Performance
These TwinStor algorithms are implemented within 3ware’s Escalade ATA RAID controllers to further enhance its impressive
level of throughput. Escalade card’s on-board CPU and firmware support this logic and combine it with its packet-switched
architecture to achieve breakthrough performance levels from standard, low-cost ATA drives. This results in performance
and affordability that can’t be attained with competing RAID solutions.
Previously, the highest performing disk architectures were implemented with SCSI drives and RAID controllers from
Adaptec, Mylex and others. While SCSI drives were higher in performance than ATA drives, the gap has now closed and
many manufacturers use identical head disk assemblies for their ATA and SCSI drives. The market dominance of ATA (87%
of the unit volume) assures that the value and availability of high-performance ATA drives will continue to exceed that of
SCSI drives.
Figure 3 is a graph comparing a single SCSI drive to a pair of drives using TwinStor technology. Both the SCSI and ATA
drives are 7200 RPM. The transfer rate of the ATA drive is slightly higher than the SCSI drive. The access time of the SCSI
drive is faster than the ATA drive. The street price of the SCSI drive is more than double that of the ATA drive, making the
TwinStor solution less expensive by about 10%. (Escalade controller prices have not been included here, as prices vary
among vendors.)
In this comparison, 3ware’s TwinStor solutions win in all categories. The streaming read and random read rates are
significantly higher than the single SCSI drive. The write rate is slightly higher for the TwinStor solution, showing that the
need to write to both disks does not reduce performance relative to writing to a single disk. Not shown is the huge advantage
in fault tolerance with the TwinStor solution. Using 9.1 GB drives, a terabyte requires just over 100 drives. With no
redundancy and an expected failure rate of one per 500K hours, the SCSI drives would be expected to have about one
failure every six months. With the data redundancy in the TwinStor solution, data loss happens only when a second drive
fails before there is a chance to repair the first failure. The mean time to failure (MTTF) of the pair is determined based on a
mean time to repair (MTTR) of three days using the standard formula: MTTFdual = MTTF2/2MTTR [3]. The chance that the
second drive will fail during the three-day repair time is extremely small (over 1,700 years per terabyte).
Figure 3. Single SCSI to TwinStor Comparison
