Dell PERC controllers run two kinds of preventive background work. Patrol read scans the drives for media problems before they cost you data. Consistency check (CC) verifies that mirror or parity data matches the data it protects. Neither replaces backups, but both catch problems while redundancy is still there to fix them. This guide explains what each does, what it skips, and how to run them, based on the Dell PERC 11 and PERC 12 user guides and the iDRAC9 7.xx User's Guide.
Before you start
- Expect some performance loss while a CC, BGI or rebuild runs. Plan manual runs outside peak hours.
- Both features only help redundant virtual disks (RAID 1, 5, 6, 10, 50, 60). CC cannot run on RAID 0.
- For iDRAC9 actions you need Login and Server Control privileges.
- Keep a current backup. A CC can only correct data while the virtual disk still has redundancy.
Patrol read, consistency check and BGI compared
| Operation | What it does | How it starts |
|---|---|---|
| Patrol read | Scans configured drives and resolves potential media problems | Automatically every seven days by default; can also be started manually |
| Consistency check | Verifies and corrects mirror or parity data, and reports inconsistencies as events | Manually (HII, iDRAC, RACADM); Dell recommends running it periodically |
| Background initialization (BGI) | Writes parity or mirror data on a newly created VD; also corrects parity but does not report it | Automatically after VD creation; cannot be started manually, and restarts within five minutes if cancelled |
What patrol read covers and skips
- It runs on all drives that belong to a virtual disk, including hot spares.
- It does not run on drives in Ready state or drives that are not part of a VD. The PERC 12 guide also excludes non-RAID disks.
- The iDRAC guide states that it does not run on SSDs. The PERC 11 guide states the seven-day default applies to SAS and SATA hard drives.
- It skips drives involved in a rebuild, replace member, full or background initialization, CC, RAID level migration or capacity expansion.
- It adapts to load: under heavy I/O it uses fewer resources, and iDRAC notes it suspends during heavy I/O and resumes afterwards.
Step 1 — Set or start patrol read in System Setup
- Press F2, open Device Settings and select the PERC.
- Go to Main Menu > Controller Management > Advanced Controller Properties and click Patrol Read.
- Use Start, Suspend, Resume or Stop as needed.
- On PERC 11, set Mode to Auto, Manual or Disabled. On PERC 12, the page also offers Rate and Correct Unconfigured Areas.
- Click Apply Changes.
Note: The PERC 12 HII steps do not list a Mode setting, and the iDRAC9 guide states that the PatrolReadMode attribute on PERC 12 controllers is immutable: a job to change it runs, but the value stays the same. Leave patrol read in its default automatic mode unless Dell support tells you otherwise.
Step 2 — Or manage patrol read from iDRAC9 and RACADM
- Go to Storage > Overview > Controllers and select the controller.
- Find the patrol read properties, choose the new value from the Action drop-down and pick an Apply Operation Mode.
- Click Apply.
RACADM equivalents from the iDRAC9 guide (start and stop work only when the mode is Manual):
racadm set storage.controller.<index>.PatrolReadMode {Automatic | Manual | Disabled}
racadm storage patrolread:<Controller FQDD> -state {start|stop}Dell notes that start and stop fail when the job runs if the controller has no virtual disks.
Step 3 — Run a consistency check
Warning: In the virtual disk Operations list, Check Consistency sits next to Delete Virtual Disk, Fast Initialization and Slow Initialization (PERC 11 HII), and the iDRAC Action list also offers Delete and Initialize. Those options erase data. Read the selection twice before clicking Go or Apply, and back up first.
From System Setup (PERC 11 and PERC 12):
- Open the PERC utility and go to Main Menu > Virtual Disk Management.
- Select the virtual disk (not RAID 0).
- From the Operations list, select Check Consistency and click Go.
- Select Confirm and click Yes.
From iDRAC9: go to Storage > Overview > Virtual Disks, select the controller, choose Check Consistency from the VD's Action drop-down, then Apply Now, At Next Reboot or At Scheduled Time. A running CC can be cancelled in real time.
From RACADM:
racadm storage ccheck:<VD FQDD>
racadm storage cancelcheck:<VD FQDD>The PERC 11 guide says recurring CC schedules are set in Dell OpenManage storage management. In HII, Abort Consistency Check on Error (under Advanced Controller Properties) decides whether a CC stops when it finds an inconsistency.
Step 4 — Tune the rates and read the results
In Advanced Controller Properties > Task Rates you can set the BGI, Consistency Check, Rebuild and Reconstruction rates (PERC 11), or BGI, Patrol Read and Consistency Check rates plus a Rebuild Operating Mode Priority on newer PERC 12 firmware. A higher rate finishes sooner but takes more from host I/O.
The PERC 12 guide documents these CC outcomes:
| Message (shortened) | Meaning and action |
|---|---|
| CC detected inconsistency parity on the virtual drive | The inconsistency was corrected. No action required |
| CC completed with uncorrectable errors on the virtual drive | Medium errors were not corrected; if the VD is not redundant or is degraded, data may be lost. Check VD and drive health and replace drives if needed |
| Inconsistency data cannot be logged, too many inconsistencies | Logging was disabled. Check VD and drive health and replace drives if needed |
If CC points to a failing drive, follow replace a failed PERC disk or predictive-failure drives.
Frequently asked questions
Do I need to run patrol read by hand?
Normally no. Dell states it runs automatically every seven days on configured drives and throttles itself under load.
Can I stop BGI to get performance back?
Only briefly. Dell notes BGI cannot be disabled permanently; if cancelled, it restarts within five minutes.
Checklist
- Patrol read left in automatic mode (or the mode change confirmed on PERC 11).
- CC run periodically on every redundant VD, outside peak hours.
- Abort Consistency Check on Error set to the behaviour you want.
- Task rates reviewed; not left at maximum on busy production servers.
- CC results read in the logs; uncorrectable errors followed up with drive checks.
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