Re: [netcdfgroup] Inquire about cache size and nslots ?

• To: Jennifer Adams <jma@xxxxxxxxxxxxx>, netcdfgroup@xxxxxxxxxxxxxxxx
• Subject: Re: [netcdfgroup] Inquire about cache size and nslots ?
• From: Ed Hartnett <ed@xxxxxxxxxxxxxxxx>
• Date: Tue, 17 Nov 2009 18:13:48 -0700

Jennifer Adams <jma@xxxxxxxxxxxxx> writes:

>
>   Thanks for asking a question for which there is an easy answer. Some of
>   the recent support issues have been real stumpers!
>
> I'm glad it wasn't too tough, and also that the answer was 'YES'.
>
>   There are functions to both set and learn about the chunk
>   cache. Changing the cache settings only applies to future opens/creates,
>   it does not affect already-open files.
>
>   These don't tell you what the file was opened/created with, just the
>   current settings. So if you change them, and want to know what they are
>   for each file you open/create, then you must remember what they were
>   when the file was opened/created.
>
> Let me see if I have understood properly ... The cache parameters are set
> on a per-file basis, but copied from global settings at the moment I open
> the file? And the original global settings are those used at compile
> time? 

Yes and yes.

> What if I open a file, query the chunk and the cache sizes, and discover
> that the cache isn't big enough to hold a single chunk and my I/O is
> doomed. Do I have to close the file, reset the cache size, then open the
> file again? 

Yes.

But does that really ever happen? The default chunk cache is 32 MB - do
you have files with chunks larger then that?

If so, you could either change the chunk size to a better default
setting before doing any opens, or you could use the --with-chunk-cache=
parameter of configure to change the default chunk cache size for that
installation of netCDF.

Is this not enough to meet all of your chunk caching needs?

Thanks,

Ed
-- 
Ed Hartnett  -- ed@xxxxxxxxxxxxxxxx

• References:
  • [netcdfgroup] Inquire about cache size and nslots ?
    • From: Jennifer Adams
  • Re: [netcdfgroup] Inquire about cache size and nslots ?
    • From: Ed Hartnett
  • Re: [netcdfgroup] Inquire about cache size and nslots ?
    • From: Jennifer Adams