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SBS Network slows down

This is an SBS 2008 server with about 15 client pcs. I

A few weeks ago a couple of users reported that that the network seemed very slow. I usually test this with a Utility called Lan_SpeedTest.exe it reports how quickly the server can be read from and written to. At that time Lan_SpeedTest gave me figures of about 30 Mbs write and 60 Mbs read (which is very slow). At the end of the day I rebooted  the server, and afterwards found that I was then getting figures 10 times faster ie 300 Mbs/600 Mbs. By the morning afterwards, the speed had roughly halved to 150 Mbs/300 Mbs, and a day later it had halved again.

Since then I've been monitoring the network speed and find that the speed is always about 300/600 after rebooting and usually it settles down to 75/150 after a couple of days. Usually it stays at this speed, but occasionally it slows further. Today is somewhat unusal. The server has been up for 10 days and I am getting 250/85 The write speed is not usually faster than the read. Looking at the server network performance graph, it is always close to 0% utilization. Can anyone suggest what is going on here?

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8/22/2022 - Mon

I thought it might be relevent also to point this out....

I If run Lan_SpeedTest from the server console I have a couple of choices. I can test the speed to a local folder, or to a network folder. For instance, on the server I have the Local folder D:\customers shared as Customers.

So when I run Lan_SpeedTest on D:\Customers, I get 175/1150 and when I run Lan_Speed test on the X: drive (which is mapped to the Customers share), I get 150/400

So at the moment, even though I am running both tests from the server console, when I go through a network share, I am getting one third the write speed compared to when I use the local drive letter.

Assuming the numbers are correct, and also correctly interpreted (i.e. file copying from client to server slows down every day), BUT you notice nothing on the server itself (for example: copying big file from C drive to D drive is super fast), then I would suspect either the switch it's connected to, or the NIC itself. Do you have a spare network switch and/or NIC to test with?
when testing the speed locally on the server, and you already notice the slowdown, of course the need to test network switch/NIC is not useful, the server itself needs to be checked out (figuring either hardware or software slowdown is the next step)

Ok I did some more trials copying a 400M file around the server.

going From D: to C: (both local drives)  5secs

going from C: to D: 5 secs

Going from C: to X (Shared folder on the D: Drive) 17 secs

I've previously rebooted the switch and found no difference to the speed. The speed goes back to 300/600 when I reboot the server. Whatever speed is observed prior to rebooting the switch is still observed after rebooting the switch. So maybe it's the NIC which is at fault. But on an SBS server, i'm frightened to make any changes.
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Are you monitoring your network overall for any abnormal traffic patterns? Between that and maybe the switch port your server is on, you might find the issue.

@masnrock:  I'm not so technical that I can do things like  Network traffic monitoring. I have have proved though, that speeds dont change when I only have my pc and the server connected to the switch. I have also tried different switch ports.

@kimputer: Ok, i'll try a different network card. It'll have to wait until the weekend though. So I can down the server for a while. So i'll close this thread and thank everyone for their input.