[Phil beat me to it]
Yes, the 0s are a result of integer division in DTrace/kernel.

On Jun 14, 2012, at 9:20 PM, Timothy Coalson wrote:

> Indeed they are there, shown with 1 second interval.  So, it is the
> client's fault after all.  I'll have to see whether it is somehow
> possible to get the server to write cached data sooner (and hopefully
> asynchronous), and the client to issue commits less often.  Luckily I
> can live with the current behavior (and the SSDs shouldn't give out
> any time soon even being used like this), if it isn't possible to
> change it.

If this is the proposed workload, then it is possible to tune the DMU to
manage commits more efficiently. In an ideal world, it does this automatically,
but the algorithms are based on a bandwidth calculation and those are not
suitable for HDD capacity planning. The efficiency goal would be to do less
work, more often and there are two tunables that can apply:

1. the txg_timeout controls the default maximum transaction group commit
interval and is set to 5 seconds on modern ZFS implementations.

2. the zfs_write_limit is a size limit for txg commit. The idea is that a txg 
be committed when the size reaches this limit, rather than waiting for the
txg_timeout. For streaming writes, this can work better than tuning the 

 -- richard

> Thanks for all the help,
> Tim
> On Thu, Jun 14, 2012 at 10:30 PM, Phil Harman <phil.har...@gmail.com> wrote:
>> On 14 Jun 2012, at 23:15, Timothy Coalson <tsc...@mst.edu> wrote:
>>>> The client is using async writes, that include commits. Sync writes do not
>>>> need commits.
>>> Are you saying nfs commit operations sent by the client aren't always
>>> reported by that script?
>> They are not reported in your case because the commit rate is less than one 
>> per second.
>> DTrace is an amazing tool, but it does dictate certain coding compromises, 
>> particularly when it comes to output scaling, grouping, sorting and 
>> formatting.
>> In this script the commit rate is calculated using integer division. In your 
>> case the sample interval is 5 seconds, so up to 4 commits per second will be 
>> reported as a big fat zero.
>> If you use a sample interval of 1 second you should see occasional commits. 
>> We know they are there because we see a non-zero commit time.


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