Ramdisk Size

Tony Travis ajt at rri.sari.ac.uk
Wed Sep 24 06:49:46 EDT 2003


Eric R Johnson wrote:

> [...]
> as possible on a system dedicated to this task.  Now that you have a 
> little more info, do you still recommend tmpfs?  If so, is there a 
> specific performance issue that leads us in that direction, because the 
> search time is all that matters in this case.

Hello, Eric.

On the basis of what you've told us, I think you would be better off 
avoiding the filesystem overhead altogether and read your database 
directly into memory within a program. Accessing RAM directly by memory 
references in a program is as fast as you can get. Anything else you do 
will incur an OS filesystem call overhead. This may not be practical, of 
course, but it would give you the fastest solution. One-step (hash) 
lookups of whatever your database contains directly from RAM is fastest.

My basic point about ramdisk/tmpfs is that I think you would be better 
off allowing the memory manager to keep your most recently accessed data 
in memory for you. However, if your entire job, including the database, 
fits into physical memory (RAM) then no swapping to disk will occur with 
tmpfs. I think you really should be looking for a solution that gives 
you the best performance improvement for the least effort - i.e. tmpfs.

Forgive me if I'm teaching you to suck eggs, but if search time is all 
that matters then your search algorithm is important. There is little 
point accessing your database as fast as possible using a poor search 
algorithm: Indeed, you may find that your concern about 'fast' access to 
your data on ramdisk/tmpfs is not the main bottleneck after all. You 
could benchmark the database problem on harddisk and tmpfs to find out.

	Tony.
-- 
Dr. A.J.Travis,                     |  mailto:ajt at rri.sari.ac.uk
Rowett Research Institute,          |    http://www.rri.sari.ac.uk/~ajt
Greenburn Road, Bucksburn,          |   phone:+44 (0)1224 712751
Aberdeen AB21 9SB, Scotland, UK.    |     fax:+44 (0)1224 716687

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