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From Asaf Mesika <asaf.mes...@gmail.com>
Subject Re: Uneven write request to regions
Date Sat, 16 Nov 2013 19:16:01 GMT
First I forgot to mention that <customerId> in our case is
MD5(<customerId>).
In our case, we have so much data flowing in, that we end up having a
region per <customerId><bucket> pretty quickly and even that, is splitted
into different regions by specific date duration (timestamp).

We're not witnessing a hotspot issue. I built some scripts in java and awk,
and saw that 66% of our customers use more than 1Rs.

We have two main serious issues: primary and secondary.

Our primary issue being the slow-region vs fast-region. First let's be
reminded that a region represents as I detailed before a specific
<customerId><bucket>. Some customers gets x50 times more data that other
customers at a specific time frame (2hrs - 1 day). So in a one RS, we have
regions getting 10 write requests per hour, vs 50k write requests per hour.
So the region mapped to the slow-filling customer id, doesn't get to the
256MB flush limit and hence isn't flushed, while the regions mapped to the
fast-filling customer id, are flushing very quickly since they are filling
very quickly.
Let's say the 1st WAL file contains the put of a slow-filling customerId.
the fast-filling customerId, fills up the rest of that file. After 20-30
seconds, the file gets rolled, and another file fills up with fast filling
customerId. After a while, we get to 32 WAL Files. The 1st file wasn't
deleted since its region wasn't flushed. The 32 limit makes HBase go into
stress mode, and dump all involving regions contains in those 32 WAL Files.
In our case, we saw that it flushes 111 regions. Lots of the store files
are 3k-3mb sized. So our compaction queue start filling up with those store
files needs to be compacted.
At the of the road, the RS gets dead.

Our secondary issue is those of empty regions - we get to a situation where
a region is mapped to a specific <customerId>, <bucket>, and date range
(1/7 - 3/7). Those when we are in August (we TTL set to 30 days), those
regions gets empty and will never get filled again.
We assume this somehow wrecks havoc in the load balancer, and also MSLAB
probably steals 1-2 GB of memory for those empty regions.

Thanks!



On Sat, Nov 16, 2013 at 7:25 PM, Mike Axiak <mike@axiak.net> wrote:

