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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 21:52:16 -0400
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On 10/19/2024 9:48 PM, Lawrence D'Oliveiro wrote:
> On Sat, 19 Oct 2024 21:31:51 -0400, Arne Vajhøj wrote:
> 
>> 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.
> 
> You can say the same for applications keeping data in their own RAM
> buffers. It’s a meaningless objection.

The context is one where data loss is not acceptable. So data must be
persisted so that they can survive system crash and power failure.

File system cache and application cache are both no good in that case.

So it is raid controller with battery backup cache or no cache.

The first gives better performance than the second.

Very meaningful.

Arne