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From: lyndat3@your-mail.com
To: "J. Bruce Fields" <bfields@fieldses.org>
Cc: linux-nfs@vger.kernel.org
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 <20150401175437.GD3040@fieldses.org>
Subject: Re: file xfer over NFSv4 with 'sync' ~300X slower than with 'async' ?
Date: Wed, 01 Apr 2015 11:02:17 -0700
Sender: linux-nfs-owner@vger.kernel.org

> There's no maximum sync/async ratio.  You could make that ratio lower or
> higher by varying dd's block size, for example.
> 
> The way I'd look at it, your dd of a 100MB file above is doing 3072
> writes, and taking about 8*60/3072 =~ .16 seconds per write.
> 
> That does sound high.  Things to look at to understand why might include
> the round-trip ping time to the server, and the time for the server's
> disk to do a synchronous write.
> 

After comments from one of the nfs client maintainers, it turns out the slowness issue is simply one of not-quite-MIS-configuration.

As helpfully commented here

  http://serverfault.com/questions/499174/etc-exports-mount-option/500553#500553

, IIUC there are two *separate* syncs to consider -- at the server, and at the client.

'sync' on the EXPORT, and 'async' on the MOUNT is the sane approach; That config also appears to return the performance.

The many recommendations online to use 'sync' for data integrity are IIUC for sync on the server.

With 'async' set on the moount, and 'sync' on the exports, It appears integrity of writes is properly assured and I've got rsync-over-NFSv4 performance back in the ~30-60 MB/s range.

LT