About Verification

3DM2™ Online Help

About Verification
This topic is organized into the following:

| Overview of Verification

| What Verification Does

| Verification of Non-Redundant Units

| Verification of Redundant Units

| How Errors Are Handled

Overview of Verification
The verify feature confirms the validity of redundant data on redundant units and performs media scans on non-redundant units.
Regular weekly verification is a good idea, as it can provide early warning of a disk drive problem or failure. This allows you to replace drives before they fail.
You can start a verify manually or regular verification can be done automatically by enabling Auto-Verify. (See Starting a Verify Manually and Using Auto Verification.)
During verification, I/O continues normally, but with a slight performance loss, depending on your verify rate setting. You can adjust how much verification will slow performance by setting a rate at which it occurs. (See Setting Background Task Rate.) You can also postpone verification until a scheduled time. (See Scheduling Background Tasks.)

| Note: Not verifying the unit periodically can lead to an unstable unit and may cause data loss. It is strongly recommended that you schedule a verify at least 1 time per week. You can take advantage of the the Auto-Verify and Basic Verify Schedule to accomplish this.

What Verification Does
For a RAID 1 or RAID 10 unit, a verify compares the data of one mirror with the other. For RAID 5, RAID 6, and RAID 50, a verify calculates parity and compares it to what is written on the disk drive. For non-redundant units, it performs a media scan.
Verification checks each sector on a drive. This is important, because day-to-day use of the media may leave many sectors on a drive unused or unchecked for long periods of time. This can result in errors occurring during user operation. Periodic verification of the media allows the disk drive firmware to take corrective actions on problem areas on the disk, minimizing the occurrence of uncorrectable read and write errors.
Verifies can be performed through either 3BM (BIOS) or through 3DM2. In addition, they can be scheduled to run at preferred times, through 3DM2 or through the CLI, or can be run automatically during the Verify schedule window, if scheduling and the Auto-Verify feature are enabled.

| Mac User Note: Disregard the paragraph above, and replace with “Verifies can be scheduled to run at preferred times or can be run automatically during the Verify schedule window, if scheduling and the Auto Verify feature are enabled.”

Verification of Non-Redundant Units
Verification of non-redundant units (single disks, spares, and RAID 0 units) read each sector of a drive, sequentially. If a sector can’t be read, it is flagged as unreadable, and the next time the controller writes to that location, the drive reallocates the data to a different sector.
Verification of Redundant Units
Verification of redundant units reads each sector, working from lowest block to highest block. If verification cannot read data in a sector, dynamic sector repair is used to recover the lost data from the redundant drive or drives; this recovered data is written to the problem sector. This forces the drive to reallocate the defective sector with a good spare sector.
If the verify unit process determines that the mirrored drives are not identical or the parity is not correct, the error is corrected. For RAID 1 and 10, this involves copying the miscompared data from the lower port(s) to the higher port(s) of the mirror. For RAID 5, RAID 6, and RAID 50, this involves recalculating and rewriting the parity that was incorrect. AEN 36 (“Verify detected and fixed data/parity mismatch”) is posted to the Alarms page.
For RAID 1 and 10, verification involves copying the data from the lower port(s) to the higher port(s) of the mirror. For RAID 5 and RAID 50, this involves recalculating and rewriting the parity for the entire unit. If the unit is not redundant, a file-system check is recommended to correct the issue. If the errors persist and cannot be overwritten from a backup copy, perform a final incremental backup. You will need to replace the defective drive, recreate the unit, and reinstall the data.
How Errors Are Handled
Verification makes use of the same error checking and error repair techniques used during ordinary use of drives configured through 3ware RAID controllers.
When verification encounters an error, the controller typically retries the command. If there are cable CRC errors, there may be multiple retries including downgrade of the UDMA mode. If the error persists and is unrepairable (e.g., ECC errors), an error notification is issued to indicate the problem. (See AEN 0026 Drive ECC error reported.)
If the disk drive is part of a redundant unit that is in a redundant state (not degraded or rebuilding), then Dynamic Sector Repair automatically rewrites the redundant data to the error location to force the drive to reallocate the error location. A notification of repair is posted to the alarms list. The result is a restoration of drive and data integrity; the primary and redundant data are again both valid.
If the unit is not redundant, it is recommended that you perform a file-system check to correct the issue. Under Windows, you can do this by right-clicking on the Drive and choosing Properties; then on the Tools tab, click Check Now. Under Mac OS X, you can do this using the First Aid tab in the Disk Utility—select the disk on the left and then click Verify Disk. If verification encounters problems, you can then use the Repair Disk option on the same screen.
If the errors persist and cannot be overwritten from a backup copy, perform a final backup of files that have changed since your last backup. You will need to replace the defective drive, recreate the array, and reinstall the data.

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