Xref: utzoo comp.sys.att:7855 comp.unix.i386:902 Path: utzoo!attcan!uunet!mcsun!ukc!dcl-cs!aber-cs!rupert!pcg From: pcg@rupert.cs.aber.ac.uk (Piercarlo Grandi) Newsgroups: comp.sys.att,comp.unix.i386 Subject: Re: What ESDI Controller ships with the new AT&T 6386s? Message-ID: Date: 22 Oct 89 12:29:01 GMT References: <2542@infmx.UUCP> Sender: pcg@aber-cs.UUCP Organization: Coleg Prifysgol Cymru Lines: 44 In-reply-to: aland@infmx.UUCP's message of 20 Oct 89 19:28:27 GMT In article <2542@infmx.UUCP> aland@infmx.UUCP (Dr. Scump) writes: I have two basic questions: 1) do the 25 and 33 MHz ESDI models include DPT controllers?, and 2) if not, has anybody tried to use the DPT caching ESDI controller in one of these machines? Does anybody know a reason why they *wouldn't* work? Also, are there caching SCSI controllers that would work with the Model S, and how would the SCSI combination perform compared to ESDI with the DPT caching controller for ESDI? Of course since you also want reliable operation, you *MUST* *MUST* have an UPS if you use a caching controller with a database system. If not, and the system goes off before the cache has not been flushed, all the careful algorithms used by your DMBS to ensure consistency, rewcoverability, and other trivialities will have been made pointless, and your customers will be *very* angry. We want to use several of these machines in an environment where disk performance will be the biggest performance bottleneck. A caching controller does not improve disk performance *AT ALL*. It simply does the same job as the OS cache, or the DBMS cache, in a different way; it reduces the average perceived disk latency, but not the maximum one, at the price of keeping disc blocks in *VOLATILE* storage. A caching disc controller is only useful if you have no control over either (e.g. you have reached the limit of expandability of your system), and you can guarantee that its cache is not volatile. The OS or DBMS caches need not be guaranteed non volatile because the OS or DBMS can bypass them for safe writes, while the volatile storage in a caching disc controller is used even for writes to what the OS or DBMS think are raw devices. To have good disk performance, you had better rely on tuning the OS, the DBMS, their cache sizes, and get fast disc controllers and fast discs (e.g. SCSI, multiple discs with multiple controllersfor overlapped io, and discs with tranfer rates of at least 1-2 Meg/sec. and access time of less than 18 ms.). -- Piercarlo "Peter" Grandi | ARPA: pcg%cs.aber.ac.uk@nsfnet-relay.ac.uk Dept of CS, UCW Aberystwyth | UUCP: ...!mcvax!ukc!aber-cs!pcg Penglais, Aberystwyth SY23 3BZ, UK | INET: pcg@cs.aber.ac.uk