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Tag: DDRdrive

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.

A key component of the ZFS Hybrid Storage Pool is Logzilla, a very fast device to accelerate synchronous writes. This component hides the write latency of disks to enable the use of economical, high-capacity drives. In the Sun Storage 7000 series, we use some very fast SAS and SATA SSDs from STEC as our Logzilla &mdash the devices are great and STEC continues to be a terrific partner. The most important attribute of a good Logzilla device is that it have very low latency for sequential, uncached writes. The STEC part gives us about 100μs latency for a 4KB write — much much lower than most SSDs. Using SAS-attached SSDs rather than the more traditional PCI-attached, non-volatile DRAM enables a much simpler and more reliable clustering solution since the intent-log devices are accessible to both nodes in the cluster, but SAS is much slower than PCIe…

DDRdrive X1

Christopher George, CTO of DDRdrive was kind enough to provide me with a sample of the X1, a 4GB NV-DRAM card with flash as a backing store. The card contains 4 DIMM slots populated with 1GB DIMMs; it’s a full-height card which limits its use in Sun/Oracle systems (typically half-height only), but there are many systems that can accommodate the card. The X1 employs a novel backup power solution; our Logzilla used in the 7000 series protects its DRAM write cache with a large super-capacitor, and many NV-DRAM cards use a battery. Supercaps can be limiting because of their physical size, and batteries have a host of problems including leaking and exploding. Instead, the DDRdrive solution puts a DC power connector on the PCIe faceplate and relies on an external source of backup power (a UPS for example).


I put the DDRdrive X1 in our fastest prototype system to see how it performed. A 4K write takes about 51μs — better than our SAS Logzilla — but the SSD outperformed the X1 at transfer sizes over 32KB. The performance results on the X1 are already quite impressive, and since I ran those tests the firmware and driver have undergone several revisions to improve performance even more.

As a Logzilla

While the 7000 series won’t be employing the X1, uses of ZFS that don’t involve clustering and for which external backup power is an option, the X1 is a great and economical Logzilla accelerator. Many users of ZFS have already started hunting for accelerators, and have tested out a wide array of SSDs. The X1 is a far more targeted solution, and is a compelling option. And if write performance has been a limiting factor in deploying ZFS, the X1 is a good reason to give ZFS another look.

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