[ fbsd_quest ] file_caching and hd caches

[spellberg_robert]

thanks, warren [ love your dot_com, btw ] ---



Warren Block wrote:
> 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.

now that i have a handle on today's prices,
   the choice of retailer is very important.

it also appears [ from my reading of manufacturer's literature ] that
   the hd internal_cache is used as a write_buffer
   for the benefit of the chip_set,
   then the drive can take its own sweet time writing to its notion of "sector"s.

therefore,
   for a mobo that is stuffed_to_the_gills with ram
   [ relative to the apps that it is running ],
   if i read you correctly,
   then reads will tend to come from mobo_ram and
   the hd_cache is mostly a write_buffer.
i suspect that the hd_cache would be more important for
   an os that doesn't do its own caching
   [ until its notion of "idle"ness occurs ].



> 
>> something else that i just thought up while typing this:
>>
>>  q:  are hd internal_caches non_volatile ?
> 
> 
> No.

not surprised.



> 
>> id est,
>>
>>  q:  do the cache contents survive a power_cycle ?
> 
> 
> No.  You may want to look at SSDs.

understood.



> 
>> [ 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.

understood.



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

not important.
thanks for the thought, though.



to summarize,
   it looks like, for freebsd, i should
   "get a good price from a reputable retailer
   on a high_quality product from a reputable manufacturer".
then, i can save my worrying_time for really important subjects, like
   "the determination of the correct yardarm height for the hanging of pirates".



> 
> -Warren Block * Rapid City, South Dakota USA

nice part of the country, that.
chicago & north western territory.



rob
mchenry county, illinois