[Info-vax] Databases versus RMS

[Jan-Erik Soderholm]

Johnny Billquist wrote 2012-04-20 02:17:
> On 2012-04-19 21:18, Dirk Munk wrote:
>> Johnny Billquist wrote:
>>> On 2012-04-19 14.53, abrsvc wrote:
>>>> On Thursday, April 19, 2012 8:37:52 AM UTC-4, Paul Sture wrote:
>>>>> On Wed, 18 Apr 2012 18:08:52 +0200, Dirk Munk wrote:
>>>>>
>>>>>> Many hours, sometimes even a few days. Usually the disks are not
>>>>>> powered
>>>>>> by the batteries, only the RAM memory of the cache.
>>>>>
>>>>> I was given a demo of a RAM disk with real disk backup in the early
>>>>> 1990s. The battery in that was supposed to last long enough to copy the
>>>>> RAM contents to the physical disk in the event of a power fail.
>>>>>
>>>>> I never saw any of these deployed anywhere though. I wouldn't be
>>>>> surprised if they were too expensive.
>>>>>
>>>>> --
>>>>> Paul Sture
>>>>
>>>> Digital did work on SSD devices (Solid State Disk). I recall working
>>>> on the performance testing of these. The prototype for testing
>>>> simulated an RA81 disk and actually was breadboarded using the front
>>>> panel of the disk. I recall laughing at the drive's requirement to
>>>> "spin up" before it was available.
>>>
>>> DEC did SSD long before that.
>>> Anyone remember the ML11? It's a SSD for the Massbus, looking like any
>>> other massbus disk (well, apart from the fact that every massbus device
>>> have it's own identification, so you can see what it is, and thereby
>>> know the capacity of the device).
>>>
>>> Johnny
>>
>> That's right. We had a similar device connected to PDP11 at the
>> newspaper I worked at that time. But these were not SSD's, they could
>> not store data without power present. They were RAM disks.
>
> You cannot store data to a modern SSD today without power either...
> SSD - Solid State Device. Ie. no moving parts, no mechanics, just solid
> state electronics.
>
> But maybe you meant retain data without power. That might be. DEC was fond
> of putting batteries on memories to make them keep their contents, to get
> the same effect as for core memory.
> Not sure if the ML11 used that.
>
> RAM disk is normally a term used for when you take some of your ram, and
> implement a device driver to make it look like a disk to the OS.
>
> The ML11 looks like a disk to everything. It has a massbus interface, it
> only reads and writes disk blocks, doing DMA, using the normal massbus
> controller, and so on.
> But I can see the potential for calling it a ram disk.
>
>> And indeed there also was an SSB device with a RAM disk and a real disk
>> as backup. It had a battery to be able to write the data to disk in case
>> of a power failure.
>
> Not sure what a SSB device is. But what you descibe sounds pretty much like
> the cache every disk have nowadays.
>
> Johnny
>

The DECs SSD in SSB's (StorageWorks disks) was RAM based as far as I know.
That is, did not retain data at power loss. Was often used as one part
of a schadow-set so that all reads was doen from the RAM disks and all
writes went to both (and thus stored permanently).

Modern SSD (Solid State Disks) are more or less always Flash-based and
thus retain data at a power loss. Many uses a Flash SSD as their only
disks in theirs laptop replacing the original rotating hard disk.

Jan-Erik.