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

[Dirk Munk]

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.