[AusNOG] [AUSNOG] Disk wear & Foucault Period

[Paul Wilkins]

Thread out.

Kind regards

Paul Wilkins


On Thu, 22 Aug 2019 at 14:12, Matthew Moyle-Croft <mmc at mmc.com.au> wrote:

> I preface this by saying - “I recommend that all my competitors spend
> their engineering time and effort on solving for this problem”.
>
> > 3 - If we figure a drive is good for 1M restarts, then you'd expect
> precession to cause 0.2% of disks to fail over a 5 year lifespan
>
> So, let’s look at the Backblaze numbers (
> https://www.backblaze.com/blog/2018-hard-drive-failure-rates/). Their DCs
> are in LA and AZ are approximately as north as Adelaide and Sydney are
> south.
>
> They see a 1.27% Annual failure rate across their fleet. If you look at
> their numbers the variation is by far due to the model than anything else.
> If you’re claiming 0.2% over FIVE years (ie. not annually but across 5
> years), as before, this isn’t going to be a significant impact to the
> fleet. It’s also worth noting that they don’t seem to look at controller
> failure vs mechanical failure.
>
> An extra 40 drives per year against 4455 failures per annum. 40 drives per
> annum is around USD$8k ($200/drive at scale). If you said to BackBlaze, I’d
> like to save you $8k, but you’ve got to spend a few million to realign the
> racks, then, well, it’s not going to be a business case I’d defend. (Mostly
> cost would be rebuilding the DCs so the cooling was arranged properly as
> well as all the cabling, hot aisle containment etc etc).
>
> At 0.2% across 5 years you’d have to have more than 1000 drives to make
> this something worth caring about (1000 drives means 2 extra failed drives
> over 5 years).
>
> > 4 - Whether this shows up in MTBF depends on measurement techniques, and
> whether the effect is above the random noise
>
> See above.
>
> Feel free to argue how I’ve butchered probability etc, but I doubt it
> makes the business case “better”.
>
> MMC
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