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USB Drives as backup - speed issues

I

Iain

n00b
Joined
Jan 12, 2007
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2
First, my qualifications! The target machine is a dual twin core amd 64 bit machine with 16GB of core and running Windows Server 2003 x64 R2.

In it I have two pairs of 250 GB (unformatted) mirrored seagate drives ST3250823AS and a 750 GB mirror running ST3750640AS drives. I have another pair of 750 GB to installed when I've finished my current task.

Finally, I've a pair of Lacie external USB hard drives running 500GB samsung HD501LJ whose main purpose is backup.

My problem is the external drives are just plain slow! I was expecting to be able to write at about 50MB/s, but I'm only achieving 10% or so of that.

The source is the 750GB mirror and the target one of the Lacie drives running through a USB 2.0 hub (usb 2.0 all the way) which has 2 other devices attached - one the other Lacie drive (not being used) and the second a DVD rewriter (not being used).

I'm trying to backup up 330 GB or so which is in a bit over a million files, most small but the majority of the volume (95%?) in around 150K files running up to 100s of MB.

I find that the USB drives are set to 'optimise for quick removal' (so no write cachiing), but I'm not sure this is the key problem (though it depends if windows is smart enough to preallocate the file in the directory and then stream the data or if it writes it and updates the directory in chunks).

AM I doing something wrong? How much difference will turning on write caching make? Do I have the wrong drives? Would I be significantly better buying an internal hot swap cage and doing it that way?

In passing, in my first attempt at backup I got a whole set of Delayed Write Errors, culminating in a system crash (after I removed the drive, reinstalled it and started the copy again). I believe this happened because the screen saver started up - certainly it hasn't this time after I disabled the damn thing.

Any advice appreciated!

Iain
 
Iain said:
My problem is the external drives are just plain slow! I was expecting to be able to write at about 50MB/s, but I'm only achieving 10% or so of that.
Click to expand...
USB2.0 is limited to ~24 MB/s for bulk transfers. Bulk transfers are such that the data integrity is guaranteed. USB2.0 can only transfer isochronous frames at 480Mbps.
here[/quote]
The USB specification defines four types of pipe transfer:

* Control—intended to support configuration, command, and status communication between the host software and the device. Control transfers support error detection and retry.
* Interrupt—used to support small, limited-latency transfers to or from a device such as coordinates from a pointing device or status changes from a modem. Interrupt transfers support error detection and retry.
* Isochronous—used for periodic, continuous communication between the host and the device, usually involving time-relevant information such as audio or video data streams. Isochronous transfers do not support error detection or retry.
* Bulk—intended for non-periodic, large-packet communication with relaxed timing constraints such as between the host software and a printer or scanner. Bulk transfers support error detection and retry.

Pipes also have a transfer direction associated with them. A control pipe can support bidirectional communication but all other pipes are strictly uni-directional. Therefore, two-way communication requires two pipes, one for input and one for output.
Click to expand...

With bulk transfers, the size and frequency of moving data is limited so that the theoretical max is somewhere in the 20MiB/s territory.
 
>>USB2.0 is limited to ~24 MB/s for bulk transfers. Bulk transfers are such that the data >>integrity is guaranteed. USB2.0 can only transfer isochronous frames at 480Mbps.

Fair enough, but I'm only seeing 7 MB/s average . I should surely see double to treble that?

Is firewire any better?

Now my backup is finally completed, I shall run some tests!
 
From my limited experience, Firewire is quite a bit faster.

I know what you mean with 7MB/s via USB2. I tried to move a large (240GiB) amount of Data to a WD mybook pro (has a WD5000YS hdd inside) and the windows performance monitor saw ~7 MiB/s average. To say the least, I was more than disappointed.

I am not sure if it is still the case, but "back in the day" the reliability and speed of USB devices was greatly related to the chipset being used. If could be that both my WD "mybook" and your enclosures use cheap chipsets, hence exhibit low transfer rates. Maybe the large asymmetry between the source's read speed and the medium transmission rate has a negative impact on the average file transfer rate? Maybe both of us moved a lot of small files and transferring these via USB is very inefficient?

I am sorry that I do not have a solution for you. Maybe see how fast you can move a large (multi-100 MB) file and how that transfer rate compares to a small file?

Edit: I would try and get eSATA connection for your backup drives.
 
My first post here..

USB bandwidth is actually shared...so if you have a lot of USB devices or use a USB HUB... you have less bandwidth for the HDD, which may explain the slow speed.
 
I had the same problem. I moved my external drives into SATA enclosures. Now my external drives are just as fast as my internal one. :cool:
 
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