Remove an obsolete comment in devstat(3) about the accuracy of the
milliseconds per transaction (DSM_MS_PER_TRANSACTION) calculation. The comment was accurate many years ago when the kernel didn't record I/O times on a per-I/O basis, but now that we do collect that information in most areas, it isn't correct. The milliseconds per transaction values are correct, assuming the I/O duration has been recorded. Sponsored by: Spectra Logic MFC after: 3 days
This commit is contained in:
parent
020a664210
commit
999efd901b
@ -1487,22 +1487,9 @@ devstat_compute_statistics(struct devstat *current, struct devstat *previous,
|
||||
*destld = 0.0;
|
||||
break;
|
||||
/*
|
||||
* This calculation is somewhat bogus. It simply divides
|
||||
* the elapsed time by the total number of transactions
|
||||
* completed. While that does give the caller a good
|
||||
* picture of the average rate of transaction completion,
|
||||
* it doesn't necessarily give the caller a good view of
|
||||
* how long transactions took to complete on average.
|
||||
* Those two numbers will be different for a device that
|
||||
* can handle more than one transaction at a time. e.g.
|
||||
* SCSI disks doing tagged queueing.
|
||||
*
|
||||
* The only way to accurately determine the real average
|
||||
* time per transaction would be to compute and store the
|
||||
* time on a per-transaction basis. That currently isn't
|
||||
* done in the kernel, and would only be desireable if it
|
||||
* could be implemented in a somewhat non-intrusive and high
|
||||
* performance way.
|
||||
* Some devstat callers update the duration and some don't.
|
||||
* So this will only be accurate if they provide the
|
||||
* duration.
|
||||
*/
|
||||
case DSM_MS_PER_TRANSACTION:
|
||||
if (totaltransfers > 0) {
|
||||
@ -1512,11 +1499,6 @@ devstat_compute_statistics(struct devstat *current, struct devstat *previous,
|
||||
} else
|
||||
*destld = 0.0;
|
||||
break;
|
||||
/*
|
||||
* As above, these next two really only give the average
|
||||
* rate of completion for read and write transactions, not
|
||||
* the average time the transaction took to complete.
|
||||
*/
|
||||
case DSM_MS_PER_TRANSACTION_READ:
|
||||
if (totaltransfersread > 0) {
|
||||
*destld = totaldurationread;
|
||||
|
Loading…
Reference in New Issue
Block a user