Handy Tech & Associates LLC

 

 
Storage issues

Many factors affect storage subsystem performance. If the bus is too narrow or controller bandwidth is too low, read/write requests will be delayed and the disk drives might be underutilized. If the drives themselves are too slow, read/write responses will be delayed.

If both bandwidth and the drives are experiencing excessive utilization, then the overall system is approaching saturation. Excessive page file hits can reduce the I/O bandwidth needed for other transactions.

If an array controller is present, its read/write priority and stripe-size setting can also have an effect. The type and speed of the drive are also factors.

Physical and logical I/O

To optimize the storage subsystem, it is important to understand the difference between physical and logical I/O.

  • Physical I/O — The number of I/O requests the array controller sends to the disk drives, using the array controller as the reference point. This calculation includes RAID overhead when applicable.
  • Logical I/O — The number of I/O requests the operating system or the application sends to the array controller for processing. It does not include RAID overhead because hardware RAID is transparent to the operating system and application.

    Example

    If the application sends 1,000 logical requests per second to the array controller with RAID 5, and 750 are reads and 250 are writes, the resulting physical I/O is as follows:

     
    • 750 physical reads resulting from 750 logical reads
    • 250 x 4 physical I/O transactions resulting from 250 logical writes
    • Total of 1,750 physical I/Os

    Each physical disk drive should not be servicing more than 150 physical random I/O requests per second. Therefore, in this example, 10 drives would be saturated by this I/O profile (1,750 physical I/Os ÷ 10 drives = 175 I/Os per second per disk drive), but 15 drives would not be saturated.

Write performance is impacted more by RAID level than read performance. RAID 0 has the least impact on write performance, and RAID Advanced Data Guard (ADG) has the most impact. The impact on write performance is caused by the data redundancy overhead associated with a RAID level:

  • RAID 0 — Provides no data redundancy, and thus has no impact on performance.
  • RAID 1+0 — Generates two physical writes for every logical write.
  • RAID 5 — Generates four physical I/O requests (two reads and two writes) for every logical write.
  • RAID ADG — Generates six physical I/O requests for every logical write.

Therefore, when monitoring the storage subsystem, you must consider the RAID level and how it impacts performance. Other factors that impact storage subsystem performance include:

  • Application environment and the associated I/O profile
  • Amount of memory and memory configuration
  • Amount of array controller cache and its configuration