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From: =?UTF-8?Q?Arne_Vajh=C3=B8j?= <arne@vajhoej.dk>
Newsgroups: comp.os.vms
Subject: Re: RX2800 sporadic disk I/O slowdowns
Date: Sat, 19 Oct 2024 21:31:51 -0400
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On 10/19/2024 9:25 PM, Lawrence D'Oliveiro wrote:
> On Sat, 19 Oct 2024 21:22:08 -0400, Arne Vajhøj wrote:
>> On 10/19/2024 9:04 PM, Lawrence D'Oliveiro wrote:
>>> Here’s another question: why does a disk controller need a battery-
> backed-
>>> up cache? Or indeed any cache at all?
>>
>> Better performance.
> 
> Think about it: the OS already has a filesystem cache in main RAM. That
> runs at main RAM speeds. Whereas the disk controller is connected through
> an interface to the CPU suitable only for disk I/O speeds. So any disk
> controller cache is on the wrong side of that interface.

That cache is toast if the system crashes. So applications
that need to be sure data are written bypass that.

If data loss with crash is acceptable then OS cache is fine.

Arne