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Why the Write Performance of the Oceanspace S2600 Storage Array Deteriorates?

Publication Date:  2012-07-22 Views:  138 Downloads:  0
Issue Description
After the Oceanspace S2600 storage array runs for a while, its performance suddenly deteriorates. The phenomenon is that data can be accessed normally but the I/O speed becomes dramatically low.
Alarm Information
Handling Process
Step 1 On the ISM, manually change the write policy from Write Through to Write Back.
Note: If other faults exist in the storage system, rectify those faults first. Otherwise, the write policy may fail to be changed.
Step 2 If the write policy is already Write Back, run the following command in debug mode to locate the slow disk.
iostat -d -x 2
Step 3 Send the displayed messages to Huawei engineers for further handling. 
Root Cause
If the previous phenomenon occurs, the LUN write policy needs to be checked. The write policy can be Write Back or Write Through. In Write Back mode, data on server is written to cache first and then to hard disks when service load is light, ensuring high system performance. However, if BBUs or UPSs are not configured, data inconsistency occurs upon power failures. In Write Through mode, data is written to hard disks directly, securing data consistency but adversely affecting system performance. It is recommended to set the write policy to Write Back. If the write policy of a storage device configured with BBUs or UPSs is set to Write Back, when a power failure occurs, the policy is automatically changed from Write Back to Write Through. However, after the system power-on, the policy remains to be Write Back, which deteriorates the system performance.
If this phenomenon occurs, the storage system may have a “slow disk” (disk that has a slow I/O processing speed). The slow disk needs to be located and replaced.
Step 1  Log in to the storage system as user admin, and check whether the write policy is Write Through.
Step 2  If the write policy is not Write Through, check whether a slow disk exists in the storage system.