System and method for improved rebuild in RAID

A method for RAID rebuilds for a plurality of drives in a storage system comprising the steps of:

• determining average rebuild IO response times in a RAID system operating rebuilds;
• computing a delay by which to delay issuance of a rebuild IO to the head of queue of a drive in the RAID according to parameters of average rebuild response times from the queue, a time allotted for rebuild compared to total time and the number of rebuild threads;
• and,
• issuing rebuild IOs to the queue using a SCSI IO tag head-of-queue if the average rebuild response time exceeds a threshold and delaying execution of the rebuild IOs by the delay.

An apparatus for RAID rebuilds for RAID rebuilds under host IO conditions comprising:

• means for measuring rebuild IO responses in a RAID storage system;
• means for queuing for storing rebuild IO and host IO requests for said RAID storage system;
• means for computing a delay to delay issuance of said rebuild IO requests to the head of said queue comprising a delay time in milliseconds;
• means for issuing rebuild IOs to head of said queue and delaying the execution of the rebuild IOs by said delay time;
• means for setting a rebuild rate for said RAID system;
• said RAID storage system means for measuring rebuild IO responses, means for computing delay and means for issuing rebuild IOs operate in a periodic manner, wherein,
• said means for measuring rebuild IO responses in a first time period computes the average rebuild response times of the RAID system when operating in a traditional manner, until such time either one of the response rate of the RAID system and the response times of the RAID system rise above predetermined values;
• said means for computing a delay in a second time period computes the optimal delay by computing the average rebuild response time when said RAID system is operating with said delay instructions added to head of said drive queue, the rebuild rate of said means for setting a rebuild rate, and the number of threads in said RAID system;
• and,
• said means for issuing rebuild IOs delays issuance of a rebuild IO request in a third time period to head of said drive queue by said optimal delay computed in said second time.