there’s a bigger question… why are you worried about hot >>>>>> spotting? Are you adding rows where the row key is sequential? Or are >>>>>> you worried about when you first start loading rows, that you are hot >>>>>> spotting, but the underlying row key is random enough that once the >>>>>> first set of rows are added, HBase splitting regions will be enough? >>>>>> >>>>>>> - Is there ever a reason to salt to more buckets than there are region >>>>>>> servers? The only reason why I think that may be beneficial is to >>>>>>> anticipate future growth??? >>>>>>> >>>>>> Doesn’t matter. >>>>>> Think about how HBase splits regions. >>>>>> Don’t take the modulo, just truncate to the first byte. Taking the >>>>>> modulo is again a dumb idea, but not as dumb as using a salt. >>>>>> >>>>>> Keep in mind that the first byte of the hash is going to be 0-f in a >>>>>> character representation. (4 bits of the 160bit key) So you have 16 >>>>>> values to start with. >>>>>> That should be enough. >>>>>> >>>>>>> - Is it beneficial to always hash against a known number of buckets >>>>>>> (ie never change the size) that way for any individual row key you can >>>>>>> always determine the prefix? >>>>>>> >>>>>> Your question doesn’t make sense. >>>>>> >>>>>>> - Are there any good use cases of this pattern out in the wild? >>>>>>> >>>>>> Yup. >>>>>> Deduping data sets. >>>>>> >>>>>>> Thanks >>>>>>> >>>>>> NOTE: Many people worry about hot spotting when they really don’t have >>>>>> to do so. Hot spotting that occurs on a the initial load of a table is >>>>>> OK. Its when you have a sequential row key that you run in to problems >>>>>> with hot spotting and regions being only half filled. >>>>>> >>>>> >>>> >>> >> >
