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Thread: Disk I/O question
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04-26-2011, 12:50 PM #1Web Hosting Guru
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Disk I/O question
I just ran the command -
dd if=/dev/zero of=test bs=64k count=16k conv=fdatasync
and got this output:
16384+0 records in
16384+0 records out
1073741824 bytes (1.1 GB) copied, 27.9404 seconds, 38.4 MB/s
Good? Bad? Ugly? What do you think? What is a good disk I/O? What is bad disk I/O indicative of?
Thank you in advance for any insight / input!
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04-26-2011, 01:03 PM #2Web Hosting Master
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May I know the disk setup when you run this command?
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04-26-2011, 01:09 PM #3Web Hosting Guru
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Would love to give you that info! Uh... how do I find it? *blush* I am new to this and am still in the learning stage. Thanks for your help and patience!
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04-26-2011, 01:10 PM #4Web Hosting Evangelist
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It's acceptable. If it's a budget VPS, you can't expect them to have fast SAS drives in h/w RAID10.
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04-26-2011, 01:18 PM #5Web Hosting Guru
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Disk I/O varies? I ran it yesterday and got a much faster result. So I am curious as to how it works exactly and what factors in.
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04-26-2011, 01:25 PM #6Web Hosting Master
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disc IO very much depends on the setup which you can always get from your provider.
The values of IO generated from the command line is realtime data for IO speed. If you provider over loaded the server, the IO value will be affected.
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04-26-2011, 01:31 PM #7Web Hosting Master
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I don't that there is any "normal" IO value, its going to be purely dependent on the IO load at any given time. You only need as much IO as you can use. It would be more accurate to speak in terms of minimum expectations.
Keep in mind that dd is one test, hardly representative of a typically more random real-world scenario. Its a good barometer, but I would personally be more concerned about random IO.
Good/bad/ugly IO is relative, as long as IO wait is not causing problems for you, I wouldn't be too concerned about absolute benchmarks.
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04-26-2011, 01:47 PM #8Web Hosting Evangelist
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04-26-2011, 01:51 PM #9Web Hosting Guru
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04-26-2011, 02:09 PM #10Web Hosting Master
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04-26-2011, 02:10 PM #11Web Hosting Master
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This has been explained in layman words.
"Normal" here is referring to none overloaded server and without simultaneous high load processes run in each individual VPS.
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04-26-2011, 05:28 PM #12Junior Guru Wannabe
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I don't think you can make the statement, if its "budget VPS" you can't expect them to have SAS/Raid1+0. Not advertising at all, but, we, have SAS drives and Raid 10 in almost every server and we have a few "budget" level packages. Still not in the five dollar a month range, but you pay for what you get.
dd if=/dev/zero of=test bs=64k count=16k conv=fdatasync
on nine of our nodes and the slowest was:
16384+0 records in
16384+0 records out
1073741824 bytes (1.1 GB) copied, 4.96544 s, 216 MB/s
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04-26-2011, 09:43 PM #13Web Hosting Master
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High dd bench values just proves either:
a) your users aren't hitting your disk frequently enough at any given time to force the seek head off its current position
or
b) you have write-back cache enabled
To kjetterman, if your noticing that values fluctuate, then you're likely in the opposite of situation (a) where there are enough users at that time of day using the disk as well. No big deal.
The only time I'd worry with dd benchmarks is if it consistently falls below 10 MBps. That level still might not be bad enough to affect most users, but its generally considered a sign that the host is not doing enough to proactively manage their IO allocation.
Most will try to keep it at least above 50 just so people won't incorrectly complain that they have slow 'IO'.
Your iowait times is more indicative of how much slow-io is actually impacting you, not dd bench.
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