[Info-vax] Byte range locking - was Re: Oracle on VMS

[Craig A. Berry]

On 11/21/16 11:43 AM, David Froble wrote:

<discussion of SSIO and byte range locking snipped>

> Well, as I understand it, today's disks can have a minimum cluster size
> of maybe 8 blocks, but, while I'm not familiar how VMS does it, you can
> still read, and update, individual 512 byte blocks.  VMS has always had
> to work with disk cluster size.  I'm thinking that I need to get some
> understanding of how this is implemented.  Possibly the same technique
> could be used for updates not on disk block boundaries?
>
> This is starting to grab my attention.  Maybe I should be afraid?

There are some slides dated 2012 at:

<http://de.openvms.org/TUD2012/opensource_and_unix_portability.pdf>

that describe some of the reasons for SSIO and provide a high-level
description of the initial implementation. There's more detail here than
in anything else I've seen on the topic. The fact that the initial
implementation didn't work well enough to support PostgreSQL could mean
a lot of of things. SSIO involves a pretty complex interaction among
RMS, the XQP, the CRTL, and XFC, so there are a lot of things that could
go wrong. I believe VSI looked at it long enough to say "we're not doing
that right now" but they've kept it on their list of research projects.