Re: [RFC v2 0/2] New RAID library supporting up to six parities

On Tue, Jan 14, 2014 at 1:25 AM, David Brown [off-list ref] wrote:

On 06/01/14 19:13, Mark Knecht wrote:

quoted

Andrea,
   As others have said it looks interesting. Thanks for the efforts.

   Question: As an end-user type who currently uses mdadm RAID6, if I
wanted to set up a dedicated machine to do some SnapRAID testing then
what's the minimum disk hardware I'd need assuming Linux is just on
it's own disk?

1) 1 drive for Linux
2) 4 (or possibly 3) drives for a SnapRAID RAID6 device?
3) 3 drives for a SnapRAID RAID5 device? (If RAID5 can even use
SnapRAID. Not sure it can)

   Are there any known issues using both mdadm and SnapRAID devices in
the same system? Again, I don't care if there are and the machine dies
a brutal death (which I doubt from your announcement) but just asking.

Cheers,
Mark

If you want to do testing (once there is mdadm support for making the
multi-raid devices), then an easy way is to make some big empty files on
an existing disk and set them up as loopback devices.  Then you can use
these "fake" drives for the arrays.  You can then test a 6+4 quad parity
array using 10 1G files rather than needing 10 physical drives, and you
can play with resyncing, fault testing (using the md "faulty" layout),
reshaping, etc.

Of course, you can't test real-world speed this way.  However, you /can/
test parity generation/recovery speeds nicely by putting the loopback
files in a tmpfs system in memory - that eliminates the hard disk speeds
and lets you do hard testing of the raid code.

Thanks David. I'll save this email and give it some thought. I've not
used loopback devices before but some quick reading makes your
suggestion seem quite interesting.

Cheers,
Mark