[ fbsd_quest ] file_caching and hd caches

[Warren Block]

On Wed, 8 Apr 2009, spellberg_robert wrote:

> howdy, y'all ---
>
> so, i was looking over the offerings of the on_line retailing "usual 
> suspects",
>  when i got to thinking:
>
>  q:  to what extent does freebsd cache recently_used hard_drive files ?

To the extent that RAM is available.

>  q:  under freebsd, to what extent are
>        hard_drive internal_caches and their sizes [ e. g., 2mb, 8mb, 16mb ]
>        important ?

It depends on workload.

> i am not so much looking for a history_ and theory_of_operation as
>  i am looking for a "yes/no" to the question:
>
>  q:  should i pay up for hd_cache, if the other hd parameters are the same ?

Again, depends on workload.  Also the difference in price for relatively 
small differences in cache RAM on the hard drive.

> something else that i just thought up while typing this:
>
>  q:  are hd internal_caches non_volatile ?

No.

> id est,
>
>  q:  do the cache contents survive a power_cycle ?

No.  You may want to look at SSDs.

> [ some supplementary "fyi"s:
>
>    yes, i am aware that
>      hd access_times are a relative "eternity" to a chip_set's hd_port.
>
>    i am not thinking about ram_size and swap_size and "thrashing";
>      all of my boxen have plenty of ram.
>
>    i know i have to read it in the first time.
>    rather, i am thinking about opening and reading
>      some file that i recently wrote and closed.

FreeBSD is pretty good at that.  For example, reboot and start Firefox. 
Then close it and start it again.

There may be ways of prioritizing what's kept in cache, although I don't 
know them.

-Warren Block * Rapid City, South Dakota USA