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From Randall Leeds <randall.le...@gmail.com>
Subject Re: Why do my CouchDB databases grow so fast?
Date Tue, 01 Jun 2010 01:02:56 GMT
I don't think so, but check his script on github. Here's a link to the lines.
http://github.com/konrad/couchdb-benchmarking/blob/master/couchdb-benchmarking.rb#L84

On Mon, May 31, 2010 at 17:39, Nicholas Orr <nicholas.orr@zxgen.net> wrote:
> Ahh yes, I see that now...
>
> Is that what the grey faint line on the end of those graphs represent?
> I actually didn't notice that the first time, just thought the black
> line going up was it...
>
> On Tue, Jun 1, 2010 at 10:13 AM, Randall Leeds <randall.leeds@gmail.com> wrote:
>> If you look at the script the compaction is only performed at the end
>> and not on each iteration.
>>
>> On Mon, May 31, 2010 at 16:58, Nicholas Orr <nicholas.orr@zxgen.net> wrote:
>>> 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
>>>> >
>>>> >
>>>> >
>>>> >
>>>> >
>>>> >
>>>>
>>>
>>
>

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