Re: Raw devices (was Re: Berkeley DB license)u

> On Sat, 20 May 2000, Bruce Momjian wrote:
>
> > And there is the problem of cache wiping, where a large sequential scan
> > removes all other cached blocks from the buffer. I don't know a way to
> > prevent that one, though we could have large sequential scans reuse
> > their own buffer, rather than grabbing the oldest buffer.
>
> On some systems, you can specify (or hint) to the kernel that the file you
> are reading should not be buffered.

Well, I was actually thinking of the cache wiping that happens to our
own PostgreSQL shared buffers, which we certainly do control.

> The only (completely) real solution for this is to use raw devices,
> uncached by the kernel, without any filesystem overhead...

We are not sure if we want to go in that direction. Commercial vendors
have implemented it, but the gain seems to be minimal, especially with
modern file systems. Trying to duplicate all the disk buffer management
in our code seems to be of marginal benefit. We have bigger fish to
fry, as the saying goes.

--
Bruce Momjian | http://www.op.net/~candle
pgman(at)candle(dot)pha(dot)pa(dot)us | (610) 853-3000
+ If your life is a hard drive, | 830 Blythe Avenue
+ Christ can be your backup. | Drexel Hill, Pennsylvania 19026