Chris George from DDRdrive put together a great presentation at the OpenStorage summit looking at the ZFS intent log (ZIL), and how their product is particularly well-suited as a ZIL device. Chris did a particularly interesting analysis of the I/O pattern ZFS generates to ZIL devices (using DTrace of course). With writes to a single ZFS dataset, writes are almost 100% sequential, but with activity to multiple datasets, writes become significantly more non-sequential. The ZIL was initially designed to accelerate performance with a dedicated hard drive, but the Hybrid Storage Pool found a significantly better ZIL device in write-optimized, flash SSDs.
In the 7000 series, the performance of these SSDs — called Logzillas — aren’t particularly sensitive to random write patters. Less sophisticated, cheaper SSDs are more significantly impacted by randomness in that both performance and longevity can suffer.
Chris concludes that NV-DRAM is better suited than flash for the ZIL (Oracle’s Logzilla built by STEC actually contains a large amount of NV-DRAM). I completely agree; further, if HDDs and commodity SSDs continue to be target ZIL devices, ZFS could and should do more to ensure that writes are sequential.