Pool X2 Replication

[DAXAR]

Hi All,

I just have a quick question. I have two 8 TB drives in my pool with X2 replication /protection set. All has been working well for a long time. I'm now starting to run out of pool capacity. So if I add a third drive to the pool, let's say another 8 TB drive, how will the X2 replication be handled with three 8TB drives?

Thanks...

[Shane]

Hi Daxar. If you add a third drive but keep the duplication at x2, which drives any given file ends up on will depend on how you have the balancers set.

The default set of balancers includes the Duplication Space Optimizer; its job is to optimize the pool to make the most use of your free spacefor duplicating files, so it will want to move some of the existing files to the third drive to free up space for more duplicated files.

[DAXAR]

Thanks for that explanation Shane. So the balancer will try to redistribute all the files amongst the 3 drives evenly to gain equal amount of free space on each of the 3 drives, and this would include X2 replication? So the X2 replication data on each of the 3 drives would end up on one or the other 2 drives?

[Shane]

I don't actually know the exact algorithm it uses; with three drives and the goal of maximising free space on multiple drives to fit as many duplicated (x2) files as possible I'd expect it should (attempt to) end up with one drive full and the other two with a roughly equal amount of free space.

For example if you have drives A____ B____ and filled them with x2 duplicated files 1 2 3 4 then you'd end up with A1234 B1234. If you then added drive C____ the balancer could try to arrange them something like A1234 B12__ C34__ so it'd then have the most room for additional x2 duplicated files.

Something I could play around with to test if I had more minutes of free time lately.

[DAXAR]

Thanks again Shane, what you're saying here makes sense. My total pool free space is now a little less than 900 GB. So I will try to add at least another 8 TB drive to the pool if I can find a bargain price somewhere. The prices of hard drives are so crazy high these days unfortunately. 

Thanks again ....

Cheers.

 

[Christopher (Drashna)]

To clarify, new files are placed on the drive with the most available free space. This is checked for the absolute value, not for the relative value.

Adding the 3rd drive would usually put new files on the new drive, and alternate between the original two (assuming they were equally filled). 

 

Also, likely, you'll have a good chunk of "unusable for duplication" space after adding the third drive.  In this case, the "Duplication Space Optimizer" balancer will attempt to move files around to eliminate this "unusable for duplication" space.  This would mean moving a good chunk of data to the 3rd drive, until that "unusable for duplication" space is gone.  (most likely until the two original drives have half as much free space as the new drive, but this depends on capacity of the drives, if there are just 2 drives before, etc).

[DAXAR]

Thanks Christopher for that detailed explanation. I hope to bite the bullet, for the high prices theses days, and buy an 8 TB HDD to add to the pool. I will report back when that happens. Cheers.

[Christopher (Drashna)]

You are very welcome!  

And for the drives, if you don't already have them, the sooner you can manage, the better. Since prices are only going up.