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From "Geir Magnusson Jr." <g...@pobox.com>
Subject Re: Faster updates, optional ACID
Date Mon, 05 Jan 2009 18:59:45 GMT

On Jan 5, 2009, at 12:54 PM, Damien Katz wrote:

> It was brought to my attention that commits on OS X were very slow  
> with the latest releases of Erlang. After I upgraded to the most  
> recent version, I found them to be indeed slow, slowing the tests  
> down to point it was painful to run them. It appears, any disk sync  
> from Erlang now takes somewhere between 50 and 100ms, up from the  
> previous times of ~5ms. This is almost certainly due to the  
> F_FULLFSYNC flag the erlang file handling now uses on darwin based  
> systems, but it's surprising how bad performance on OS X. A little  
> investigation had shown other database engines have similar issues  
> on OS X.

One user, though,  reported a huge performance difference between 0.8  
and trunk *using the same version of Erlang*.

http://mail-archives.apache.org/mod_mbox/couchdb-user/200901.mbox/%3C49619897.7000104@gmail.com%3E

To me, this hints that something changed in CouchDB that triggers  
usage of fcntl(F_FULLFSYNC).

I haven't tried to duplicate but will.  If this can be verified, I  
think that this is an important mystery to solve.

>
>
> To address this problem, I implemented delayed commit functionality.  
> We had always intended to implement delayed commit for performance  
> reasons, but hadn't had the need until now. This makes updates much  
> faster in the general case, but with the caveat they aren't flushed  
> completely to disk right way. If you can't tolerate the possible  
> loss of recent updates, you can use the "full commit" option for  
> ACID commits.

So this bring up a question.  There is no way to get the same  
durability semantics on linux as you can get on OS X with F_FULLSYNC.   
This means that the "full commit" option really gives you different  
levels of durability, depending on whether or not you are on OS X.

And thinking more about what appears to be the perf bug/slowdown in  
CouchDB code, might his warrant three options?

1) delayed commit (what you did last night)
2) fsync() commit (what I suspect Couch did on and around 0.8)
3) optional F_FULLSYNC commit, on OS X and any other platform that  
provides this level of commit

>
>
> For full acid commit, add a header field to the doc PUT or  
> _bulk_docs POST like this:
> X-Couch-Full-Commit:true
>
> Then couchdb will completely commit the change before returning.


That's cool but it puts the burden on the client - why not make it a  
config option, so that the db admin can choose the durability level in  
general, and let clients that know they are talking to couch override  
w/ a header?


geir

>
>
> Also, if you have several delayed updates and you want to make sure  
> they all made it to disk, you can invoke POST /db/ 
> _ensure_full_commit and all outstanding commits are flushed to disk.
>
> The view engine has been already modified to deal with delayed  
> commits too, it ensures it never fully commits it's own indexes to  
> disk if the documents indexed aren't already committed to disk.
>
> The last remaining work item is db server crash detection, so that  
> clients can detect when a server has crashed and potentially lost  
> updates. This is pretty simple, each db server just needs a unique  
> ID generated at it's startup. Client retrieve this value at the  
> beginning of the writes and then checks that the value is the same  
> once down a flushed to disk. If not, we know we maybe have lost some  
> updates and we redo the replication from the last known good commit.
>
> Right now the default is to delay the commits, because I think that  
> will be the most common use case but I'm really not sure. I  
> definitely want the commits delayed for the test suite, to keep  
> things running fast.
>
> -Damien
>
>


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