Re: Raw device I/O for large objects

Index organized tables would do this and it would be a generic capability.

- Luke

Msg is shrt cuz m on ma treo

-----Original Message-----
From: Georgi Chulkov [mailto:godji(at)metapenguin(dot)org]
Sent: Monday, September 17, 2007 11:50 PM Eastern Standard Time
To: Tom Lane
Cc: pgsql-hackers(at)postgresql(dot)org
Subject: Re: [HACKERS] Raw device I/O for large objects

Hi,

> We've heard this idea proposed before, and it's been shot down as a poor
> use of development effort every time. Check the archives for previous
> threads, but the basic argument goes like this: when Oracle et al did
> that twenty years ago, it was a good idea because (1) operating systems
> tended to have sucky filesystems, (2) performance and reliability
> properties of same were not very consistent across platforms, and (3)
> being large commercial software vendors they could afford to throw lots
> of warm bodies at anything that seemed like a bottleneck. None of those
> arguments holds up well for us today however. If you think you want to
> reimplement a filesystem you need to have some pretty concrete reasons
> why you can outsmart all the smart folks who have worked on
> your-favorite-OS's filesystems for lo these many years. There's also
> the fact that on any reasonably modern disk hardware, "raw I/O" is
> anything but.

Thanks, I agree with all your arguments.

Here's the reason why I'm looking at raw device storage for large objects only
(as opposed to all tables): with raw device I/O I can control, to an extent,
spatial locality. So, if I have an application that wants to store N large
objects (totaling several gigabytes) and read them back in some order that is
well-known in advance, I could store my large objects in that order on the
raw device.* Sequentially reading them back would then be very efficient.
With a file system underneath, I don't have that freedom. (Such a scenario
occurs with raster databases, for example.)

* assuming I have a way to communicate these requirements; that's a whole new
problem

Please allow me to ask then:
1. In your opinion, would the above scenario indeed benefit from a raw-device
interface for large objects?
2. How feasible it is to decouple general table storage from large object
storage?

Thank you for your time,

Georgi

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