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Frequently Asked Questions (FAQ)
iSCSI Standards

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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