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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: Sun, 20 Oct 2024 21:32:05 -0400
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On 10/20/2024 9:28 PM, Lawrence D'Oliveiro wrote:
> On Sun, 20 Oct 2024 21:17:06 -0400, Arne Vajhøj wrote:
>> On 10/20/2024 9:10 PM, Lawrence D'Oliveiro wrote:
>>> On Sun, 20 Oct 2024 20:32:41 -0400, Arne Vajhøj wrote:
>>>> On 10/20/2024 8:28 PM, Lawrence D'Oliveiro wrote:
>>>>> Transaction resilience is a standard thing with databases (and
>>>>> journalling filesystems) going back decades.
>>>>
>>>> Yes.
>>>>
>>>> But they can't do miracles.
>>>
>>> They can ensure, to a high degree of confidence, that the on-disk
>>> structure is consistent. That is to say, each transaction is either
>>> recorded as completed or not recorded at all, nothing in-between.
>>
>> Only if it can rely on a successful write not being lost.
> 
> In other words, that the disk controller is not lying to you when it says
> a write has completed?

Just that is is not lying when it says that it got it.

>>>> To be sure to come up ok after a system crash it is either write to
>>>> plates or write to a cache that will survive the system crash (raid
>>>> controller cache with battery backup).
>>>
>>> Unfortunately, that controller cache can’t guarantee any of these
>>> things: it can’t do miracles either, all it does is add another point
>>> of failure.
>>
>> Yes - it can.
>>
>> It is not impacted by a system crash.
> 
> Now you are really starting to sound like a believer in miracles ...

A system crash and restart will blank RAM and wipe out all OS
and filesystem caches - it will not impact the cache in the
RAID controller.

Arne