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From Randall Leeds <randall.le...@gmail.com>
Subject Re: few doubts
Date Mon, 04 Jul 2011 20:55:23 GMT
On Mon, Jul 4, 2011 at 13:34, Randall Leeds <randall.leeds@gmail.com> wrote:
>
> 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?

Ah. Of course. The kernel parameter is /sys/proc/vm/swappiness on Linux systems.
1 - Dump cached pages in favor of keeping applications in memory
...
...
100 - Swap applications in favor of keeping pages cached in memory

I suspect there's no aggressive dumping of pages just because files
have been closed, though I haven't found any information one way or
another yet. In other words, while your warm-up query might do things
for MySQL's internal caching that could have benefits, doing something
similar in CouchDB would probably just force out whatever useful pages
your OS has cached from before the restart. I would not recommend
trying to do anything like this.

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