Brandon Simmons wrote:
On Fri, Mar 12, 2010 at 1:33 PM, Trond Myklebust
<trond.myklebust@xxxxxxxxxx> wrote:
On Fri, 2010-03-12 at 13:22 -0500, Brandon Simmons wrote:
I am using tiobench to test performance of an NFS mounted volume, and
notice that Sequential Reads are much slower than Random Reads. This
isn't the behavior when I run the same test on the disk mounted
locally.
For random reads I'm getting:
50 MB/s over NFS
v.s
384 MB/s when mounted locally
This is in comparison to the benchmark for _Random Reads_, in which I get:
288 MB/s both over NFS _and_ when directly mounted
The other benchmarks seem to be in line with what I would expect, but
I'm fairly new to NFS. Why would sequential reads over NFS be sooo
much slower than random reads over NFS?
They're not usually. My guess is that this is an artifact of your test.
What is tiobench doing prior to the sequential read?
Trond
I'm wondering if this is caused by caching. my benchmark does
Sequential Reads first, then it does Random Reads. Is it possible that
the random reads could be working on cached data?
That would mean my sequential reads aren't too slow, rather the random
reads are extraordinarily fast because of caching. Looking at running
some different benchmarks.
I think so too - since your throughput are way too good (when compared
with other I/O-bound numbers). Using simple benchmark(s) such as
tiobench can generate very misleading results if you don't know how to
steer away from cache effects. For NFS workload, SPECsfs is a much
better benchmark but it is cumbersome to run (and you have to pay for
it, I think).
In general, however, I find sequential read performance hard to predict
in a virtual environment. This is because the data can be (physically)
scattered around. It takes non-trivial efforts for system software to
get its pre-fetch logic right.
-- Wendy
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