[Info-vax] Databases versus RMS

[Johnny Billquist]

On 2012-04-18 14:57, David Froble wrote:
> Johnny Billquist wrote:
>> On 2012-04-18 12:51, Dirk Munk wrote:
>>> VAXman- @SendSpamHere.ORG wrote:
>>>> In article<4f8dc739$0$1688$c3e8da3$50776f34 at news.astraweb.com>, JF
>>>> Mezei<jfmezei.spamnot at vaxination.ca> writes:
>>>>> {...snip...}
>>>>>
>>>>>> And, generaly speaking, Rdb is more "safe" then a plain RMS
>>>>>> based "database".
>>>>>
>>>>> An RMS write tends to result in an immediate physical write to disk.
>>>>> (unless hidden by a storage array which delays writes).
>>>>
>>>> That's not true!
>>>>
>>> Indeed it is not. Cobol for instance has the deferred write option.
>>
>> Even beyond any language issue, or RMS details, the OS can cache and
>> defer actual writes to the disk without you ever knowing about it. Not
>> to mention that disks also cache things...
>>
>> Johnny
>>
>
> If Cobol is doing a deferred write, ie; don't write the buffer until
> necessary, then that is NOT a "write to disk".

Right. And important to understand, as "RMS write" might mean different 
things to different people.

> It's my understanding that VMS does a "write through cache", which would
> result in a directive to the storage device to "write" the data. Now, if
> you're using a storage device that doesn't do as it's told ....

Unfortunately I have forgotten (if I ever knew) how VMS caching works. 
In RSX, writes can be deferred by the caching layer, which is located 
beneath the QIO processing in the OS.
So, even when you have done the actual QIO to write the disk block, it 
might still not be written out to disk yet. It's basically a write back 
cache.

 From a performance point of view, I'd be surprised in VMS could not 
offer the same, but from a data safety point of view, write back is 
preferable...

	Johnny

-- 
Johnny Billquist                  || "I'm on a bus
                                   ||  on a psychedelic trip
email: bqt at softjar.se             ||  Reading murder books
pdp is alive!                     ||  tryin' to stay hip" - B. Idol