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From: lyndat3@your-mail.com
To: linux-nfs@vger.kernel.org
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Subject: file xfer over NFSv4 with 'sync' ~300X slower than with 'async' ?
Date: Tue, 31 Mar 2015 07:21:16 -0700
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Narrowing doen the issue, NFSv4 file xfer with 'sync' appears, here, to be ~ 300X slower than with 'async'.

(1) for NFSv4 mount with 'sync'

	grep NAS1 /etc/auto.nfs4 
		NAS1   -fstype=nfs4,_netdev,rw,proto=tcp,sync,...  xen01.loc:/

a 100MB file xfer takes ~8 minutes

	rm -f /mnt/NFS4/NAS1/file.out && \
	 time dd if=/dev/zero of=/mnt/NFS4/NAS1/file.out  bs=32K count=3K
		3072+0 records in
		3072+0 records out
		100663296 bytes (101 MB) copied, 485.721 s, 207 kB/s

		real    8m5.861s
		user    0m0.012s
		sys     0m0.250s

(2) Change mount 'sync' -> 'async',

	vi /etc/auto.nfs4 
	-	NAS1   -fstype=nfs4,_netdev,rw,proto=tcp,sync,...  xen01.loc:/
	+	NAS1   -fstype=nfs4,_netdev,rw,proto=tcp,async,...  xen01.loc:/
	systemctl restart autofs

the same 100MB file xfer takes ~ 2 seconds

	rm -f /mnt/NFS4/NAS1/file.out && \
	 time dd if=/dev/zero of=/mnt/NFS4/NAS1/file.out  bs=32K count=3K
		3072+0 records in
		3072+0 records out
		100663296 bytes (101 MB) copied, 1.65577 s, 60.8 MB/s

		real    0m1.658s
		user    0m0.000s
		sys     0m0.089s


I'd expect 'sync' to be slower than 'async', but 300X ?

Is there additional config that cures, or at least drastically improves, this slow down?

Some very old (10+ years) posts suggested kernel bugs, but those were fixed ages ago.  Maybe reemerged?

LT