什麼是 iowait

IOWait in this context is the measure of time over a given period that a CPU (or all CPUS) spent idle

because all runnable tasks were waiting for a IO operation to be fulfilled.

當 High iowait 時

CLI Tools

# -m     Display statistics in megabytes per second.

iostat -m 5

...
avg-cpu:  %user   %nice %system %iowait  %steal   %idle
           5.19    0.00   10.68   63.28    0.00   20.85

Monitoring tool: Netdata

Log: Centos 7 - /var/log/message

... backup1 kernel: ata4.00: failed command: READ FPDMA QUEUED
... backup1 kernel: ata4.00: cmd 60/08:e0:10:11:be/00:00:52:00:00/40 tag 28 ncq 4096 in#012
                                 res 50/00:08:a0:11:be/00:00:52:00:00/40 Emask 0x10 (ATA bus error)
... backup1 kernel: ata4.00: status: { DRDY }
... 一堆堆重復 Error ...
... backup1 kernel: ata4: hard resetting link
... backup1 kernel: ata4: SATA link up 6.0 Gbps (SStatus 133 SControl 300)
... backup1 kernel: ata4.00: configured for UDMA/133
... backup1 kernel: sd 4:0:0:0: [sdd] FAILED Result: hostbyte=DID_OK driverbyte=DRIVER_SENSE
... backup1 kernel: sd 4:0:0:0: [sdd] Sense Key : Illegal Request [current] [descriptor]
... backup1 kernel: sd 4:0:0:0: [sdd] Add. Sense: Unaligned write command
... backup1 kernel: sd 4:0:0:0: [sdd] CDB: Read(16) 88 00 00 00 00 00 52 7d d9 10 00 00 00 08 00 00
... backup1 kernel: blk_update_request: I/O error, dev sdd, sector 1383979280
... backup1 kernel: sd 4:0:0:0: [sdd] FAILED Result: hostbyte=DID_OK driverbyte=DRIVER_SENSE
... backup1 kernel: sd 4:0:0:0: [sdd] Sense Key : Illegal Request [current] [descriptor]
... backup1 kernel: sd 4:0:0:0: [sdd] Add. Sense: Unaligned write command
... backup1 kernel: sd 4:0:0:0: [sdd] CDB: Read(16) 88 00 00 00 00 00 52 7d d9 20 00 00 00 08 00 00
... backup1 kernel: blk_update_request: I/O error, dev sdd, sector 1383979296
... backup1 kernel: ata4: EH complete