incubator-couchdb-user mailing list archives

Site index · List index
Message view « Date » · « Thread »
Top « Date » · « Thread »
From Nicholas Orr <nicholas....@zxgen.net>
Subject Re: Why do my CouchDB databases grow so fast?
Date Mon, 31 May 2010 23:58:05 GMT
This all makes sense except the OP says a compaction step is being
performed.
A compaction is essentially a copy/paste/delete/rename operation, so the on
disk size should be fairly constant as the data copied is just the info
required isn't it?

Nick

On Tue, Jun 1, 2010 at 9:39 AM, Randall Leeds <randall.leeds@gmail.com>wrote:

> Hi Konrad,
>
> I'll take a stab at this and if I'm wrong hopefully someone will correct
> me.
>
> The on disk BTree is written in an append only fashion rather than
> modified in place. Append only updates mean that every inner node of
> the BTree along the path from the root to the new update has to be
> re-written each time. Initially, when there are very few inner nodes,
> the amount of disk space used for each new update is relatively
> constant. Since the tree has a large fan-out the depth does not change
> much at first. In the second graph you are seeing a tree that has a
> depth of 1 (just the root) being written over an over again to disk
> and the corresponding expected linear growth results. However, when
> you have a higher revision limit the old revisions are kept in the
> tree and the tree grows taller and fatter with each update. As you
> make more updates more inner nodes need to be rewritten for each
> update which causes the growth to accelerate. Eventually, you hit the
> revision limit and old revisions are discarded, the tree stops getting
> any taller or fatter and the number of inner nodes that need to be
> changed for each update remains relatively constant (but greater than
> in the case of rev_limit=1). I suspect that the first graph becomes
> linear above 1000 updates and does not continue to accelerate.
>
> Cheers,
> Randall
>
> 2010/5/31 Konrad Förstner <konrad@foerstner.org>:
> > Hi,
> >
> > I have an issue with CouchDB and posted the question on stackoverflow
> > [1] but did not get any helpful answer. I would be great if somebody
> > could answer this here or a stackoverflow (there I also had a problem
> > with the compaction which was just a timing issue as explaint in the
> > comment)
> >
> > I was wondering why my CouchDB database was growing to fast so I wrote
> > a little test script [2]. This script changes an attributed of a CouchDB
> > document 1200 times and takes the size of the database after each
> > change. After performing these 1200 writing steps the database is
> > doing a compaction step and the db size is measured again. In the end
> > the script plots the databases size against the revision numbers. The
> > benchmarking is run twice:
> >
> > * The first time the default number of document revision (=1000) is used
> (_revs_limit).
> >
> > * The second time the number of document revisions is set to 1.
> >
> > The first run produces the following plot
> > http://www.flickr.com/photos/konradfoerstner/4656011444/
> >
> > The second run produces this plot second run
> > http://www.flickr.com/photos/konradfoerstner/4656012732/
> >
> > For me this is quite an unexpected behavior. In the first run I would
> > have expected a linear growth as every change produces a new
> > revision. When the 1000 revisions are reached the size value should be
> > constant as the older revisions are discarded.
> >
> > In the second run the first revision should result in certain database
> > size that is then keeps during the following writing steps as every
> > new revision leads to the deletion of the previous one.
> >
> > I could understand if there is a little bit of overhead needed to
> > manage the changes but this growth behavior seems weird to me. Can
> > anybody explain this phenomenon or correct my assumptions that lead to
> > the wrong expectations?
> >
> > Many thanks in advance
> >
> > Konrad
> >
> > [1]
> http://stackoverflow.com/questions/2921151/why-do-my-couchdb-databases-grow-so-fast
> > [2] http://github.com/konrad/couchdb-benchmarking
> >
> >
> >
> >
> >
> >
>

Mime
  • Unnamed multipart/alternative (inline, None, 0 bytes)
View raw message