I'm seeing a very strange behavior: If I run a scan during major compaction, I can see both the modified Delta Key Value (which contains the aggregated values - e.g. 9) and the other two delta columns that were used for this aggregated column (e.g, 3, 3) - as if Scan is exposed to the key values produced in mid scan. Could it be related to Cache somehow?
I am modifying the KeyValue object received from the InternalScanner in preCompact (modifying its value). On Feb 12, 2013, at 11:22 AM, Anoop Sam John wrote: >> The question is: is it "legal" to change a KV I received from the >> InternalScanner before adding it the Result - i..e returning it from my own >> InternalScanner? > > You can change as per your need IMO > > -Anoop- > > ________________________________________ > From: Mesika, Asaf [[email protected]] > Sent: Tuesday, February 12, 2013 2:43 PM > To: [email protected] > Subject: Re: Custom preCompact RegionObserver crashes entire cluster on OOME: > Heap Space > > I am trying to reduce the amount of KeyValue generated during the preCompact, > but I'm getting some weird behaviors. > > Let me describe what I am doing in short: > > We have a counters table, with the following structure: > > RowKey = A combination of field values representing group by key. > CF = time span aggregate (Hour, Day, Month). Currently we have only for Hour. > CQ = Round-to-Hour timestamp (long). > Value = The count > > We collect raw data, and updates the counters table for the matched group by > key, hour. > We tried using Increment, but discovered its very very slow. > Instead we've decided to update the counters upon compaction. We write the > deltas into the same row-key, but a longer column qualifier: > <RoundedToTheHourTS><Type><UniqueId>. > <Type> is: Delta or Aggregate. > Delta stands for a delta column qualifier we send from our client. > > in the preCompact, I create an InternalScanner which aggregates the delta > column qualifier values and generates a new key value with Type Aggregate: > <TS><A><UniqueID> > > The problem with this implementation that it consumes more memory. > > Now, I've tried avoiding the creation of the Aggregate type KV, by simply > re-using the 1st delta column qualifier: simply changing its value in the > KeyValue. > But from some reason, after a couple of minor / major compactions, I see data > loss, when I count the values and compare them to the expected. > > > The question is: is it "legal" to change a KV I received from the > InternalScanner before adding it the Result - i..e returning it from my own > InternalScanner? > > > > > > > On Feb 12, 2013, at 8:44 AM, Anoop Sam John wrote: > >> Asaf, >> You have created a wrapper around the original InternalScanner >> instance created by the compaction flow? >> >>> Where do the KV generated during the compaction process queue up before >>> being written to the disk? Is this buffer configurable? >> When I wrote the Region Observer my assumption was the the compaction >> process works in Streaming fashion, thus even if I decide to generate a KV >> per KV I see, it still shouldn't be a problem memory wise. >> >> There is no queuing. Your assumption is correct only. It is written to the >> writer as and when. (Just like how memstore flush doing the HFile write) As >> Lars said a look at your code can tell if some thing is going wrong. Do you >> have blooms being used? >> >> -Anoop- >> ________________________________________ >> From: Mesika, Asaf [[email protected]] >> Sent: Tuesday, February 12, 2013 11:16 AM >> To: [email protected] >> Subject: Custom preCompact RegionObserver crashes entire cluster on OOME: >> Heap Space >> >> Hi, >> >> I wrote a RegionObserver which does preCompact. >> I activated in pre-production, and then entire cluster dropped dead: One >> RegionServer after another crashed on OutOfMemoryException: Heap Space. >> >> My preCompact method generates a KeyValue per each set of Column Qualifiers >> it sees. >> When I remove the coprocessor and restart the cluster, cluster remains >> stable. >> I have 8 RS, each has 4 GB Heap. There about 9 regions (from a specific >> table I'm working on) per Region Server. >> Running HBase 0.94.3 >> >> The crash occur when the major compaction fires up, apparently cluster wide. >> >> >> My question is this: Where do the KV generated during the compaction process >> queue up before being written to the disk? Is this buffer configurable? >> When I wrote the Region Observer my assumption was the the compaction >> process works in Streaming fashion, thus even if I decide to generate a KV >> per KV I see, it still shouldn't be a problem memory wise. >> >> Of course I'm trying to improve my code so it will generate much less new KV >> (by simply altering the existing KVs received from the InternalScanner). >> >> Thank you, >> >> Asaf
