[Info-vax] Databases versus RMS

[Jan-Erik Soderholm]

Johnny Billquist wrote 2012-04-18 22:17:
> On 2012-04-18 20:53, Jan-Erik Soderholm wrote:
>> Johnny Billquist wrote 2012-04-18 19:53:
>>> On 2012-04-18 15:43, Bob Koehler wrote:
>>>> In article<jmm94e$atj$1 at Iltempo.Update.UU.SE>, Johnny
>>>> Billquist<bqt at softjar.se> writes:
>>>>>
>>>>> 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...
>>>>>
>>>>
>>>> OS cache? Are you dreaming UNIX? VMS has never cahced any data I
>>>> cared about without my explicitly telling it to do so.
>>>
>>> Really? I find it hard to believe. RSX have done caching of things
>>> from the
>>> disk at that low level since day one. F11ACP do some caching for the
>>> system, keeping the most recently referenced directories around in a
>>> cache
>>> in the ACP.
>>>
>>> Newer versions of RSX also do caching of individual blocks fully
>>> transparent at the OS level. It's selectable if you want writes to be
>>> write-back or write-through.
>>>
>>> That VMS would have none of this sounds unlikely, as well as very bad for
>>> performance.
>>>
>>>> Disks that cache should use a non-volatile memory to do so, or
>>>> contain internal power storage sufficient to flush the cache.
>>>
>>> Right. But that is still a cache that can cause surprising results,
>>> although I freely admit that the caches in the disks are normally very
>>> reliable.
>>>
>>> Johnny
>>>
>>
>> You are mixing read and write cache. Make sure you are talking
>> about the same thing before arguing... :-)
>
> Huh? They are not separate! If they were, you'd get some really bad results.
>
> But even disregarding that, I do not understand what you were commenting.
> Please explain.
>

I read a lot of discussing about "caching" but do not understand in
every case if it is read or write caching that is ment.

As an example (the part that triggered my comment) :

 >>>> OS cache? Are you dreaming UNIX? VMS has never cahced any data I
 >>>> cared about without my explicitly telling it to do so.
 >>>

This comment is obvisouly about *write* caching.
(Of course VMS caches *reads* of different things without
explicitly telling it to do to).

 >>> Really? I find it hard to believe. RSX have done caching of things
 >>> from the disk at that low level since day one. F11ACP do some
 >>> caching for the system, keeping the most recently referenced
 >>> directories around in a cache in the ACP.

And this (as a reply to the statement above) is abviously about
*read* caching.

Since they comment on two different things (read vs. write cache)
the whole discussion becomes a bit weird... :-)

Jan-Erik.


> Johnny
>