[Info-vax] VMS and the (lack of the) TRIM facility.

[johnwallace4 at yahoo.co.uk]

On Saturday, 20 June 2015 10:28:45 UTC+1, Jan-Erik Soderholm  wrote:
> Dirk Munk skrev den 2015-06-20 10:28:
> > I suppose most of you will know what the TRIM facility is, but just to make
> > sure I will explain it.
> >
> > With flash memory you can do two things, you can write empty cells, or you
> > can erase written cells. You can not rewrite cells, like you would rewrite
> > written block on a disk.
> >
> > When you delete a file on a disk, not very much will happen. A directory
> > entry will be deleted, and somewhere the blocks of the file will be marked
> > as free. The actual contents of the blocks will remain unchanged, that's
> > why you sometimes can undelete them. When a new file needs those blocks,
> > they will be simply rewritten.
> >
> > If you would try to do the same thing with flash, you would get a problem.
> > Before the cells can be written again, they would need to be erased. So it
> > is an erase followed by a write procedure, and that is a rather lengthy
> > process. Your fast flash memory would suddenly become rather slow. That is
> > what happened in the early days of flash.
> >
> > You should also keep in mind that there is no direct correlation between
> > disk block numbers and the actual cells that are being used. The reason is
> > very simple, some blocks/cells would be rewritten (=erased + written) very
> > frequently, so they would reach their maximum number of erase/write cycles
> > very soon. The SSD has its own housekeeping to link disk blocks numbers to
> > cells.
> >
> > The solution the industry came up with is called TRIM. Now when you delete
> > a file, the SSD will get a list of blocks/cells it can erase, so on the
> > background all the cells of the deleted files will be erased, and when a
> > new file needs them they can be written immediately. If a block in a
> > database needs to be rewritten, the SSD will use erased cells to write the
> > new data, and will erase the cells previously used.
> >
> > As far as I'm aware VMS doesn't have TRIM yet, not even the Itanium
> > version. So that would make SSD's not very suitable for VMS.
> >
> > It becomes even more difficult when you're using an emulator, and your VMS
> > disks are in fact container files on a disk of the host operating system. A
> > delete operation of the guest operating system is not seen as a delete
> > operation by the host operating system, so even if the host supports the
> > TRIM facility, it can not use it if the guest issues a file delete. This
> > applies to all operating systems of course, not just to VMS.
> >
> > In my view the only way to use SSD's with VMS is never to allocate the full
> > size of the SSD for a VMS disk. Suppose you have a 256GB SSD, then you
> > would for instance allocate a 200GB disk on that SSD. That way there is
> > always 56GB of erased space available. As soon as the SSD needs to rewrite
> > a disk block, it can use cells from the 56GB of erased space, write the new
> > data, and erase the cells that were formerly in use. This also works with
> > emulated disks (container files) on the disk of the host operating system.
> > The disk of the host operating system should never allocate the full size
> > of the SSD, that way there is always enough erased cell capacity on the SSD.
> 
> What about large SAN's? When we moved our VMS disks over from the
> old HSZ80/HSG80 storage to a large IBM DS8000 system, the sysman
> there said that he first thought about putting "our" volumes on
> flash-only storage "since they was so small anyway".
> 
> Now, the SAN doesn't work with "container files", as far as I know.
> Each VMS file is seen as a file by the SAN also. And I would guess
> that any TRIM facility would be builtin into the SAN controllers...
> 
> Or is the TRIM command something that *must* be sent from the OS
> itself?
> 
> Jan-Erik.

SANs surely don't see "files", they see blocks?

I'm struggling with the sentence "Each VMS file is seen as a
file by the SAN also". In a UNIX or Windows setup, a NAS box
might see a file as a file (using NFS or SAMBA or whatever).
Not obvious how that applies in a block-oriented VMS+SAN setup.

Also, TRIM deals in blocks.

Have a think about that and see if you can rephrase your question?