> Hi,
>
> One new key pattern that we're starting to use is a salt based on a shard.
> For example, let's take your key:
>
>   <customerId><bucket><timestampInMs><uniqueId>
>
> Consider a shard between 0 and 15 inclusive. We determine this with:
>
>  <shard> = abs(hash32(uniqueId) % 16)
>
> We can then define a salt to be based on customerId and the shard:
>
>  <salt> = hash32(<shard><customerId>)
>
> So then the new key becomes:
>
>  <salt><customerId><timestampInMs><uniqueId>
>
> This will distribute the data for a given customer across the N shards that
> you pick, while having a deterministic function for a given row key (so
> long as the # of shards you pick is fixed, otherwise you can migrate the
> data). Placing the bucket after the customerId doesn't help distribute the
> single customer's data at all. Furthermore, by using a separate hash
> (instead of just <shard><customerId>),  you're guaranteeing that new data
> will appear in a somewhat random location (i.e., solving the problem of
> adding a bunch of new data for a new customer).
>
> I have a key simulation script in python that I can start tweaking and
> share with people if they'd like.
>
> Hope this helps,
> Mike
>
>
> On Sat, Nov 16, 2013 at 1:16 AM, Ted Yu <yuzhihong@gmail.com> wrote:
>
> > bq. all regions of that customer
> >
> > Since the rowkey starts with <customerId>, any single customer would only
> > span few regions (normally 1 region), right ?
> >
> >
> > On Fri, Nov 15, 2013 at 9:56 PM, Asaf Mesika <asaf.mesika@gmail.com>
> > wrote:
> >
> > > But when you read, you have to approach all regions of that customer,
> > > instead of pinpointing just one which contains that hour you want for
> > > example.
> > >
> > > On Friday, November 15, 2013, Ted Yu wrote:
> > >
> > > > bq. you must have your customerId, timestamp in the rowkey since you
> > > query
> > > > on it
> > > >
> > > > Have you looked at this API in Scan ?
> > > >
> > > >   public Scan setTimeRange(long minStamp, long maxStamp)
> > > >
> > > >
> > > > Cheers
> > > >
> > > >
> > > > On Fri, Nov 15, 2013 at 1:28 PM, Asaf Mesika <asaf.mesika@gmail.com>
> > > > wrote:
> > > >
> > > > > The problem is that I do know my rowkey design, and it follows
> > people's
> > > > > best practice, but generates a really bad use case which I can't
> seem
> > > to
> > > > > know how to solve yet.
> > > > >
> > > > > The rowkey as I said earlier is:
> > > > > <customerId><bucket><timestampInMs><uniqueId>
> > > > > So when ,for example, you have 1000 customer, and bucket ranges
> from
> > 1
> > > to
> > > > > 16, you eventually end up with:
> > > > > * 30k regions - What happens, as I presume: you start with one
> region
> > > > > hosting ALL customers, which is just one. As you pour in more
> > customers
> > > > and
> > > > > more data, the region splitting kicks in. So, after a while, you
> get
> > > to a
> > > > > situation in which most regions hosts a specific customerId, bucket
> > and
> > > > > time duration. For example: customer #10001, bucket 6, 01/07/2013
> > > 00:00 -
> > > > > 02/07/2013 17:00.
> > > > > * Empty regions - the first really bad consequence of what I told
> > > before
> > > > is
> > > > > that when the time duration is over, no data will ever be written
> to
> > > this
> > > > > region. and Worst - when the TTL you set (lets say 1 month) is over
> > and
> > > > > it's 03/08/2013, this region gets empty!
> > > > >
> > > > > The thing is that you must have your customerId, timestamp in the
> > > rowkey
> > > > > since you query on it, but when you do, you will essentially get
> > > regions
> > > > > which will not get any more writes to them, and after TTL become
> > zombie
> > > > > regions :)
> > > > >
> > > > > The second bad part of this rowkey design is that some customer
> will
> > > have
> > > > > significantly less traffic than other customers, thus in essence
> > their
> > > > > regions will get written in a very slow rate compared with the high
> > > > traffic
> > > > > customer. When this happens on the same RS - bam: the slow region
> > Puts
> > > > are
> > > > > causing the WAL Queue to get bigger over time, since its region
> never
> > > > gets
> > > > > to Max Region Size (256MB in our case) thus never gets flushed,
> thus
> > > > stays
> > > > > in the 1st WAL file. Until when? Until we hit max logs file
> permitted
> > > > (32)
> > > > > and then regions are flushed forcely. When this happen, we get
> about
> > > 100
> > > > > regions with 3k-3mb store files. You can imagine what happens next.
> > > > >
> > > > > The weirdest thing here is that this rowkey design is very common
-
> > > > nothing
> > > > > fancy here, so in essence this phenomenon should have happened to
a
> > lot
> > > > of
> > > > > people - but from some reason, I don't see that much writing about
> > it.
> > > > >
> > > > > Thanks!
> > > > >
> > > > > Asaf
> > > > >
> > > > >
> > > > >
> > > > > On Fri, Nov 15, 2013 at 3:51 AM, Jia Wang <ramon@appannie.com>
> > wrote:
> > > > >
> > > > > > Then the case is simple, as i said "check your row key design,
> you
> > > can
> > > > > find
> > > > > > the start and end row key for each region, from which you can
> know
> > > why
> > > > > your
> > > > > > request with a specific row key doesn't hit a specified region"
> > > > > >
> > > > > > Cheers
> > > > > > Ramon
> > > > > >
> > > > > >
> > > > > > On Thu, Nov 14, 2013 at 8:47 PM, Asaf Mesika <
> > asaf.mesika@gmail.com>
> > > > > > wrote:
> > > > > >
> > > > > > > It's from the same table.
> > > > > > > The thing is that some <customerId> simply have less
data saved
> > in
> > > > > HBase,
> > > > > > > while others have x50 (max) data.
> > > > > > > I'm trying to check how people designed their rowkey around
it,
> > or
> > > > had
> > > > > > > other out-of-the-box solution for it.
> > > > > > >
> > > > > > >
> > > > > > >
> > > > > > > On Thu, Nov 14, 2013 at 12:06 PM, Jia Wang <ramon@appannie.com
> >
> > > > wrote:
> > > > > > >
> > > > > > > > Hi
> > > > > > > >
> > > > > > > > Are the regions from the same table? If it was, check
your
> row
> > > key
> > > > > > > design,
> > > > > > > > you can find the start and end row key for each region,
from
> > > which
> > > > > you
> > > > > > > can
> > > > > > > > know why your request with a specific row key doesn't
hit a
> > > > specified
> > > > > > > > region.
> > > > > > > >
> > > > > > > > If the regions are for different table, you may consider
to
> > > combine
> > > > > > some
> > > > > > > > cold regions for some tables.
> > > > > > > >
> > > > > > > > Thanks
> > > > > > > > Ramon
> > > > > > > >
> > > > > > > >
> > > > > > > > On Thu, Nov 14, 2013 at 4:59 PM, Asaf Mesika <
> > >
> >
>

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