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From Randall Leeds <randall.le...@gmail.com>
Subject Re: few doubts
Date Mon, 04 Jul 2011 20:34:02 GMT
On Jul 4, 2011 11:11 AM, "Jens Alfke" <jens@mooseyard.com> wrote:
>
>
> On Jul 3, 2011, at 11:41 PM, Matthew John wrote:
>
> > how does CouchDB perform
> > when compared to MySql in performing the Warm-up (lets define Warm-up as
> > filling the memory with Disk data in a judicial manner).
>
> No idea. But really, you could perform something very similar by doing
something like
>        cat /path/to/my_database.couch > /dev/null
> since that would have the same effect of loading the file contents into
the VM cache. So it’s not really that specific to CouchDB itself.
>
> How large is your data-set? How many rows, and what’s the disk space
occupied by the MySQL tables?
>
> —Jens

It should also be noted that CouchDB does not do any caching of its own. I
don't know if MySQL does, but if yes then your warm-up query is possibly
letting MySQL cache some index headers or something, but nothing exactly
like this happens in CouchDB. The only caching is at the filesystem/OS
level. In fact, if your OS hasn't decided to dump the page cache, it may be
beneficial _not_ to do anything that might disturb it between restarts,
since some of that file data might still be around. This behavior would
depend on your OS keeping the pages around after all file descriptors are
closed. I don't know whether that happens or when. Maybe there is a kernel
setting for this behavior?

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