Storage Spaces something not adding up?

[Jay2001]

Hello all I have a single storage space created consisting of now six 1TB drives. I started with less drives (four-1TB drives) using drives I had on hand, well The storage space was set up with a two-way mirror type resiliency and still is. after a while it complained that I needed to add two drives to the pool because it was running low on space (over the 70% threshold) So I did it took care of the warning and life went on well I have been doing some investigating and I have noticed that (unless I'm missing something) something is a miss.

 

First when I check the amount of disk space being used by just the files themselves on the storage space. Viewing that data by dashboard> storage> hard drives> manage storage spaces> view files> right click server folders> properties. results in size on disk = 1.27 TB.

 

Ok now under manage storage spaces the bar graph shows Using 3.79 TB of 5.45 TB pool capacity.

 

Seems fine except when I do some figuring 1.27 x 2 = 2.54 TB where is 3.79 TB coming from? Unless I'm not understanding how a two-way mirror works. seems to work when I multiply by (3) 1.27 x 3 = 3.81 But its not a 3 way mirror. Again am I missing something?

 

Now when I go into Server Manager> File and Storage Services> Volumes> Storage Pools Under virtual disks right click on storage spaces > properties yields the following data: allocated space = 1.9 TB Used Pool space = 3.8 TB

 

Ok cool 1.9 x 2 = 3.8TB but now where is it coming up with 1.9? 1.27 x 2 = 2.54 1.27 x 3 = 3.81 again close but why times 3 its not a three-way mirror.

 

Verified its a two-way mirror by observing dashboard> storage> hard drives> manage storage spaces....under storage spaces lists it as a two-way mirror in black and white text. 

 

Bottom line I'm confused can somebody make sense of this, Is this normal behavior? if so could someone explain it to me? (Feeling stupid)

Edited February 28, 2015 by Jay2001

[Drashna Jaelre]

Well, when the disk was formatted, what was the "allocation unit size" (aka cluster size). This matters, as NTFS divides your volume into equally sized chunks called clusters, which are typically 4096 bytes in size. If your file doesn't fit neatly into these clusters then there’s going to be some space at the end of the file that’s wasted. 

 

Also, directories take up space, as to NTFS metadata files, such as alternate data streams.

 

Depending on what is actually on the disk, and how it's being stored, this may be normal.

 

 

Also, utilities like "WinDirStat" may be a good idea to run, as they can index the entire drive and display it's contents.

 

 

Also, it may be a good idea to see about running a defrag pass (you can use the built in "defrag" program, and I'd recommend using the "/x" switch if you do).

[Jay2001]

Thank you I will look into this.

[Drashna Jaelre]

Not a problem. 

How Windows uses disks can be tricky and confusing. We (covecube/stablebit) get questions like this all  the time.

[Jay2001]

Is it safe to use the /x switch in defrag on a thin provisioned space two-way mirror?

[Drashna Jaelre]

I believe so. 

Though, if it's thin provisioned, you may want to use the "/k" switch as well or instead of.