Database Reference
In-Depth Information
team does not need to be extended either. In short, Fiber Channel is a great solution for established data centers
to quickly get some of the benefits of flash memory. Using Fiber Channel to address “SSD” inside an array is a bit
slower, though—the Sane SAN 2010 paper explains the difference in great detail. Essentially the round trip time of
Fiber Channel is higher than the blazing fast Remote Direct Memory Access available with Infiniband's SCSI RDMA
Protocol (SRP).
Some vendors exploit the capabilities of PCIe and allow the external array to be connected via PCIe card plugged
into the database server to the array instead of Fiber Chanel. All storage communication has to use that wire, and the
storage system behaves as if it were directly plugged into a PCIe slot. Thus the storage system benefits greatly from
PCIe's high bandwidth and low latency properties. Although again a very fast solution, this design does not allow
the array to be shared between hosts, the same limitation as with directly attached PCIe cards. The best, although
probably most expensive, way to attach an external array to the database hosts is to use Infiniband and the previously
mentioned SCSI RDMA Protocol or SRP.
USING INFINIBaND tO traNSpOrt SCSI COMMaNDS
the Small Computer System interface has proven remarkably versatile over the many years of its existence. SCSi
has mostly been used in expensive enterprise hardware initially to address hard disks or tape drives when it came
out in the 1980s. Like so many protocols it started out as a parallel bus but has been replaced by a serial version,
which is known as Serially attached SCSi, or SaS for short.
SaS has emerged as the de-facto standard for enterprise-class direct attached storage, in which the stricter rules
about signaling and cable-length do not matter as much. the current implementations of the SCSi protocol do not
allow for more than 10-20m cable length.
the cable-length limitations have been addressed with the introduction of Fiber Channel. FC is a networking
protocol, and despite being capable of transporting many workloads, it has found its niche as the predominant
form of transporting SCSi commands over a distance. almost every single modern database server is
“San-connected,” which means it has its storage provided by an external array. Fiber Channel is divided into a
number of protocol layers that remind you of the iSo/oSi layers; the upper level layer is primarily concerned with
mapping SCSi to the new method of transportation.
Fiber Channel has evolved over time from gigabit Fiber Channel in 1997 to 16 gB Fiber Channel available in 2011.
Since changing from one generation to another requires investment in supporting infrastructure, it is expected
that 8 gB FC will remain mainstream for a few more years.
Some users thought that the evolution of Fiber Channel as the method of choice for transporting SCSi commands
did not happen quickly enough. alternative approaches to the lower level protocols of Fiber Channel are Fiber
Channel over ethernet, internet SCSi (iSCSi), and other less widely used ones. Most of these additional protocols
use layers of the tCp/ip stack. the idea behind that approach is to be able to make use of existing know-how and
potentially infrastructure—ethernet is well understood.
one of the fastest (but also most expensive) ways to transport SCSi commands today is by using remote direct
Memory access, rdMa. rdMa is often implemented as infiniband, which has seen a lot more widespread use
since the introduction of oracle's exadata. rdMa allows zero-copy networking between hosts, bypassing many
parts of the operating system, taking load off the Cpus, and considerably benefitting the latency of operations.
of the many uses infiniband permits, the SCSi rdMa protocol is one and is most useful for general purpose
storage solutions.
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