[mythtv-users] Mirroring root partition on SSD to a hard drive -- bad idea?

[Harry McGregor]

Hi,


On 01/15/2016 08:48 PM, Stephen Worthington wrote:
> On Fri, 15 Jan 2016 18:02:54 +0000, you wrote:
>
>
>> Regular HDD's are a lot more forgiving about read errors.  True, a
>> controller failure can stop the drive cold, but you can always buy an
>> identical model, swap the board and get your data back.
> Not for quite a while now - the electronics boards are now tied to the
> physical drive and swapping them does not help at all.  I think there
> is an ID written to a special area of the drive that has to match one
> in the electronics.  I think this is part of the standard for the
> "secure erase" feature.  Secure erase, if not complete, will
> automatically restart when a drive is powered up again, and that could
> be defeated if you could swap the electronics boards.  So my guess
> would be that any drive that has the secure erase feature will not
> allow swapping of the electronics.  Which means pretty much all drives
> shipped in the last few years.

Just to add to this, the PCB swap only worked for certain failure modes.

Yes, SSDs tend to have a more binary failure mode (one minute they are 
working, next they are not) then hard disk drives, but in either case if 
your planning on "I will be able to rescue the data" as your backup 
plan, your not doing it right.

I work in enterprise disk storage, what ever can go wrong, usually 
will.  The only way to have data backup, is to have data backup. That is 
not mirroring, that is not replication, that is offline backup.  The 
others help with data availability, but they don't help with the fully 
catastrophic.

My personal preference for home and small business use is a combination 
of CrashPlan (paid cloud backup), and dirvish to my own devices.  
Crashplan gets you geographic diversity, but very slow restores.

Dirvish works well over SSH, and you can put a small computer (even a 
raspbery pi 2) and large external USB disk at a friends house across 
town, and due to the magic of hard links, it does not take much space to 
keep multiple iterations (space of a incremental backup, but it looks 
like a full backup).  http://dirvish.org/  and it's in Ubuntu and Debian 
distros.

Not using an SSD for "high write" areas of your system to try and 
preserve the SSD is silly.  Those are the areas that you will get the 
best performance gain out of the SSD.  SSDs in general have far more 
write life then people expect.  Most failures I have seen have not been 
due to write cycles, but instead due to controller faults or firmware 
defects in the drive.  If you are really worried about it, use Dirvish 
to backup your SSD to a mirrored set of HDDs.

IMHO, your mysql db, root FS, var (including logs), and swap should all 
be on SSD.

-Harry

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