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Luca Bert, director of architecture and strategy for DAS storage at LSI, says, “You should not mix SAS and
SATA in the same logical drive, or virtual disk. You need to create separate virtual disks.” He also recommends
keeping the different drive types as physically separate as possible-ideally in different enclosures-to avoid the
rotational vibration problems.
“Mixing SAS and SATA drives is definitely the wave of the future, and we haven’t run into any problems, but
we do encourage users to put the different drive types in different enclosures,” says Mike Joyce, senior director
of marketing at Promise Technology.
Controller and subsystem vendors are also addressing the RAID reliability issue with software that increases
reliability by proactively scanning and monitoring disks, and detecting-and potentially fixing-errors before they
occur. Xyratex, for example, uses a “predictive drive cloning” feature that clones a drive that may have errors
on it.
Despite the controversy and caveats associated with mixing drive types in the same enclosure, clear-cut benefits
exist. Mixing SAS and SATA drives enables in-the-box tiered storage where users can put frequently accessed
data on high-speed SAS drives and infrequently accessed data on lower-cost, lower- performance SATA drives.
Although this may require manual migration of data in some instances, a variety of vendors have software that
automates the migration of data between different drive tiers based on administrator-defined policies according
to, say, access frequency or by file/ data types. However, intermixing SAS and SATA drives in the same
enclosure is still rare among end users. But certain vertical markets have more of a need to intermix drive types,
and adoption of the practice in those markets is picking up. One example is the entertainment market and
applications such as collaborative, real-time editing of graphics, animation, and special effects, which can
require a combination of high-speed disk access and very high capacities for storing inactive files or data.
In these markets, some vendors are going beyond two-tier schemes (e.g., SAS and SATA drives) into three-tier
configurations. For example, at last month’s Siggraph show, Apace Systems introduced its fxStor RAID-5 disk
arrays, which allow users to mix 15,000rpm SAS drives, 7,200rpm SATA drives, and very high-speed solid-
state disks, or SSDs. (Specifically, fxStor arrays include 32GB of DRAM configured as SSDs, or RAM disks.)
A combination of firmware and software manages migration of data among the three tiers.
Apace’s fxStor NAS arrays come with Gigabit Ethernet interfaces that can be upgraded to 10Gbps Ethernet via
cards from vendors such as Chelsio. Two pre-configured models, both of which are based on enclosures from
AIC, are available:
•
The 3U FX3000-3U6T (based on AIC’s RSC-3E chassis) includes four 147GB SAS drives (588 GB
total capacity), 12 500GB SATA drives (6TB), 32GB of RAM, two dual-core 64-bit CPUs, and four
Gigabit Ethernet ports in a RAID-5 configuration.
•
The 4U FX3000-4U8T (based on AIC’s RSC-4E enclosure) includes eight 147GB SAS drives (1.2TB),
16 500GB SATA drives (8TB), 32GB of RAM, two dual-core Opteron CPUs, and six Gigabit Ethernet
ports in a RAID-5 configuration.
Software-based RAID
