# iSCSI terms

> The iSCSI and storage words BeamDisk and storage servers use, explained: initiator, target, LUN, IQN, portal, CHAP, discovery, persistent reservation, digest, thin provisioning, zvol, extent, sector size, SLOG, jumbo frames, IOPS, latency percentiles and flushes.

## iSCSI

### iSCSI

A standard way to use a disk over a network. The server provides a block device; the computer formats and uses it like a local disk.

### Initiator

The software on the computer that connects to iSCSI storage. BeamDisk is an initiator for the Mac.

### Target

What the server or SAN offers: a named endpoint with one or more disks behind it.

### LUN

A logical unit: one disk behind a target, numbered from 0. On TrueNAS, usually a zvol.

### IQN

An iSCSI qualified name, the address-like name of an initiator or target, such as `iqn.2005-10.org.freenas.ctl:projects`. Targets allow initiators by their IQN. BeamDisk calls this Mac's IQN its **initiator name**.

### Portal

The address and port where a target listens, usually port 3260.

### Discovery (SendTargets)

Asking a portal which targets it offers. BeamDisk's Find Disks does this.

### Session

A target's login from this Mac. BeamDisk opens one session per target, and a disk's page shows how long it has been up.

### CHAP and mutual CHAP

Login authentication with a shared secret. With mutual CHAP, the server also proves itself to the Mac.

### Persistent reservation

A claim on a disk, kept by the target, that stops other computers using it. BeamDisk uses it to give each disk to one Mac at a time.

### Digest

A checksum (CRC32C) on each command's header or data, to catch corruption on the network.

## Storage

### Thin provisioning

A disk that only takes space on the server as it's written. Deleted files' space goes back when it's reclaimed (SCSI UNMAP).

### Zvol

A ZFS volume: a block device carved from a ZFS pool, which TrueNAS offers as a LUN. A sparse zvol is thin-provisioned.

### Extent

TrueNAS's word for what a target serves: a zvol (or a file), with settings such as its sector size.

### Sector size

The smallest unit a disk reads and writes, set on the server: 512 bytes or 4 KiB. 512-byte disks are readable by every initiator.

### SLOG

A separate, fast log device for a ZFS pool. It shortens the wait when an app asks for data to be safely stored.

### Hold time

How long BeamDisk holds writes while a server isn't responding, before apps see errors: 60 seconds unless you change it.

## Network and performance

### MTU and jumbo frames

The largest packet a network carries: 1,500 bytes by default. Jumbo frames (MTU 9000) carry more data per packet, if every device on the path supports them.

### Throughput

How much data moves per second, in MB/s (1,000,000 bytes, as Finder counts), MiB/s (1,048,576 bytes) or Gb/s.

### IOPS

Input/output operations per second: how many reads and writes a disk completes each second. Small random requests are measured this way.

### Latency, p50 and p99

How long a read or write takes. p50 is the median: half finish faster. p99: all but 1 in 100 finish faster, which shows the slow outliers.

### Flush

A request to the server to save what it has received so far, so it survives a power cut. Apps and macOS send them to make data safe.