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From: lm@slovax.Sun.COM (Larry McVoy)
Newsgroups: comp.unix.wizards
Subject: Re: RAM disk.
Message-ID: <143359@sun.Eng.Sun.COM>
Date: 4 Oct 90 06:24:53 GMT
References: <900908.7074@franklin.com> <1990Sep12.084002.5575@hq.demos.su> <1223@tardis.Tymnet.COM> <1990Sep13.002300.15266@mlb.semi.harris.com> <6167@titcce.cc.titech.ac.jp> <143190@sun.Eng.Sun.COM> <18560@rpp386.cactus.org>
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Reply-To: lm@sun.UUCP (Larry McVoy)
Organization: Sun Microsystems, Mountain View
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In article <18560@rpp386.cactus.org> jfh@rpp386.cactus.org (John F. Haugh II) writes:
>In article <143190@sun.Eng.Sun.COM> lm@slovax.Sun.COM (Larry McVoy) writes:
>>In article <6167@titcce.cc.titech.ac.jp> mohta@necom830.cc.titech.ac.jp (Masataka Ohta) writes:
>>>If you use elaborated and complicated memory disk, it can be only as
>>>slow as ordinary disk, but not faster.
>>
>>Bullsh*t.  Tmpfs is orders of magnitude faster than a disk.  
>
>I suspect this was a typo - if tmpfs can only be as slow, and not
>any faster, then it must be the same speed.

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):

$ 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.

>As for 5MB/sec transfer limitations - does the SS1 have a DMA
>setup that can handle memory to memory transfers?  I would hope
>that the memory subsystem can handle more than 5MB/sec ...

Not by much.  A SS1 can't bcopy the data much faster than that.

---
Larry McVoy, Sun Microsystems     (415) 336-7627       ...!sun!lm or lm@sun.com