Re: [RFC] nfs: use 4*rsize readahead size

From: John Stoffel
Date: Tue Mar 02 2010 - 12:34:18 EST

>>>>> "Trond" == Trond Myklebust <Trond.Myklebust@xxxxxxxxxx> writes:

Trond> On Tue, 2010-03-02 at 11:10 +0800, Wu Fengguang wrote:
>> Dave,
>> Here is one more test on a big ext4 disk file:
>> 16k 39.7 MB/s
>> 32k 54.3 MB/s
>> 64k 63.6 MB/s
>> 128k 72.6 MB/s
>> 256k 71.7 MB/s
>> rsize ==> 512k 71.7 MB/s
>> 1024k 72.2 MB/s
>> 2048k 71.0 MB/s
>> 4096k 73.0 MB/s
>> 8192k 74.3 MB/s
>> 16384k 74.5 MB/s
>> It shows that >=128k client side readahead is enough for single disk
>> case :) As for RAID configurations, I guess big server side readahead
>> should be enough.

Trond> There are lots of people who would like to use NFS on their
Trond> company WAN, where you typically have high bandwidths (up to
Trond> 10GigE), but often a high latency too (due to geographical
Trond> dispersion). My ping latency from here to a typical server in
Trond> NetApp's Bangalore office is ~ 312ms. I read your test results
Trond> with 10ms delays, but have you tested with higher than that?

If you have that high a latency, the low level TCP protocol is going
to kill your performance before you get to the NFS level. You really
need to open up the TCP window size at that point. And it only gets
worse as the bandwidth goes up too.

There's no good solution, because while you can get good throughput at
points, latency is going to suffer no matter what.

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