Storage device performance deteriorated的警告

目前我maintain的機房,是以VMWare ESXi 5.1為虛擬平台+Synology DS3611xs(群暉NAS)的iscsi target做為storage.但用vSphere Client去看event,發現有很多如下的warning:

Device naa.600140540eb1ab2d4bced370adb60dde performance has
deteriorated. I/O latency increased from average value of 5774 microseconds
to 139667 microseconds.

但虛擬機運作一切正常.我去VMWare的KB看,是說:
Some of the possible reasons why the device latency increased are:
1. Changes made on the target
2. Disk or media failures
3. Overload conditions on the device
4. Failover

看來是在怪storage有錯誤,或負載過重.我有warning的潔癖,請問要如何才能找出問題的根源呢?謝謝!