Site
!!! Q: How are capacity calculations are carried out and reported? Assume the following system:
!! Why are the pool and filesystem capacity numbers different?
Q: How are capacity calculations are carried out and reported? Assume the following system:
GB-2000 with 22 x 1TB drives
4 x 5 drive RAIDz vdevs
2 x Spares
The pool capacity reported as 18.1TB after RAID configuration, but in the top level Filesystem properties only 14.3TB is reported. The dedupe and compression ratios are both 1.0x.
Where is the missing 3.8TB of capacity ?
A: What is confusing about this is the way zfs reports sizes.
First, 1T disks are actually 1,000,000,000,000 bytes, which is actually about 0.9TB. Thus, when you have 20, you get 18TB of raw storage.
Second, the pool gives you the raw storage numbers, but the filesystem reports usable storage. They are using RaidZ, which gives you (n-1)/n actually storage because of parity. In this case, the RaidZ have 5 drives, so you have to multiply the raw storage by 4/5 = 0.8 to get the actual storage. This brings the filesystem size down to 18TB * 0.8 = 14.4TB, which is what they are seeing.
Another way of looking at it is that they are losing 1 disk from each RaidZ for redundancy, so 4 * 0.9TB = 3.6TB, which is basically the difference you are seeing.
- JohnP
Usage Tips
Site.UsageTips History
Show minor edits - Show changes to output
June 13, 2012, at 08:40 PM
by - Post of the definitive email from John Podeska on the topic
Deleted line 0:
Changed lines 3-4 from:
Q: How are capacity calculations are carried out and reported? Assume the following system:
to:
!!! Q: How are capacity calculations are carried out and reported? Assume the following system:
Changed line 13 from:
A: What is confusing about this is the way zfs reports sizes.
to:
!!! A: What is confusing about this is the way zfs reports sizes.
Added lines 1-21:
!! Why are the pool and filesystem capacity numbers different?
Q: How are capacity calculations are carried out and reported? Assume the following system:
GB-2000 with 22 x 1TB drives
4 x 5 drive RAIDz vdevs
2 x Spares
The pool capacity reported as 18.1TB after RAID configuration, but in the top level Filesystem properties only 14.3TB is reported. The dedupe and compression ratios are both 1.0x.
Where is the missing 3.8TB of capacity ?
A: What is confusing about this is the way zfs reports sizes.
First, 1T disks are actually 1,000,000,000,000 bytes, which is actually about 0.9TB. Thus, when you have 20, you get 18TB of raw storage.
Second, the pool gives you the raw storage numbers, but the filesystem reports usable storage. They are using RaidZ, which gives you (n-1)/n actually storage because of parity. In this case, the RaidZ have 5 drives, so you have to multiply the raw storage by 4/5 = 0.8 to get the actual storage. This brings the filesystem size down to 18TB * 0.8 = 14.4TB, which is what they are seeing.
Another way of looking at it is that they are losing 1 disk from each RaidZ for redundancy, so 4 * 0.9TB = 3.6TB, which is basically the difference you are seeing.
- JohnP