[GTALUG] New Desktop PC -- debian Linux - Proposed 2 TB HDD Partitioning;

Lennart Sorensen lsorense at csclub.uwaterloo.ca
Wed Apr 18 09:42:20 EDT 2018

On Wed, Apr 18, 2018 at 06:35:57AM -0500, Russell wrote:
> It wrote a bunch of zeros to a virtual file. Perhaps even touching a tmp file along the way. Even if it didnt touch tmp, it wrote the zeros someplace in order to perform the count.

No it passed a bunch of zeros through a pipe to md5sum.  All ram.
No virtual files involved in pipes at all.

> I was just trying to comment on the speeds of the two installs relative to the respective  disks the OS runs from. I'm sorry you didn't understand that. Perhaps I should have said running the OS from the two different drives, irrespective of all the other disk writes which may happen when the OS operates normally when calling dd from a GUI.

Well nothing in the test depends on the disk.  The kernel is in ram,
so /dev/zero is in ram, dd reads from ram, writes to a pipe (ram), which
is read by md5sum (from ram).  It does not create the whole 1GB of zeros
before passing it to md5sum.  pipes pass data as it is ready and ussually
only allow a few KB of buffering between processes.  It does not buffer
to a file.  If md5sum can't keep up, dd will be blocked and pause until
there is more room in the pipe.

> Often tests provide side channel results which are not part of the expected normal metric but quantifiable data arises none the less.

Well that test is essentially a ram speed test.

I tried it on my machine with a bunch of slow disks, and got just a
touch less.  I guess your ram is newer than mine.

Len Sorensen

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