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I want to start backing up my home PCs (mix of XP and Vista) to a spare machine (XP).

All have recent SATA 7200rpm drives (no RAID), 100Mbit network cards, except one laptop connected over 802.11g.

I'd only expect to backup one machine at a time, using SyncBackSE, and these will be a mix of large and small files: text, documents, photos, video, virtual machine images.

I know the maximum speed of the network should be 12.5 megabytes/sec, but given overheads, what kind of actual transfer speeds in rough terms of gigabytes/hr should I expect both for the wired and wireless machines?

Edit: having done a few test runs, although I do indeed approach 12.5 megabytes/sec for individual large files over the wired connection, the time overhead per file seems to be very significant. It's around 0.5 seconds per file, and thus while transferring small files the actual transfer speed is much less - it seems to stay at < 5% of the maximum for much of the time.

Is this to be expected, is there any way of benchmarking this, and is there anything I can do to improve matters?

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up vote 4 down vote accepted

The network is going to be your bottleneck, a 100mbit connection has a maximum throughput of 12.5 MByte/second. So in ideal conditions you would see 45GByte / hr.

Of course real world conditions aren't quite ideal, depending on your network and devices you might see 90% of that throughput or 60% of that (or any other amount).

Wireless will of course be slower yet, plus the advertised speed is a bit of a pipe-dream. Ideal wireless connectivity means only one powered device talking to the router, no interferance from other RF devices (wifi shares the same spectrum of RF as most other wireless consumer devices, 2.4GHz).

There's too many variables to tell you what you can expect in terms of overhead. Provided your wired machines are in good working order, not under load, without other network activity, and with a decent switch between them and decent CAT-5 that isn't damaged, too long, coiled near anything that might emit high EMI, and properly crimped then you should see close to full throughput. Wireless is much more of a crapshoot. If you build a faraday cage and put both your hotspot and laptop inside it then you might get close to it's full throughput.

...long story short is to find a large file (30gb VM drive, 8GB ISO image, etc) and time its transfer.

The overhead you're seeing on small files is normal, and it's not unique to the network transfer. For example, unzipping 1 1GB file may take 90 seconds, whereas unzipping 1024 1MB files might take 180 seconds (the numbers are pulled from my backside).

I can't point a finger at a single culprit, but typically a large file on a non-fragmented drive is stored in a relatively continuous block--small files are likely to be scattered. This means that when the operating system wants to transfer files 1 thru 1000 it has to find the start of each file on the drive and seek to that location.

Transferring small files across the network might amplify the slowdown, in which case you would want/need to 'roll' them together into an archive using a tool such as Zip/Rar/Tarball/etc...

disclaimer the bottom-line that lots of small files are slower to move than a few large files is based on lots of experience--the specific reasons that cause this slowdown go beyond my knowledge, there's likely several factors beyond just the drive seek-time.

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Thanks for this - please see edits to original question, as I'm getting near the maximum for large files, but time overhead per file is very significant. – e100 Oct 26 '09 at 17:27

You'll probably just feel the pain on your initial backup. Most modern backups use deltas (that is, they only backup the changes).

That said, you may want to look at upgrading your wired LAN to gigabyte ethernet and (if an option) upgrade your wireless to the full N standard (granted, not much gear out if you're excluding draft N items).

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I take your point on deltas, but my backup software and all others I've seen backup the whole of each changed file, although I guess this might not be the case for more specialist/corporate software? So monolithic VM drives particularly will mean a lot of data to backup. However, as per edits to original question, it's the small files that are taking the time, and I'm not sure upping the speed of the network is going to make a significant difference. – e100 Oct 26 '09 at 17:16

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