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

[terry+googleblog at tmk.com]

On Saturday, June 20, 2015 at 11:36:22 AM UTC-4, Stephen Hoffman wrote:
> That's XQP-level, and VMS would need to flag those controllers that had 
> TRIM, as well as then issuing the TRIM request for the erasure or the 
> deletion.   OpenVMS already has something similar.  (See below.)

It should be perfectly fine to always pass that info down to the appropriate driver. The driver can then simply discard the trim request (returning successful completion) if none of the devices it talks to supports TRIM, or it can pass the request along to a device if that device is known to support TRIM.

Drives that support TRIM can be queried for that status, and give the nature of VMS device support (if it isn't on the SPD, it isn't supported even if it might work), dealing with devices that get wrong is less of an issue than on Linux, Windows, etc. which usually maintain a list of devices with quirks in order to handle them differently.

On other operating systems the overhead of getting the TRIM requests all the way down to the driver level is miniscule, and I don't see that VMS would be different.

> Generic TRIM support can sometimes be dicy, as some controllers and 
> some SSDs don't do that very well and there have been cases of data 
> loss.  In short, it'd probably be on approved controllers by default, 
> and enabled by site-specific override on others.  (See link below.)

SSDs have gotten a lot better since they were introduced. If a drive gets handed a block number in a TRIM request, it had better do The Right Thing these days. I have been using flash-based SSDs for nearly 10 years and other than a race condition where a specific model of SSD doesn't show up on a fast reboot* I haven't had any problems. Earlier drives definitely had their problems (if the firmware failed, they'd report being something completely different and not allow access to the data, and sometimes a mandatory firmware upgrade would require a full reformat). I haven't seen any of that lately.

I've used non-rust storage on VMS systems for years - I had a Unibus something-or-other from Dataram years ago on a VAX 8600, and I used a more modern device (Emerald Systems, I think, though the memory has faded) on a DS20 which was a HVD SCSI device with 8GB of RAM, a battery, and a hard drive which stored the contents of the "disk" RAM in case of power failure.

Neither of those needed TRIM, of course.

* On some Windows systems, the BIOS can be configured for "quick boot" where almost all tests are skipped if this is a warm reboot of the system. In that case, one specific model of SSD would occasionally not finish its self-test before the BIOS tried to boot, giving the scary "No bootable media found" error.