On 10/28/11 00:04, Mark Wolek wrote:

Still kicking around this idea and didn’t see it addressed in any of the threads before the forum closed. 


If one made an all ssd pool, would a log/cache drive just slow you down?  Would zil slow you down?  Thinking rotate MLC drives with sandforce controllers every few years to avoid losing a drive to “sorry no more writes aloud” scenarios. 




Interesting question. I don't think there's a straightforward answer. Oracle uses write optimised log devices and read optimised cache devices in it's appliances. However, assuming all the SSDs are the same then I suspect neither a log nor a cache device would help:

If there is a log then it is solely used, and can be written to in parallel with periodic TXG commit writes to the other pool devices.  If that log were part of the pool then the ZIL code will spread the load among all pool devices, but will compete with TXG commit writes.  My gut feeling is that this would be the higher performing option though.  I think, a long time ago, I experimented with designating one disk out of the pool as a log and saw degradation on synchronous performance. That seems to be the equivalent to your SSD question.

Similarly for cache devices the read would compete at TXG commit writes, but otherwise performance ought to be higher.


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