What is the exact checksum size overhead?

[Bidule0hm]

Wow, very detailed and clear post, many thanks ;)

"ZFS always reserves 1/64th of the pool for all the metadata so you can put that into the calculator." it's already in the app (actually it's named checksums overhead but I'll change that to metadata overhead) :D let's call this overhead calculus done :p

So now let's see about the blocks overhead: it's calculable (in percentage or ratio)? And also I thought that the rule of the power of two doesn't apply anymore because of the compression?

The allocation overhead? I didn't think about that. Thanks for the formula, I should be able to implement it pretty easily.

"Also realize that this is just for ashift=12" No problem, the app assumes ashift=12, recordsize=128k and no compression (I should add that in the other thread though) ;)

In case you wonder: "the app" and the "other thread" refers to the RAID size calculator I've made (the link is in my sig).

 

[cyberjock]

I'm pretty sure the behavior of padding has changed and those numbers aren't 100% anymore. But you'd have to jump in IRC to talk to one of the ZFS guys in #freenas. There's one I have in mind, but he's not on right now. :(

 

[SirMaster]

So now let's see about the blocks overhead: it's calculable (in percentage or ratio)? And also I thought that the rule of the power of two doesn't apply anymore because of the compression?

It's freespace calculation. When you make a brand new empty zpool, the amount of free space capacity will be determined by the block overhead. It applies before any data or compression of that data is put on the pool. Compression will end up saving a lot more space than how much is lost to this padding.

You can create test zpools and see in zfs list the starting AVAIL capacity with various configurations. The AVAIL number currently matches the formula explanation unless there is some reason I don't know why the AVAIL is not a number we should be looking at or care about as much anymore.

 

[Bidule0hm]

@cyberjock Ok, I'll ask on IRC ASAP, thanks ;)

@SirMaster Interresting, there is still some benefit to use the power of two rule. Free space calculation? yeah but I can't use that in my app (or I don't understand what you said) :P