Quantum 6-00360-15 manual Configuration, Unit of I/O, Converting MB/sec to I/O/ sec

Appendix D Quality of Service Guide Configuration

Configuration

Unit of I/O

The primary configuration for QOS is in the FSM configuration file. No client configuration is required, although there is a QOS tuning parameter that can be specified when the file system is mounted.

Real-time I/O is based on well-formed I/O. This means that for the purposes of determining bandwidth rates, well-formed I/O is characterized as being a stripe width in size. This makes the best utilization of the disks in the stripe group and maximizes the transfer rate. Internally, non-real-time I/O is tracked by number of I/O operations per second. An I/O operation is a minimum of a file system block size, and a maximum of the file system block size multiplied by the stripe breadth

(FsBlocksize * StripeBreadth).

FsBlockSize <= I/O <= (FsBlocksize * StripeBreadth)

Converting MB/sec to I/O/ sec

Typically, it is easier to qualify an I/O subsystem in terms of MB/sec that can be sustained. However, internally the file system tracks everything on an I/O/sec basis. Note that the file system tracks only non-real-time I/

O(that is, it gates only non-real-time I/O). An I/O is a minimum of the file system block size, and is typically the point at which the file system hands the request off to the disk driver (IoCallDriver in Windows, or a strategy call in UNIX).

The file system counts the number of I/Os that have taken place during a given second. If the number exceeds that which is allotted, the request is pended until I/O becomes available (typically in the next second). I/O is honored in FIFO fashion; no priority is assigned.

To convert between I/Os and MB/sec, SNFS uses a somewhat unique formula that quantifies I/O as well-formed. The rationale behind this is due to the way in which many video applications make real-time I/O requests. To optimize the disk subsystem, real-time I/Os are well-formed so they saturate the disks. In SNFS terminology, this would be an I/O that covers all of the disks in a stripe. This can be expressed as follows:

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