Date: Thu, 26 Nov 2015 23:17:42 -0600 From: Adam Vande More <amvandemore@gmail.com> To: "Eugene M. Zheganin" <emz@norma.perm.ru> Cc: "freebsd-stable@freebsd.org" <freebsd-stable@freebsd.org> Subject: Re: high disk %busy, while almost nothing happens Message-ID: <CA%2BtpaK03027Va8WAw72BQQJXL6AbZy14oeTsNcfUBHvuxHbU6A@mail.gmail.com> In-Reply-To: <5657E031.4030703@norma.perm.ru> References: <5656CE96.9000103@norma.perm.ru> <5657E031.4030703@norma.perm.ru>
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On Thu, Nov 26, 2015 at 10:46 PM, Eugene M. Zheganin <emz@norma.perm.ru> wrote: > 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. > Data which isn't very compressible isn't a very great on a transparently compressed filesystem. Gzip is particularly bad at this. LZ4 may have had only a slight impact. Setting gzip-1 would have also been less overhead than the default gzip which I believe is gzip-6. > 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. > Well that's basically what you had. And %busy is not really meaningful. L(q) and ops are the ones to keep an eye on. -- Adam
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