What Is iSCSI? Hardware Requirements and Why It Runs Slow
iSCSI carries storage traffic over ordinary Ethernet, which makes shared block storage possible without a separate fibre channel network. This covers what it is, what it needs from your hardware, and where it goes wrong.
What is iSCSI?
A protocol that carries SCSI storage commands over TCP/IP, so a server can use storage on another device as though it were a local disk.
The server runs an initiator; the storage device presents a target. The server sees a block device it can partition and format, exactly as it would a local drive. The difference is that the drive is somewhere else on the network.
That distinction matters: iSCSI gives block storage, which the server formats itself. A NAS protocol gives file storage, where the NAS owns the filesystem. Our storage architecture guide covers the difference and when each fits.
What hardware does iSCSI need?
Less than people expect, and more than the minimum suggests.
Standard Ethernet adapters work. No special hardware is required, which is iSCSI’s main advantage over fibre channel.
Bandwidth matters more than anything. 1GbE works for light use and becomes the bottleneck quickly. 10GbE is the practical minimum for anything doing real work, and 25GbE where the storage is fast. Our 10GbE adapter guide covers the hardware.
A dedicated network, or at minimum a dedicated VLAN. Storage traffic competing with general network traffic produces latency that looks like a storage fault. This is the most common iSCSI performance problem and it is a design error rather than a hardware one. Our segmentation guide covers separating it.
A switch with adequate backplane capacity for every port running at once, not just the port speed. Our switch guide covers what to check.
Do I need iSCSI HBAs or TOE adapters?
Usually not.
Dedicated iSCSI HBAs and TCP offload adapters move protocol processing off the host processor. On modern servers with plenty of cores, the software initiator built into the operating system performs well and the offload hardware is rarely worth its cost or its compatibility constraints.
Where offload still matters: very high throughput on hosts where processor headroom is genuinely tight, and boot-from-iSCSI configurations that need the adapter to work before the operating system loads.
Redundancy: multipath
A single network path to storage is a single point of failure, and losing storage is worse than losing a network link.
Multipath uses two network paths from host to storage, through separate adapters and ideally separate switches, with the operating system managing failover. Configure it properly and a switch failure is an event rather than an outage.
Two points. Two paths without multipath software configured is worse than one, because the operating system sees the same storage twice as separate devices. And the storage device needs two controllers for full redundancy, or the path is redundant up to a single point that is not. Our multipath guide covers the equivalent question on direct-attached storage.
Where iSCSI goes wrong
Four causes, in order of frequency.
Shared network. Storage traffic on the general network. Separate it.
Insufficient bandwidth. 1GbE for a workload that needs more.
Jumbo frames configured inconsistently. Larger frames reduce overhead and help throughput, but every device in the path — host, switch, storage — must agree. One device configured differently produces intermittent, hard-to-diagnose problems.
No multipath, or multipath configured wrongly.
Notice that three of the four are network configuration rather than storage. Our network monitoring guide covers spotting these.
iSCSI or fibre channel?
iSCSI where you want shared block storage on your existing Ethernet skills and equipment, at lower cost, with adequate performance for most workloads.
Fibre channel where you need the lowest latency, have an existing FC fabric, or run workloads sensitive enough to justify a separate dedicated network. Our fibre guide covers the cabling side.
For most organisations building shared storage today, iSCSI on dedicated 10GbE is the sensible default.
Frequently asked questions
What is iSCSI used for?
Presenting block storage from one device to a server over ordinary Ethernet, so the server can use it as though it were a local disk. It is the low-cost route to shared block storage without a fibre channel network.
What is the difference between iSCSI and NAS?
iSCSI gives block storage that the server formats itself. NAS protocols give file storage where the NAS owns the filesystem. Applications that expect a local disk generally need block storage.
Do I need special hardware for iSCSI?
No. Standard Ethernet adapters work with the software initiator built into the operating system. Dedicated iSCSI HBAs are rarely worth their cost on modern servers with cores to spare.
Is 1GbE enough for iSCSI?
For light use only, and it becomes the bottleneck quickly. 10GbE is the practical minimum for anything doing real work, and 25GbE where the storage itself is fast.
Why is my iSCSI storage slow?
Most often storage traffic sharing the general network. Then insufficient bandwidth, then jumbo frames configured inconsistently across host, switch and storage. Three of the four common causes are network configuration rather than storage.
Do I need multipath for iSCSI?
For anything where losing storage matters, yes: two paths through separate adapters and ideally separate switches. But two paths without multipath software configured is worse than one, because the OS sees the storage twice as separate devices.
Tell us your host count and the workload, and we will specify the adapters, switch and cabling for a storage network that will not be the bottleneck.
