FYI: Special-device size on PBS in practice

[tuxis]

Hi,

saw a thread last week speculating about the required size for a 'special' device in a PBS ZFS Pool. We now have about 8TB of active backups in the pool, and every time I calculated the special-device size for this pool, it was about 0.5%.

So, to calculate the max size of your special-device in a PBS setup, here's the formula:
MAXPOOLSIZE*0.005 = MAXSPECIALDEVICESIZE

 

[Cookiefamily]

it was about 0.5%.

MAXPOOLSIZE*0.05 = MAXSPECIALDEVICESIZE

Is it 5% or 0.5% now?

 

[tuxis]

Let us not talk about this anymore.

 

[Cookiefamily]

Thank you for your observations, that is really not a lot of special device usage... awesome!

 

[aaron]

Thanks for the experience report

I can confirm this, though not for a PBS installation but on my personal NAS at home I have about 2.5TiB in the pool and the special device uses about 4.5GiB of data currently. This results in ~0.2% special device usage compared to pool size.

Considering that a PBS datastore probably has a lot more files than my NAS with all the tens of thousands of chunk directories and chunks themselves, it should use the special device more than my NAS to store all that metadata. But it really is surprisingly little.