Re: [gentoo-user] Raid 5 creation is slow - Can this be done quicker?

[J. Roeleveld]

On Monday 01 February 2010 14:20:28 Stroller wrote:
> On 1 Feb 2010, at 11:58, J. Roeleveld wrote:
> > ...
> > I am currently installing a new server and am using Linux software
> > raid to
> > merge 6 * 1.5TB drives in a RAID5 configuration.
> >
> > Creating the RAID5 takes over 20 hours (according to " cat /proc/
> > mdstat ")
> >
> > Is there a way that will speed this up? The drives are new, but
> > contain random
> > data left over from some speed and reliability tests I did. I don't
> > care about
> > keeping the current 'data', as long as when the array is reliable
> > later.
> >
> > Can I use the " --assume-clean " option with mdadm and then expect
> > it to keep
> > working, even through reboots?
> > Or is this a really bad idea?
>
> It wasn't my intention to chide you - I don't use software RAID
> myself, and your question piqued my curiosity - but the first three
> Google hits for "assume-clean" indicate that this isn't safe to use
> with RAID5.
>
> The 4th Google hit contains an extract from the manpage:
>
>                ... It can
>                also be used when creating a RAID1 or RAID10 if you want
>                to avoid the initial resync, however this practice --
>                while normally safe -- is not recommended. Use this
>                only if you really know what you are doing.

I did find the same results on Google, but not really a proper explanation as 
to why it's a "bad idea". Unfortunately, my budget doesn't extend to a 
hardware raid solution. (The cheap cards offload it to the CPU anyway and are 
generally considered slower in various benchmarks)

> I kinda expected this 20 hours to be spent verifying that the disks
> contain no bad sectors, which would really hose you if it were the case.

True, but I already ran "badblocks" twice on each disk to verify that the 
disks are fine. (No badblocks found).

> But OTOH, 20 hours does not seem an outrageous amount of time for
> building a 7.5TB array. You're not going to do this often, and you
> want it done right.

Good point, and I agree, which is why I will let it finish it's course, but I 
also expected it could be done quicker.

> It would be interesting to know whether hardware RAID would behave any
> differently or allow the sync to perform in the background. I have
> only 1.5TB in RAID5 across 4 x 500gb drives at present; IIRC the
> expansion from 3 x drives took some hours, but I can't recall the
> initial setup.

I'm hoping someone with more knowledge about RAID-systems can throw in his/her 
2cents.

Thanks,

Joost