On machines with large numbers of SCSI adapters and disks a significant amount of time can be spent probing for disks. By default Linux probes disks serially, but there are options to parallelise this.
The two phases of disk probe that can be parallelised are:
- Adapter probe. This is where each adapter is probed, reset and allowed to settle. This can be parallelised with the “scsi_mod.scan=async” boot option.
- Disk probe. This is where each disk behind an adapter is probed. The disk is spun up if it isn’t already spinning. This can be parallelised with the “fastboot” boot option.
To highlight the importance of parallelising both parts of disk probe, I ran three tests and measured the time it took to get to userspace. I used a POWER5 system with 4 SCSI controllers and 13 disks as a benchmark system. The disks were not spinning when Linux was booted.
- Serial adapter and disk probe: 88 seconds
- Parallel adapter probe, serial disk probe: 67 seconds
- Parallel adapter and disk probe: 15 seconds
On this system, full parallelisation is over 5 times faster.