OT/drift: when is a RAMdisk an appropriate solution
- From: Chris Torek <nospam@xxxxxxxxx>
- Date: 29 Aug 2006 07:39:06 GMT
Flash Gordon wrote:
[RAM-disk creation and manipulation is] no more difficult and no
less system specific than it was back then.
Well, no less system-specific, to be sure. Sometimes "more (or
less) difficult" depending on the system. (Aside: in vxWorks, just
include the "ram disk" component in your project. The default name
is "/ram0" but you can change it. If you want more than one, it is
somewhat more difficult.)
The difference is that if you are doing it to speed things up you
are quite possibly wasting your time and effort.
Indeed.
In article <H5GIg.4036$y61.1915@fed1read05>
jmcgill <jmcgill@xxxxxxxxxxxxxxxxx> wrote:
It still makes sense on diskless clients that boot over NFS and so on.
It's also indicated in some applications where security is sensitive, or
for devices that need to operate in environments where moving parts are
impractical.
Sometimes, a physical (as opposed to virtual, in-kernel-only) RAM
disk is appropriate. That is, you have a piece of hardware you
plug in, which looks and acts like a disk as far as the system is
concerned, but actually stores data in RAM.
While many systems (including vxWorks, as mentioned above) do
include in-main-memory pseudo-disk devices, in a purely theoretical
sense, there is *never* [%] any reason to use one: whatever you
are doing that is going through some sort of "file system" layer,
only to wind up going directly to regular old ordinary memory, is
going through a "unnecessary" manipulation. File systems do a lot
of work in order to deal with structured, and somewhat klunky, disk
drive hardware, where memory exists in the form of "disk blocks" or
"sectors" that can only be rewritten in entire units, cannot be
moved or resized, and is generally very slow to access (about 5 or
6 decimal orders of magnitude slower than RAM). Real RAM does not
share most of these characteristic drawbacks, and going through
a (usually quite heavy) software layer that simply pretends to add
some of them back is just silly.
[% One obvious exception is when the RAM-disk is being used for
testing the file system software. Here, even in theory, the RAM
drive is useful. :-) ]
In practice, the reason for using a RAM disk is that the stubborn
software (written by some stubborn and/or or long-gone programmer)
insists on using the file system interface, and the system provides
no way to short-circuit this. For instance, in C -- to get at
least marginally back on topic -- one might have a program (or
large library) that does all its I/O to a "stdio" stream. Since
ISO C has no way to "open memory as a stream", one is often forced
to supply an actual, on-disk file. To get decent performance, one
may wish to use an "on-RAM-disk file" instead of "on-actual-disk".
Note that it can make sense to use a "RAM file system" rather than
a "RAM disk" in this case. Going through a *real* file system,
you may do a whole lot of extra work trying to avoid talking too
much to a slow (real) disk -- i.e., spend large amounts of CPU time
to avoid waiting for the device. But if the "device" is fake, and
actually is very fast (relative to real disks), this CPU time is
being wasted. It may be better to go through a fake file system
(one which uses RAM), instead of a real file system on a fake disk.
Of course, it would be even better still if C had some sort of
memory-oriented stdio streams -- or something like the 4.4BSD
"funopen", which is even more general. (I am pretty sure that
funopen() did get proposed for C99; but we might consider ourselves
lucky to have gotten even snprintf(). :-) )
--
In-Real-Life: Chris Torek, Wind River Systems
Salt Lake City, UT, USA (40°39.22'N, 111°50.29'W) +1 801 277 2603
email: forget about it http://web.torek.net/torek/index.html
Reading email is like searching for food in the garbage, thanks to spammers.
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