[Info-vax] Databases versus RMS

[VAXman- at SendSpamHere.ORG]

In article <jmmds1$cfl$1 at Iltempo.Update.UU.SE>, Johnny Billquist <bqt at softjar.se> writes:
>On 2012-04-18 14:48, VAXman- @SendSpamHere.ORG wrote:
>> In article<jmm94e$atj$1 at Iltempo.Update.UU.SE>, Johnny Billquist<bqt at softjar.se>  writes:
>>> 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...
>>
>> Ignoring writeback caching, when the $QIO has completed the data should
>> be on the disk or, at least, in the hands of a storage controller; what
>> happens to it from that point on is not a VMS issue per se.  The answer
>> I posted was directed directly at the comment about RMS and immediately
>> writing to disk.  There are way too may details in RMS that made that a
>> completely wrong statement.  MBC, MBF, WBH, process and global buffers,
>> etc.  I've been dealing with all of this for more than 2 years whilst I
>> have been working on CDC.  I'm thankful that RMS does do all of this or
>> it would have been impossible (short of performing extra I/O, both ugly
>> and a performance issue) to obtain a before image!
>
>Hmm. If that is so, then VMS works rather different from RSX. In RSX, 
>disk caching in the OS happens under the QIO layer. Do VMS not have disk 
>caching? Or how does it deal with sharing between different processes, 
>where I/O is hitting the same actual disk blocks?


...and in VMS.  There are various caches in VMS but mostly, due to the 
"belts and suspenders" (I attribute that one to Hein) approach to data
integrity, these are write-through.



>Or was that what you meant by the writeback caching comment?

Yes.  If you want the speed and data corruption of a UNIX file system,
you can configure that.  However, in most applications of VMS systems,
the most important and valuable asset is the data!

-- 
VAXman- A Bored Certified VMS Kernel Mode Hacker    VAXman(at)TMESIS(dot)ORG

Well I speak to machines with the voice of humanity.