Hey, all! Using iozone (with the sequential read, sequential write, random read, and random write categories), on a Sun X4240 system running OpenSolaris b104 (NexentaStor 1.1.2, actually), we recently ran a number of relative performance tests using a few ZIL and L2ARC configurations (meant to try and uncover which configuration would be the best choice). I'd like to share the highlights with you all (without bogging you down with raw data) to see if anything strikes you.
Our first (baseline) test used a ZFS pool which had a self-contained ZIL and L2ARC (i.e. not moved to other devices, the default configuration). Note that this system had both SSDs and SAS drive attached to the controller, but only the SAS drives were in use. In the second test, we rebuilt the ZFS pool with the ZIL on a 32GB SSD and the L2ARC on four 146GB SAS drives. Random reads were significantly worse than the baseline, but all other categories were slightly better. In the third test, we rebuilt the ZFS pool with the ZIL on a 32GB SSD and the L2ARC on four 80GB SSDs. Sequential reads were better than the baseline, but all other categories were worse. In the fourth test, we rebuilt the ZFS pool with no separate ZIL, but with the L2ARC on four 146GB SAS drives. Random reads were significantly worse than the baseline and all other categories were about the same as the baseline. As you can imagine, we were disappointed. None of those configurations resulted in any significant improvements, and all of the configurations resulted in at least one category being worse. This was very much not what we expected. For the sake of sanity checking, we decided to run the baseline case again (ZFS pool which had a self-contained ZIL and L2ARC), but this time remove the SSDs completely from the box. Amazingly, the simple presence of the SSDs seemed to be a negative influence - the new SSD-free test showed improvement in every single category when compared to the original baseline test. So, this has lead us to the conclusion that we shouldn't be mixing SSDs with SAS drives on the same controller (at least, not the controller we have in this box). Has anyone else seen problems like this before that might validate that conclusion? If so, we think we should probably build an SSD JBOD, hook it up to the box, and re-run the tests. This leads us to another question: Does anyone have any recommendations for SSD-performant controllers that have great OpenSolaris driver support? Thanks! -Gray -- Gray Carper MSIS Technical Services University of Michigan Medical School gcar...@umich.edu | skype: graycarper | 734.418.8506 http://www.umms.med.umich.edu/msis/
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