How long does shred take? [solved]

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psutokth · Post by **psutokth** » Tue Jul 17, 2007 7:09 pm

Hi, I'm just curious how much longer my processor will be running full speed and my second drive will be inaccessible.

I ran:

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shred /dev/sda

about 3 hours ago on a 120GB usb 2.0 drive with a measly 2MB buffer. Does anyone have a guess as to how long this will take or a way I can check the progress?

blu3bird · Post by **blu3bird** » Tue Jul 17, 2007 7:26 pm

it has a --verbose parameter to show the progress. No way to check if you have started it without that parameter.

How long it takes depends on your hardware but it should take about 28hours.

my formular: (size of harddrive in mb) / (harddrive write speed in mb/s) * (number of times the harddrive is overwritten, default is 25) / 60 / 60 = hours it will take
120000 / 30 * 25 / 60 / 60 = 27.7

timeBandit · Post by **timeBandit** » Tue Jul 17, 2007 7:31 pm

Well, you can compute an estimate yourself: press Ctrl+Z to suspend shred, then (as root):

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# hdparm -tT /dev/sda

/dev/sda:
 Timing cached reads:   1774 MB in  2.00 seconds = 887.14 MB/sec
 Timing buffered disk reads:   20 MB in  3.15 seconds =   6.36 MB/sec

(I did this in a virtual machine so the numbers are wacky.)
Afterwards, give the fg command to resume shred.

Divide your disk size (120GB) by the buffered read figure (6.36MB/s, above) to get the best-case time, in seconds, to read the entire drive once. Since we're talking rough estimates, assume write speeds are similar. In the example this yields 120GB * (1024 MB/GB) / (6.36 MB/s) ~= 19320 s, or about 5h 20m for one pass. A more realistic speed of 40MB/s yields ~51m for one pass, and again that's best case (one big streaming chunk). Figure on actual speed being quite a bit slower.

Now for the really bad news: by default, shred overwrites a file 25 times....

Edit: I see someone already replied while I was checking my figures. Ah well.

psutokth · Post by **psutokth** » Tue Jul 17, 2007 7:35 pm

Oh wow, there wasn't even anything incriminating on it, but I want to freecycle it. Thanks for the help guys, I think I'll be stopping it long before the 25th pass.

x22 · Post by **x22** » Wed Jul 18, 2007 9:22 am

That calculations work only if CPU (generating random data) can keep up with drive writing speed.

eccerr0r · Post by **eccerr0r** » Wed Jul 25, 2007 11:22 pm

I've seen around 20MB/sec hdparm reads from most of my USB hard disks that are larger than 20GB. The best I've seen is around 25MB/sec. That 2MB buffer doesn't affect USB read/writes much. My 2M, 8M disks all behave pretty much the same behind a USB-IDE bridge...

Most of the data on my machines are not really that critical. I tend to do three pass wipes on my disks - two random garbage/data and then a zero pass. I figure that if someone really wants to recover data after that, they're fairly rich and really need not be after my poor data...