Jose Luis Barquín Guerola wrote:
Hello.
I have a question about how ZFS works with "Dinamic Stripe".
Well, start with the next situation:
- 4 Disk of 100MB in stripe format under ZFS.
- We use the stripe in a 75%, so we have free 100MB. (easy)
Well, we add a new disk of 100MB in the pool. So we have 200MB free but only
100MB will have the speed of 4 disk and, the rest 100MB will have the speed of
1 disk.
The questions are:
- Have ZFS any kind of reorganization of the data in the stripe that change
this situation and become in 200MB free with the speed of 5 disks?
- If the answer is yes, how is it does? in the background?
Thanks for your time (and sorry for my english).
JLBG
When you add more vdevs to the zpool, NEW data is written to the new
stripe width. That is, when data was written to the original pool, it
was written across 4 drives. It now will be written across 5 drives.
Existing data WILL NOT be changed.
So, for a zpool 75% full, you will NOT get to immediately use the first
75% of the new vdevs added.
Thus, in your case, you started with a 400MB zpool (with 300MB of data).
You added another 100MB vdev, resulting in a 500MB zpool. 300MB is
written across 4 drives, and will have the appropriate speed. 75% of
the new vdev isn't immediately usable (as it corresponds to the 75%
in-use on the other 4 vdevs), so you effectively only have added 25MB of
immediately usable space. Thus, you have:
300MB across 4 vdevs
125MB across 5 vdevs
75MB "wasted" space on 1 vdev
To correct this - that is, to recover the 75MB of "wasted" space and to
move the 300MB from spanning 4 vdevs to spanning 5 vdevs - you need to
re-write the entire existing data space. Right now, there is no
background or other automatic method to do this. 'cp -rp' or 'rsync' is
a good idea.
We really should have something like 'zpool scrub' do this automatically.
--
Erik Trimble
Java System Support
Mailstop: usca22-123
Phone: x17195
Santa Clara, CA
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