Slower than before... after adding more discs...

[delirial]

Hi there,

 

I recently built my first UnRAID box. The hardware is the following:

 

CPU:        AMD X2 5600+

Memory:  2GB DD2

Mobo:      ASUS M4A78L-M

HDD:      3x WD20EARS (all with the 7-8 jumper in place.. 2.54mm Jumpers with open shunts, in case it makes a difference)

 

The desktop and the UnRAID box are connected to a Gigabit switch (TrendNET), the other computers (and the switch) are connected to a Netgear Wireless N routers (Gigabit also). They all get the same speed.

 

I built the system with 2x WD20EARS initially, then bought two more but have only added one of them to the array. The fourth drive is connected and detected by UnRAID, but still not in the drive pool (takes about 8 hours to do the clear, I'll be doing that tonight). I was getting around 37MB/s write from Windows before adding the new drives. Not spectacular, but not bad either. Now, after adding the third disc to the array, I'm getting 26MB/s write. Reading from the array usually begins at 60MB/s then gradually drops to 46MB/s. This is not bad, and it hasn't changed with the addition of the new drives.

 

So, does adding new drives decrease UnRAID's speeds (even if the drives are exactly the same)? What other possible causes are there for the decrease in speed?

 

thanks,

del

 

[BRiT]

If I'm reading correctly, you had a 2 Data Disk setup and now you have a 2 Data Disk and 1 Parity Disk setup that is soon to be a 3 Data Disk with 1 Parity Disk setup? Writes without a parity drive are always faster than with a parity drive.

 

What size is the data you're writing to the drive? Previously the data might have been able to be fully contained in RAM hence masking most of the time it takes to write.

 

Also, what are you reading the data from, it could be a limit on the source side by RAM or disk too. The same applies with reading from unRAID, it can be limited on the destination side by RAM or disk.

 

Also, if the disks you're writing to are nearly full, the write speed is slower based on how the ReiserFS works.

 

Also, how physical disks work, the fastest speeds are early on the drives and progress to slower speeds later on the drives as the cylinder head moves across the media.

 

Have you done any tests to eliminate the other PC and the network from the equation?

dd if=/dev/zero of=/mnt/disk1/test1.dd count=4G
dd if=/dev/zero of=/mnt/disk2/test2.dd count=4G

dd of=/dev/null if=/mnt/disk1/test1.dd
dd of=/dev/null if=/mnt/disk2/test2.dd

 

And once you add in your 3rd data disk you can try:

dd if=/dev/zero of=/mnt/disk3/test3.dd count=4G
dd of=/dev/null if=/mnt/disk3/test3.dd

 

[delirial]

Silly me.

 

You were right. The problem was a limitation on the source drive. External USB drive (and apparently, a really crappy one at that).

 

thanks,

del