iSCSI terms
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
Section titled “Initiator”The software on the computer that connects to iSCSI storage. BeamDisk is an initiator for the Mac.
Target
Section titled “Target”What the server or SAN offers: a named endpoint with one or more disks behind it.
A logical unit: one disk behind a target, numbered from 0. On TrueNAS, usually a zvol.
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
Section titled “Portal”The address and port where a target listens, usually port 3260.
Discovery (SendTargets)
Section titled “Discovery (SendTargets)”Asking a portal which targets it offers. BeamDisk’s Find Disks does this.
Session
Section titled “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
Section titled “CHAP and mutual CHAP”Login authentication with a shared secret. With mutual CHAP, the server also proves itself to the Mac.
Persistent reservation
Section titled “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
Section titled “Digest”A checksum (CRC32C) on each command’s header or data, to catch corruption on the network.
Storage
Section titled “Storage”Thin provisioning
Section titled “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).
A ZFS volume: a block device carved from a ZFS pool, which TrueNAS offers as a LUN. A sparse zvol is thin-provisioned.
Extent
Section titled “Extent”TrueNAS’s word for what a target serves: a zvol (or a file), with settings such as its sector size.
Sector size
Section titled “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.
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
Section titled “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
Section titled “Network and performance”MTU and jumbo frames
Section titled “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
Section titled “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.
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
Section titled “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.
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.