[TriLUG] upgrading a software (md) RAID

[Michael Hrivnak]

Be careful with big drives in RAID.  The fault tolerance comes out to be not 
great, and when you are syncing an array, one bad read can be detrimental.

For example, checking the specs on a Seagate 1TB 7200.12 SATA drive, the 
maximum "nonrecoverable read errors" is "1 per 10^14 bits read".  My math says 
that at their standard, for every 11.4TB read, there will likely be a read 
failure.  So if your array needs to resync, a 2-drive array has a 9% chance of 
failing the sync.  Worse yet, a degraded 3-drive RAID5 array would have an 18% 
chance of getting a read failure during sync, which could fail out a second 
drive and leave you in real hot water.

Of course this assumes that the drives are right at the edge of their 
reliability standard, but it is possible that average drive reliability is 
better.  If you decide to risk it, at least keep this in mind.

Michael

On Monday 05 October 2009 04:06:42 pm Charles Mangin wrote:
> i've currently got my home network server running a three drive RAID
> system. it's running md (software RAID) under Fedora 11.
>
> the drives in there are 500 gigs, and with the prices of 1+ Tb drives
> coming down so low, i'd like to increase capacity. ideally, i'd like
> to do it without having to migrate my data to some intermediary device
> while i build a whole new RAID.
>
> is there a good tutorial (that i've yet to find by googling) that has
> a good step-by-step on how to do this? the process, as i understand
> it, is to replace one drive at a time, rebuilding the array each time.
> once all the larger drives are in place, the final rebuild will show
> the new capacity. there's bound to be some mdadm fiddling necessary as
> well...
>
>
>
>
>
> Charles Mangin
> Option8, LLC - Making Macs happy since 1999.
> option8 at option8.com | http://www.option8consulting.com
> mobile: 919.368.7167
> skype: option8llc
>
>
>
>
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