Re: Best strategy to incrementally replace smaller HDDs

["Michał Sawicz" <michal@sawicz.net>]

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Dnia 2011-09-09, pią o godzinie 14:34 +0200, David Brown pisze:
> On 09/09/2011 12:48, Michał Sawicz wrote:
> > Hi all, given the configuration below:
> >        * 8 x 1TB HDDs
> >        * 2 x 2TB HDDs
> >
> > On which I currently have:
> >        * (10 x 1TB) RAID6 - persistent storage
> >        * (2 x 1TB) system / temporary storage etc.
> >
> 
> By this do you mean that you have 8 x 1TB drives with a 1 TB partition 
> on each, and 2 x 2T drives with 2 x 1TB partition on each?  So that the 
> two big disks are shared with both raids?
Yes, that wasn't like that before, but that's a result of exchanging two
1TB drives to 2TB ones. 

> > I want to replace the 1TB drives for 2TB ones on an as-needed basis,
> > what strategy would you recommend?
> 
> This sounds like you are thinking that you can replace a single disk in 
> your RAID6 array and get more storage - changing 10 x 1 TB raid6 = 8TB 
> into 9 x 1TB + 1 x 2TB raid6 = 9 TB.  It doesn't work like that.  You 
> will have to replace /all/ your 1 TB devices with 2 TB devices (and move 
> the second raid off the two existing 2TB devices) - all members of the 
> raid6 must be the same size.
Actually no, I know how RAIDs work, so the three schemes I described
below account for that.

> To help you plan your upgrades, it is also useful to know your 
> partitioning scheme (for example, do you use LVM?), whether you have the 
> space to put lots more drives in the system or must do it one drive at a 
> time, whether you can take the system off-line during the process, and 
> whether you need to do the upgrade quickly or can spend a week or so at 
> it (some of these are conflicting requirements).
No LVM here. The most important - big RAID6 - is a single partition. The
remaining 2 x 1TB on the big drives are split into RAID1 for boot,
RAID10 for root, RAID0 for temporary storage and swap space. I can
temporarily put more drives in there. I can take it off-line for some
periods and I can do the upgrade over an extended period of time. 

> Before you think about upgrading, however, make sure you have a solid 
> backup.  Then make sure you have a backup of that backup - and a plan 
> for how to restore everything if something goes horribly wrong.
Yeah... I plan to rely on RAID's redundancy by moving data around,
growing / shrinking the filesystems and then shuffling the HDDs. I know
that's asking for trouble, but I haven't yet found a place to rent 8TBs
of storage for a week... And that's worked for me in the past, btw.
Twice, even, IIRC.

-- 
Michał (Saviq) Sawicz <michal@sawicz.net>

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