On Thu, Feb 4, 2016 at 11:34 PM, Kouhei Kaigai <kai...@ak.jp.nec.com> wrote:
> I can agree that smgr hooks shall be primarily designed to make storage
> systems pluggable, even if we can use this hooks for suspend & resume of
> write i/o stuff.
> In addition, "pluggable storage" is a long-standing feature, even though
> it is not certain whether existing smgr hooks are good starting point.
> It may be a risk if we implement a grand feature on top of the hooks
> but out of its primary purpose.
> So, my preference is a mechanism to hook buffer write to implement this
> feature. (Or, maybe a built-in write i/o suspend / resume stuff if it
> has nearly zero cost when no extension activate the feature.)
> One downside of this approach is larger number of hook points.
> We have to deploy the hook nearby existing smgrwrite of LocalBufferAlloc
> and FlushRelationBuffers, in addition to FlushBuffer, at least.

I don't understand what you're hoping to achieve by introducing
pluggability at the smgr layer.  I mean, md.c is pretty much good for
read and writing from anything that looks like a directory of files.
Another smgr layer would make sense if we wanted to read and write via
some kind of network protocol, or if we wanted to have some kind of
storage substrate that did internally to itself some of the tasks for
which we are currently relying on the filesystem - e.g. if we wanted
to be able to use a raw device, or perhaps more plausibly if we wanted
to reduce the number of separate files we need, or provide a substrate
that can clip an unused extent out of the middle of a relation
efficiently.  But I don't understand what this has to do with what
you're trying to do here.  The subject of this thread is about whether
you can check for the presence of a block in shared_buffers, and as
discussed upthread, you can.  I don't quite follow how we made the
jump from there to smgr pluggability.

Robert Haas
EnterpriseDB: http://www.enterprisedb.com
The Enterprise PostgreSQL Company

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