ESDI caching disk controllers: Reprise

Bill Kennedy bill at ssbn.WLK.COM
Wed May 2 22:35:40 AEST 1990

In article <263E613E.23840 at> baxter at (Ira Baxter) writes:
>In article <1424 at ssbn.WLK.COM> I wrote:
>> With a 768K cache memory on one card I routinely get 32% cache hits
>> and with 4Mb on the one in ssbn I get 48-52% cache hits.

I'm sorry I didn't see this when I followed up the earlier post, I'd have
combined them.

>I have never understood this.  If a caching disk controller with 4Mb gives
>you a high hit rate, why not put the 4Mb of RAM into your CPU, and
>let UNIX use it as a cache?  The hit rate should be the same.

This is certainly true for reads and, depending on NAUTOUP, for writes.  My
system is on a UPS so I set NAUTOUP to two minutes so the buffers are not
flushed so frequently.

>The win is that the OS should be able to divide the RAM between disk
>cache and process space (I assume UNIX isn't brain-dead in this regard?).
>Then you have the best of both worlds.

I don't think it works this way, so put a question mark in if approrpiate.
You specify the size of the kernel cache with the NBUFS tunable parameter,
I don't think that the kernel sizes up or down on the fly.  It probably
does something like that with the process space when it starts swapping,
but I think the disk cache is fixed at wherever you have it set.  In my
case, I had already ascertained the "optimal" number of buffers so I was
going for the time wasted waiting on the spindle/heads to get positioned.

>Or am I missing something?
>Ira Baxter

Yes, Ira, I think you are.  I agree that things get fuzzy with regard to
reading, kernel buffers work at memory speed and the controller cache at
I/O speed.  Writing is a different story though.  The logical write is
disconnected from the physical write.  The kernel dumps off its stuff at
I/O speed without regard to seek time or rotational latency and the
controller worries about the physical write.  The kernel buffers can't
help you a bit if you have to wait on the disk mechanism to get to the
right place, a caching controller can.  The other issue (in my case) was
cost.  SIMM's for the controller are a lot cheaper than column static
main memory.
Bill Kennedy  usenet      {texbell,att,,sun!daver}!ssbn!bill
              internet    bill at ssbn.WLK.COM   or attmail!ssbn!bill

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