RAM disk.

[John F. Haugh II]

In article <143359 at sun.Eng.Sun.COM> lm at sun.UUCP (Larry McVoy) writes:
>I'm not sure if you think he made the typo or I did, but my statement stands.
>Just for grins (mydd is a little program that moves I/O like dd but has some
>other options and timing info stuck in for free):

Oops.  I meant that the other poster probably made a typo.  His statement
was that tmpfs wasn't any slower and couldn't be any faster.  By implication,
this means tmpfs is exactly as fast as diskfs (or whatever ...)  Since this
is just plain unlikely, I assumed he meant something else.

>$ mydd if=internal of=/tmp/XXX count=500 fsync=1
>4000.00 Kbytes in 0.258 seconds (15505.4 Kbytes/s)
>$ mydd if=internal of=/usr/tmp/XXX count=500 fsync=1
>4000.00 Kbytes in 5.1 seconds (784.386 Kbytes/s)
>
>Yeah, so I lied, it's 15MB / sec not 5MB / sec.  I was being conservative :-)
>Actually, slovax is a 4/470 which has bcopy hardware support, the 5MB/sec
>number is pretty close for a 20MHZ SS1.

OK.  But as I pointed out, single "small" amounts of I/O are not the proper
test.  The only valid test of I/O performance is going to be continuous I/O
at some simulated load.  For example, trying DD on this system does not show
any slowdown until I DD 512K.  Everything up to that point occurs in the
same amount of time.  This is because all of the I/O is performed in system
buffers until that point.  Testing with a larger amount of total I/O will
give a more accurate representation.
-- 
John F. Haugh II                             UUCP: ...!cs.utexas.edu!rpp386!jfh
Ma Bell: (512) 832-8832                           Domain: jfh at rpp386.cactus.org
"SCCS, the source motel!  Programs check in and never check out!"
		-- Ken Thompson