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Remove an obsolete comment in devstat(3) about the accuracy of the

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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:
Kenneth D. Merry 2015-02-27 02:44:12 +00:00
parent 020a664210
commit 999efd901b
1 changed files with 3 additions and 21 deletions
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24
lib/libdevstat/devstat.c
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@ -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;