In this document we will take a look at some common questions related to Partrol Read on DCA.
1. What is Patrol Read?
Patrol Read is used to proactively detect hard-drive media defects while the array is online and redundant, and then proceeds to recover data. A Patrol Read periodically verifies all sectors of physical disks that are connected to a controller, including the system reserved area in the RAID configured drives. Patrol read starts only when the controller is idle for a defined period of time and other background tasks are active, though it can continue to run during heavy I/O process
2. Does Patrol Read impact disk performance?
Patrol Read checks disk sectors by doing read on the disks periodically. It will use some IO resources (30% by default) which will affect the disk performance.
3. Do I need to manually schedule Patrol Read so it won't impact by GPDB or PHD performance?
Most of our customers don't report significant impact to the production by Patrol Read. But if you do have mission critical operations, you may schedule the Patrol Read at non-business hours.
4. How long we can hold the patrol read from running before starting it and without causing issue?
Background Patrol Read is designed to proactively detect hard-drive media defects. The more often you run it, the more likely you capture the media defects before the disks completely fail. By default, both DCA V1 and V2 have auto Patrol Read enabled and it runs once a week. Thus patrol read mechanism is a pre-emptive way to avoid data unavailability/data loss.
5. If we manually started Patrol Read and have to stop it before it completes, is it safe to do so?
That is ok.
6. If Patrol Read was stopped manually, when it was started again, would it continue from where it was left off or it will start from beginning?
It will restart from beginning.
7. How long does Patrol Read normally run?
It is related to the disk size and workload on the disk. You can try run the patrol read in your environment a few times and see how long it takes in general.
8. Useful Patrol Read command
on DCA V1 systems
-- Set Patrol Read mode
omconfig storage controller action=setpatrolreadmode controller=id mode=manual|auto|disable
-- Stop Patrol Read (Available when Patrol Read is set to manual)
omconfig storage controller action=stoppatrolread controller=id
-- Start Patrol Read (Available when Patrol Read is set to manual)
omconfig storage controller action=startpatrolread controller=id
DCA V2 systems
-- Get Patrol read info
CmdTool2 -AdpPR -Info -aAll
-- Stop Patrol Read
CmdTool2 -AdpPR -stop -aAll
-- Start Patrol Read
CmdTool2 -AdpPR -start -aAll
-- Disable Patrol Reads
CmdTool2 -AdpPR -Dsbl -aAll
-- Enable automatic patrol reads
CmdTool2 -AdpPR -EnblAuto -aAll
-- Set Patrol read rate (not available on V1)
CmdTool2 -AdpSetProp -PatrolReadRate -v(val) -aAll
where val is between 1 to 100; default is 5