Re: Best strategy to incrementally replace smaller HDDs

[David Brown <david@westcontrol.com>]

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?

> 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.

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).

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.


>
>       1. If I moved to 2TB RAID6 members (using RAID0 on the 1TB drives),
>          I would need to replace two of the drives just to match current
>          capacity. Each next 2TB drive would get me 1TB additional
>          capacity (but actually I'd need to replace two to gain
>          anything). That sounds to be most future-proof, but most
>          expensive.
>       2. If I moved to 2TB RAID5 members, that would reduce redundancy
>          but replacing just two would gain me 2TB capacity. Most of the
>          above still applies.
>       3. If I kept to 1TB RAID6 (two on the 2TB drives), I would reduce
>          the redundancy to just one drive if it was the 2TB drive that
>          failed, but each new drive would gain me 1TB capacity and I only
>          need to replace one-by-one. This sounds like the cheapest, but
>          worst possible approach.
>
> So, am I missing something? What do you think?



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