[ILUG] benefits of raw i/o

[David Murphy]

Quoting <Pine.LNX.4.21.0006132005060.19518-100000 at rossi.itg.ie>
by Paul Jakma <paulj at itg.ie>:

> (please preface every paragraph, nay sentence, with: AIUI)

As I Understand It?

> the biggest benefit:

> serial disk i/o is faster.. much faster. (least it is on DU and
> IRIX). (i assume linux raw i/o is character device based, as it is
> on aforementioned Unix(TM)'s)

If by 'serial disk i/o' you mean 'sequential disk i/o', then yes, it
is, and will be on any OS that uses disks. As I said yesterday, moving
the disk heads is one of the slowest operations on a system -
sequential reads need fewer, shorter seeks than random reads.

> the database can therefore also optimise writes/reads to be as
> sequential as possible. With block I/O it can't really do it, as the
> OS is second guessing it by doing it's own buffering.

It's not the buffering, it's the filesystem - as you'll recall, with
ufs, and I presume ext2fs, once a file has more than X direct blocks,
the filesystem starts allocating indirect blocks, double indirect
blocks, etc. etc. - the upshot is the bigger the file, the more
pointers you have to follow around the disk. With an extent-based
filesystem, the typical commmercial example being Veritas File System
[VxFS], you could have a 4GB file, with the FS allocating it as just
one 4GB extent. Applications can influence the way VxFS allocates
files, hence you can approach the control you have with a raw disk,
while avoiding the inconvenience of raw partitions.

This is why you should ask questions if someone tells you they're
running Oracle on UFS. If they're running Oracle over NFS, the
question would be "Have you had your head examined recently?".

The buffering issue is essentially double-caching eating all your RAM
- if Oracle is caching the data in its SGA, and your OS is caching
that same data in its VM system, you may find you don't have much RAM
left for other things, like, say, the OS 8) VxFS has a potentially
useful feature, where it can decide if a given read should be buffered
or not, just be sure you've tuned the threshold - see:
http://www.sun.com/blueprints/0400/ram-vxfs.pdf

> Also: RAID systems are optimised for long sequential seeks, which
> helps...

Again, this is more the physics of disk drives than RAID systems per
so.

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