Some times ago I bought a STEC ZeusIOPS SSD with 18GB capacity. This disks comes out from a Sun ZFS Storage 7420 system. But it’s 3.5″ large and without a server which supports 3.5″ large SAS disk drives I couldn’t test the SSD. Today I was able to test the drive on a Fujitsu Primergy RX300 S5 server. I installed five 500 GB large SATA drives and my STEC ZeusIOPS SSD. The first disk contains an OpenIndiana installation, the rpool. The remaing four SATA drives are grouped as a ZFS RAIDZ2 pool. I exported a ZFS dataset over NFS and 1 GbE to a VMware ESX system. With a Ubuntu linux virtual machine I run several benchmarks.
The results without the SSD are 75-80 MBytes/s write (850ms latency), between 40 and 65 MBytes/s rewrite and 120 MBytes/s read performance. I did different runs with bonnie++ and iozone and achieved always similar values. During the tools did their benchmarks I watched the IO with “zfs iostat”. The write and rewrite results matched the numbers above. Reading lot’s of data from disk was not necessarry due a large enough ARC mem cache. That’s why the iostat output values was lower than 10 Mybtes/s.
Then I added the STEC SSD as log device to the ZFS pool and rerun all the tests. But I couldn’t believe the values!!! My benchmarks finished with only 45-50 MBytes/s write and 35-45 MBytes/s rewrite. Read performance didn’t changed, of course. The write latency exceeded 10000ms!!! Something went wrong but I don’t know what. I did the runs again and I watched the zfs iostat output parallel. But the output of zfs iostat throwed values always above 100 Mbytes/s. Sometimes I reached even values above 170 MBytes/s, but always more than 100 Mbytes/s. This is the maximum rate for a single 1 GbE connection! But the benchmark output was very different. They didn’t reached the results of the benchmark without the SSD. I was confused. I disabled the log device with the logbias option and set it to throughput. The benchmark result and the iostat results went back to 75-80 Mbytes/s write. I reenabled the log device with logbias=latency and I had again the benchmark result of max. 50 Mybtes/s write and big latency values but with an iostat ouput always over 100 Mbytes/s!
Something is wrong, but I don’t know what. :( Do you have an idea?