Date: Fri, 27 Nov 2015 09:46:41 +0500 From: "Eugene M. Zheganin" <emz@norma.perm.ru> To: freebsd-stable@freebsd.org Subject: Re: high disk %busy, while almost nothing happens Message-ID: <5657E031.4030703@norma.perm.ru> In-Reply-To: <5656CE96.9000103@norma.perm.ru> References: <5656CE96.9000103@norma.perm.ru>
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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.
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