Re: [gentoo-user] Copying data efficiently

[Daniel Troeder]

Am 17.05.2014 11:58, schrieb Neil Bothwick:
> On Sat, 17 May 2014 09:59:08 +0200, meino.cramer@×××.de wrote:
>
>>> 3. Or you could use a sequential copy:
>>>
>>>   cp -a /home /dev/sdb1/ && cp -a /home /dev/sdc1
>
>> 3.) The files I want to copy are in the size of some GB each. So the
>>     cache isnt big enough to hold ALL files for the second part.
>
> Run the two copies simultaneously, start the first, switch to another
> tab, start the second. That way the data for the second copy is always
> the most recently cached.
>
> However, I expect the speed limit here may be the USB bus unless you are
> using USB 3.0 drives on different buses.
I was thinking about "how to make sure cache is used", and that two
simultanious cp won't work, because the progress for the two cp will
quickly diverge. But then I realized: there is no need to think about
the read cache - the limiting factor is always the writing side,
especially with USB! So IMO it doesn't matter at all how you do it!

I guess two simultaneous cp will be the same as two sequential cp,
except if you have two separate USB-buses. Usually you have just one
externally connectible, use "lsusb -t" to check.

If you have less that 2 times the size of your files, IMO simultaneous
cp will be worse, because Linux (don't know if USB-subsystem or cp)
creates big buffers when cp'ing (check with "free -m"), and you'll
probably get into memory trouble.


Greetings,
Daniel

PS: Quickest way is always to open USB-case and plug SATA cable from
motherboard into drive. With >80GB it's always worth the trouble.

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