[Info-vax] Databases versus RMS

[David Froble]

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".

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 ....

RMS can be interesting, using multiple I/O buffers, and with "move mode" moving the 
desired data between I/O buffers and the buffer your program sees.  A write of the 
"current bucket" moves the data back to the I/O buffer, but the I/O buffer is flushed to 
disk when the file is closed, when the I/O buffer is needed for reading more data, or due 
to a call to flush the I/O buffer(s).