Date: Mon, 2 May 2011 22:49:32 -0700 From: Jan Koum <jan@whatsapp.com> To: Jeremy Chadwick <freebsd@jdc.parodius.com> Cc: freebsd-fs@freebsd.org, Chris Peiffer <chris@whatsapp.com> Subject: Re: very strange IO issue with FreeBSD 8 and SSD Message-ID: <BANLkTinchOrXFo%2B7RqV9-pf_2zFoBtVdeQ@mail.gmail.com> In-Reply-To: <20110503041718.GA34604@icarus.home.lan> References: <BANLkTin-qEoxxFbjJkDaA_-UZMkza08NNQ@mail.gmail.com> <20110502233601.GA29710@icarus.home.lan> <BANLkTik5tXegwoRvB7XAvpEPb385KjGEtA@mail.gmail.com> <BANLkTinQt4YZiudZUSgxL0x8dJ6MJTueRw@mail.gmail.com> <20110503041718.GA34604@icarus.home.lan>
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On Mon, May 2, 2011 at 9:17 PM, Jeremy Chadwick <freebsd@jdc.parodius.com>wrote:
>
> To emulate "iostat 1", you will need to run this from inside of a while
> loop via the shell. E.g. in sh or bash:
>
> while true; do gstat -b; sleep 1; done
>
>
sure:
$ sudo gstat -b | head -2 ; while true; do sudo gstat -b | grep 'a$'; sleep
1; echo; done
dT: 1.009s w: 1.000s
L(q) ops/s r/s kBps ms/r w/s kBps ms/w %busy Name
258 56 16 42 0.2 40 312 2.2 1.0 ad4s1a
288 76 20 81 0.2 57 387 4.0 1.2 ad5a
255 208 28 76 0.4 180 1977 12.1 3.1 ad6a
276 83 26 139 0.5 58 499 6.2 3.1 ad7a
0 17 16 40 0.2 1 4 0.2 0.4 ad4s1a
0 30 28 95 5.4 2 20 0.2 15.1 ad5a
0 2943 30 139 17.9 2913 46257 261.6 40.5 ad6a
0 24 23 82 0.2 1 4 1.6 0.6 ad7a
0 791 30 137 0.5 762 6897 24.2 16.1 ad4s1a
0 858 18 68 0.2 840 8261 35.7 16.7 ad5a
0 1308 18 46 1.7 1290 13023 25.5 22.1 ad6a
0 791 21 113 1.5 771 7320 19.8 21.3 ad7a
0 3152 26 77 18.1 3126 46089 236.0 44.0 ad4s1a
0 385 30 109 10.6 355 2420 11.4 28.1 ad5a
0 1263 25 107 11.5 1239 7172 37.3 27.8 ad6a
696 761 32 159 12.2 730 4510 22.5 31.1 ad7a
0 456 26 76 0.4 430 1892 19.0 9.4 ad4s1a
0 616 14 36 0.2 602 4971 20.3 8.6 ad5a
0 811 14 46 0.3 797 6186 27.0 10.4 ad6a
0 207 19 58 2.1 188 2982 25.2 10.3 ad7a
313 467 20 76 0.2 447 3834 19.2 4.6 ad4s1a
10 33 17 96 0.2 16 123 82.7 8.8 ad5a
3 32 16 62 0.2 16 98 0.3 0.6 ad6a
1 40 20 52 0.2 20 223 0.3 0.7 ad7a
151 1624 18 77 51.6 1606 10039 106.3 69.1 ad4s1a
25 232 8 22 95.1 224 3565 94.4 64.5 ad5a
0 868 15 48 0.2 854 7438 20.7 17.7 ad6a
0 821 11 73 1.2 810 8846 26.3 17.1 ad7a
> I believe your concern point that started the thread was that
> 4MBytes/sec was considered bad performance.
sorry, not quite... i am not judging "performance" - what i am trying to
get to the bottom of is why in the world would 500KB of file updates
(write/append) per second would generate so much IO
> There are indications from
> your iostat output that occasionally the writes are buffered and come in
> "in a burst" at 10-11MByte/sec, but your overall average is around
> 4-5MByte/sec.
>
>
we see higher averages, but OK -- don't think you 4-5MB/sec is still way too
high for the little IO application is doing?
(dd doesn't really reproduce the real life usage of filesystem with multiple
directories and threads using the underlying fs)
> I can safely say the conversation is going to immediately turn to "how
> does your application work?", including people asking for full source
> code and so on.
it is a very very very simple app built on top of erlang file module:
http://www.erlang.org/doc/man/file.html
> Unless I misunderstand, that's effectively what you're
> asking: "why does our application perform so badly on these SSDs?"
>
>
not really. what i am asking is: why is there so much IO overhead? where
is it coming from?
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