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Forum Discussion
X-RAID Volume Smaller than expected
This is not a big deal, but it puzzles me, so I thought I would ask if anyone has an explanation.
My RN214 was configured with 2x8 TB + 2x1.5 TB drives, and it reported a total volume size of 9.9 TB. That seemed kind of reasonable, since my understanding is that the general calculation is vol = total TB minus the size of the largest drive: 19 - 8 = 11 TB. I just assumed that missing 1 TB was a rounding error in the real drive sizes vs the general size the manufacuter claimed. And I also assumed that if I upgraded the smaller drives, I would gain 100% of the new capacity in the total RAID volume capacity.
So I just replaced the two 1.5 TB drives with two new 3 TB drives (a WD30EFRX and an ST3000VN007). That added about 3 TB in total new capacity, and my largest drive did not change; therefore, I expected a new volume size close to 13 TB. But after all the resyncs, etc. were all completed, the NAS is only reporting a total volume size of 11.36 TB (about 1.5 TB less than the increase I expected). That is half of the total new capacity missing, so relatively speaking, it seems like a lot.
Looking closer at the Admin Page Volumes screen, the NAS reports these capacites for each drive: 7.3 + 7.3 + 2.7 + 2.7. Thus, using the actual capacities reported by the NAS, a total of 20 TB - 7.3 TB = expected volume size of 12.7 TB. So why is there 1.3 TB missing? This makes me wonder just what to expect if I try to expand the RAID again at a later date by replacing the two 3 TB drives with a couple of 6 or 8 TB drives?
As I said above, I THOUGHT that as long as my largest drive did not change, I would gain 100% of the new drive space in the total volume size?
8 Replies
Mauser69 wrote:Looking closer at the Admin Page Volumes screen, the NAS reports these capacites for each drive: 7.3 + 7.3 + 2.7 + 2.7.
Thus, using the actual capacities reported by the NAS, a total of 20 TB - 7.3 TB = expected volume size of 12.7 TB. So why is there 1.3 TB missing?
8TB drives are decimal so effective 7.3TB resp. 3TB are effectively 2.7TB. They sell you 1TB ==1.000.000.000.000 B (and not 1.099.511.627.776 B) As two of each are in a RAID1 it's 7.3TB+2.7TB or about 11TB effectively available - minus file system overhead.
Mauser69 wrote:And I also assumed that if I upgraded the smaller drives, I would gain 100% of the new capacity in
As I said above, I THOUGHT that as long as my largest drive did not change, I would gain 100% of the new drive space in the total volume size?
And where should the RAID redundancy come from for the added devices?
"As two of each are in a RAID1 it's 7.3TB+2.7TB or about 11TB effectively available - minus file system overhead."
Actually, I do not think that is correct, at least as far as I understand the manual. And the Admin Page, System | Volumes tab, specifically reports that this volume is X-RAID, RAID 5 (not RAID 1).
Furthermore, since the whole volume is RAID 5, with one of the largest drives reserved for the redundancy of the entire volume, no ADDITIONAL redundancy space would be needed for any increase in the size of the smaller drives.
The capacity rule is "sum the drives and subtract the largest". In your case, the adds up to 14 TB. The NAS displays TIB though, and that does work out to 12.73 TiB.
But you can't upgrade the 1.5 TB drives to 3 TB drives with XRAID. You can replace drives, or upgrade drives to the largest size in the array (or larger). But (apart from some rare exceptions) you can't upgrade a smaller drive to an intermediate size. That was why the space didn't change.
Mauser69 wrote:
Actually, I do not think that is correct, at least as far as I understand the manual. And the Admin Page, System | Volumes tab, specifically reports that this volume is X-RAID, RAID 5 (not RAID 1).
What you have was a volume comprised of 2 RAID groups -
- a 4 x 1.5 TB RAID-5 group (slightly smaller, but close enough).
- a 2 x 6.5 TB RAID-1 group
These were concatenated together into the single volume.
Note each group is a partition on the drives. So there are two data partitions on the 8 TB drives.
The reason the upgrade to 3 TB didn't work is that the system would have had to repartition the 6.5 partition on the 8 TB drives. XRAID won't do that.
If you want all the extra space, you'll need to re-create the volume and restore the files from backup.
Mauser69 wrote:My RN214 was configured with 2x8 TB + 2x1.5 TB drives, and it reported a total volume size of 9.9 TB.
Depends widely again on how this set-up came together. If this was a scratch install, StephenB theory is appropriate. If this X-RAID started life with two 1.5 TB drives only, StephenB theory is appropriate again as the 2*1.5TB RAID1 will become a 4*1.5 RAID5 plus a 2*6.5 TB RAID1.
If this started from two 8 TB drives and two 1.5 TB units were added later, the two 8 TB drives will be RAID1, and the two added 1.5 TB units will be added without re-partitioning the existing 2*8 TB RAID1.
Essentially the same what happens when replacing the 1.5 TB by two 3 TB units - the existing 1.5 TB partition on the 8 TB drives won't change, you end with with a RAID1 from 2*6.5TB, a RAID5 from 1.5TB, and a RAID1 from the "new" 2*1.5TB. As this is mostly concatenate the performance advantage by the RAIDs is mixed, writing will happen for a good part on a single disk performance minus some overhead for the RAID1, read can happen from two storage blocks in parallel.
Shoot me StephenB - you know as a QNAP geek I prefer strict RAID configs without mixing RAID1 and RAID5 ... and I'm aware of the risks of running RAID5 starting with large capacity drives - where RAID6 is mandatory but X-RAID does ignore this. So I can be bluntly wrong in my understanding of X-RAID, at least with four storage blocks.
In no way the additional TBs added will add 100% to the available capacity.
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