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Re: The old raw devices chestnut.

From: Dave Hughes <dave_nospam_at_waveform.plus.com>
Date: Tue, 13 Apr 2004 12:47:21 GMT
Message-Id: <xn0dgz63a29zwna000@usenet.plus.net>


On 2004-04-13, Mark A scribbled:

> Raw disk has the potential for best performance with large tablespace
> and index scans in a data warehouse environment. During sequential
> prefetch, DB2 can make sure that the extents are contiguous on the
> disk if the space is DMS raw disk. It makes no difference for OLTP
> systems. Decision support systems that do not frequently use large
> tablespace scans, also will show very little benefit. To the extent
> that data is already in the buffer pool, it makes no difference.

One other point to note regarding DB2's DMS (Database Managed) table-spaces is that they don't /have/ to be on raw devices - you can use file "containers" instead (just a great big file on the file-system the entire contents of which is managed by DB2). Assuming the file system has sufficient contiguous space (or you defrag after creating one) you get the benefit of contiguous data without the complexity of raw devices.

Though I haven't tried raw devices on DB2, I regularly use DMS file-based containers and have found them to be noticeably faster than the default SMS (System Managed) table-spaces (which are mostly individual files for individual database objects) with queries against massive tables.

I'd be interested to know if anyone's directly compared DB2's DMS table-spaces on raw devices and file system containers. I suspect there'd be a difference - but I doubt it'd be huge.

> Software RAID does not perform well. If striping is desired, superior
> performance can usually be achieved by placing DB2 containers on
> different physical drives (or different hardware arrays) and letting
> DB2 stripe the data across the containers.

Well, I do recall an interesting review of hardware and software RAID devices by Anandtech a while ago (all on Wintel boxes), in which the hardware RAID devices were outperformed by every single software RAID device they were trying. Still, benchmarks have never been a great way of measuring real-world performance, and this was only on Wintel, so YMMV.
> These comments relate to DB2 and are different with other DBMS'.

Same here.

-- 
Dave
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Received on Tue Apr 13 2004 - 07:47:21 CDT

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