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What
is iSCSI?
The iSCSI standard is a new protocol that layers on top of TCP and is
designed to transport SCSI commands and data across an IP network, most
commonly across Ethernet.
What
is iSCSI good for?
There are many applications for iSCSI, but all are a result of three characteristics.
First, it breaks the distance barrier of SCSI. As SCSI gets faster, the
maximum cable length goes down. The length also decreases when more devices
are attached to the bus. The iSCSI protocol allows us to use Ethernet
wiring to connect SCSI devices together. Second, iSCSI permits more devices.
SCSI is limited to either 8 or 16 devices on a bus, which is no where
near enough for enterprise networking. Third, iSCSI allows us to use the
networking characteristics of Ethernet to share a given device with multiple
hosts. These features allow us to build a wide variety of storage networks.
Who
is developing the iSCSI standard?
The iSCSI standard is being developed by a working group under the Internet
Engineering Task Force (IETF). This group is best known for its RFCs,
which are documents that set standards for the Internet. The iSCSI documents
should be released from the working group in mid-2002, and become official
RFCs about a year later.
What
is the difference between block storage
and file storage?
File storage is what the user's application asks the operating system
to do. For example, a text editor has a file of arbitrary length, and
it gives it to the operating system to store. Block storage is what is
used at levels closer to the hardware. For example, disk drives can only
read and write in 512-byte blocks. (Tape drives can use fixed or variable
length blocks.) It is the job of the file system to change file-oriented
requests into block-level commands. Common file storage protocols include
NFS and CIFS. Common block storage protocols include SCSI, Fibre Channel,
and iSCSI.
Why
hasn't block storage over
Ethernet been done before?
There are two technologies that enable block storage over Ethernet. First,
is the advent of Gigabit Ethernet. This allows data transfer at 125 MBytes/sec,
which is comparable with the 80 and 160 MBytes/sec speeds found in SCSI.
Second, is the development of TCP/IP acceleration hardware. TCP/IP, which
is commonly used over Ethernet, is relatively slow when performed on the
host CPU (typically 2 - 10 MBytes/sec). With the TCP/IP stack implemented
in hardware, vendors have been able to demonstrate wire-speed data transfers.
What
is a TOE?
TOE is the acronym
for TCP/IP Offload Engine. It is a piece of hardware that "offloads"
the work of the TCP and IP protocols from the main CPU. The result is
a speed increase of 10x to 50x. The word TOE is used by vendors that sell
chip sets, and also by manufacturers of end-user interfaces such as PCI
or SBUS cards.
What
does an iSCSI device look
like to the operating system?
From a software perspective, an iSCSI device looks just like a locally-attached
SCSI drive. The SCSI commands are intercepted and directed to the iSCSI
layer. They are encapsulated, then sent out as TCP packets. At the far
end the reverse happens, so a SCSI command arrives at the device.
What
operating systems support iSCSI?
At the moment, none. OS support is likely to appear in the next few years,
but until then the only public code is two sets of Linux drivers written
by Intel and Cisco. Hardware vendors that sell hardware accelerated cards
provide a driver that makes their card look like a SCSI card, so the OS
thinks it is simply talking to a SCSI adapter.
When
will disk drives have an iSCSI interface?
Probably never. It is very expensive for a disk drive manufacturer to
maintain multiple interfaces. Their plan is to let other manufacturers
use SCSI and ATA drives to build storage arrays. The push for iSCSI devices
is threefold: 1) Disk arrays that have multiple disk drives and maybe
RAID hardware. 2) Tape drives that support a direct iSCSI connection.
3) SCSI devices (tape, disk, and other) that are supported through a SCSI-iSCSI
gateway box.
Do
regular Ethernet switches work with iSCSI?
Yes. From the network perspective, iSCSI traffic just looks like another
IP packet. All existing Ethernet switches and IP routers work transparently.
What
do Storage Routers do differently
than a regular router?
Storage Routers have
additional features that only apply to Storage Area Networks (SANs). The
primary one being support for iSNS to facilitate device discovery, access
control, and heartbeat monitoring. Some storage routers also have the
ability to convert between interfaces, such as SCSI-iSCSI, or Fibre Channel
- iSCSI.
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