high disk %busy, while almost nothing happens

[Eugene M. Zheganin]

Hi.

On 26.11.2015 14:19, Eugene M. Zheganin wrote:
> Hi.
>
> I'm using FreeBSD 10.1-STABLE as an application server, last week I've
> noticed that disks are always busy while gstat shows that the activity
> measured in iops/reads/writes is low, form my point of view:
>
>
>   L(q)  ops/s    r/s   kBps   ms/r    w/s   kBps   ms/w   %busy Name
>      8     56     50     520  160.6      6    286  157.4  100.2  gpt/zfsroot0
>      8     56     51   1474  162.8      5    228  174.4   99.9  gpt/zfsroot1
>
> These %busy numbers arent't changing much, and from my point of view
> both disks do very little.
>
The thing is, it was the compression. As soon as I cleared the gzip
compression from busy datasets, %busy went down, almost to zero.
Affected datasets were filled with poorly compressionable files, mostly
archives or zlib-compressed data.
And this is kind of counter-intuitive: one could think that worse-case
scenario would be redundant CPU load, with constand disk i/o. In
practice, otherwise, high disk %busy happens.

Could someone explain that ?
I only found this because of the flow-capture was starting like for
years, and I started to suspect the compression setting.

Eugene.