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[Xen-users] Very slow NFS write performance from VM, but good from the X

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Subject: [Xen-users] Very slow NFS write performance from VM, but good from the XenServer console
From: Arpad KUN <akun@xxxxxxxxxx>
Date: Wed, 28 Nov 2007 00:40:34 +0100
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Hi All,

I have a disk write performance problem. I made NFS virtual disk storage, to an opensolaris machine, where is an NFS exported directory.
If I copy a file about 200Mb to the virtuald disk (on the nfs) in a Windows XP VM (xs-tools installed), the write is about 0.7-1.2MByte/sec. But! If I was reading the same file from the disk, it came about 40-50-70Mbyte/sec.

I saw in the forum, other had the same problem, that writing to nfs is very slow, and the solution is to fill up the block device with zeros.
So, I made a small virtual disk (1.5GByte) on the NFS SR, booted up a linux, made a partition, and filled up the partition with dd. The write performance was during the dd the same, about 0.7 - 1.2Mbyte/sec.

After the fillup, I detached the drive and attached it to a Windows XP machine, and copy again to the virtual disk. The write performance was a little bit better, about 1.2-1.6 Mbyte/sec. Reading the same file uploaded was again 50-70MByte/sec. (Only, if there was *no write* in progres!! If I was writing a file to the disk, the read performance was about 1-1.5MByte/sec too.)

I thought, there is a problem to write the NFS server. (Both machines has network cards with tcp offload engine, nfs server 2.66GHz core 2 duo, xenserver 2.33 quad-core xeon.)
I was searching for the nfs mount on the xenserver:

[root@xenserver ~]# mount on /var/run/sr-mount/1569d837-30a4-ac1f-c75e-3d5838aecb71 type nfs (rw,soft,timeo=66,retrans=1,addr=

So i made the following:
[root@xenserver ~]# cd /var/run/sr-mount/1569d837-30a4-ac1f-c75e-3d5838aecb71
[root@xenserver 1569d837-30a4-ac1f-c75e-3d5838aecb71]# dd bs=512 count=300000 if=/dev/zero of=image
300000+0 records in
300000+0 records out
153600000 bytes (154 MB) copied, 2.91871 seconds, 52.6 MB/s

Why is then so slow from the virtual machine?
(And I checked, I installed an iSCSI client to this windows VM, and I could write to the same solaris box, with 25-28MByte/sec to an iscsi target, next to the nfs share.)

Any help, comments would be great. Please help!

Best regards,

Arpad Kun

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