Döhr, Markus Hermeskeil wrote:
It does both...
the parameter _IOPROCS_SWITCH handles the "number" of I/Os after the
second/n'th thread gets the work to write to the harddisk. So if there's low
I/O it's indeed serialized, if one thread is too busy or the number of
concurrent I/Os is higher than _IOPROCS_SWITCH && _IOPROCS_PER_DEV is > 1
then it's executed in parallel.
this is starting to go over my head. Maybe SAP-DBs threading model makes async i/o obsolete, but I know that some databases do in fact support this feature. On Linux the only DB I know that does AIO is Sybase ASE but on other Unixes there are more, mainly because other Unixes have AIO in their standard kernels.
Here's what the people from sgi say about async i/o: > Preliminary experience with KAIO have shown over 35% improvement in > database performance tests. Unit tests (which only perform I/O) using > KAIO and Raw I/O have been successful in achieving 93% saturation with > 12 disks hung off 2 X 40 MB/s Ultra-Wide SCSI channels. We believe > that these encouraging results are a direct result of implementing a > significant part of KAIO in the kernel using split-phase I/O while > avoiding or minimizing the use of any globally contented locks. (http://oss.sgi.com/projects/kaio/)
As of Linux kernel 2.6 async i/o will be part of the standard kernel. Appearantly it will at some point be the standard Linux i/o:
> Asynchronous I/O: Linux Implementation
> • NEW: Effort led by Ben LaHaise http://www.kvack.org/~blah/aio/
> • Includes libaio
> • New file_operations struct elements
> aio_read(), aio_write(), aio_fsync()
> • Eventually all I/O will fall under AIO
> • Earlier implementation in Red Hat 7.3
> • Docs at: http://lse.sourceforge.net/io/aionotes.txt
(http://www.axian.com/pdfs/linuxkernel2.pdf)
Maybe AIO is a new feature for the 8.x release of SAP-DB, at least on Linux :-)
Arne
-- Institute of Shipping Economics and Logistics http://www.isl.org/ Arne Gehlhaar mailto:[EMAIL PROTECTED] Universitaetsallee GW1 Block A phone:+49.421.22096.49 D-28359 Bremen, Germany fax:+49.421.22096.55
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