Chuck Lever wrote:
On Jun 12, 2008, at 5:41 PM, Olga Kornievskaia wrote:
Olga Kornievskaia wrote:
Trond Myklebust wrote:
On Wed, 2008-06-04 at 12:40 -0400, Olga Kornievskaia wrote:
While testing NFSv4 performance over the 10GE network, we are
seeing the following behavior and would like to know if it is
normal or a bug in the client code.
The server offers the max_write of 1M. The client mounts the
server with the "wsize" option of 1M. Yet during the write we are
seeing that the write size is at most 49K. Why does client never
come close to 1M limit?
I have a feeling that is due to some crap in the VM. I'm currently
investigating a situation where it appears we're sending 1 COMMIT for
every 1-5 32k WRITEs. This is not a policy that stems from the NFS
client, so it would appear that the VM is being silly about things.
I'm specially suspicious of the code in get_dirty_limits() that is
setting a limit to the number of dirty pages based on the number of
pages a given BDI has written out in the recent past. As far as I can
see, the intention is to penalise devices that are slow writers,
but in
practice it doesn't do that: it penalises the devices that have the
least activity.
I think we are seeing larger than usual number of COMMIT messages.
Using Chuck's nfs-iostats to monitor an NFS write I can see that each
operation writes about 830MB. Why is so much small than wsize=1M?
I assume you mean 830KB.
Remember that nfs-iostats reports an average transfer size, so you may
be seeing a lot of 1MB writes on the wire, and just enough small
writes to reduce the average. Or, the client may not be writing 1MB
at all.
You have to look at a network trace to see which.
On the other hand, 830KB is still very large.
Apologizes, yes, it is 830KB. If you say it's an average write then my
question is why is NFS breaking down 1M writes into smaller chunks? When
I say 1M write I'm referring the the user land (dd) calling write() with
1M buffer.
I'm trying to understand why NFS has poor write performance. I had 2
leads to pursue (1) nfs-iostats shows that each write operation is
>100KB smaller than a read operation and (2) I see that during a write
nfs-iostats reports fewer operations per second than during a read. The
latter can be due to the COMMIT problem.
If nfs client is writing less amount of data on each operation and it is
not able to write fast enough, wouldn't that explain it's poor performance.
Current read performance is 590MB/s
Current write performance is 230MB/s
-O
--
Chuck Lever
chuck[dot]lever[at]oracle[dot]com
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