Brandon High wrote:
I might have this mentioned already on the list and can't find it now,
or I might have misread something and come up with this ...
Right now, using hot spares is a typical method to increase storage
pool resiliency, since it minimizes the time that an array is
degraded. The downside is that drives assigned as hot spares are
essentially wasted. They take up space & power but don't provide
usable storage.
Depending on the number of spares you've assigned, you could have 7%
of your purchased capacity idle, assuming 1 spare per 14-disk shelf.
This is on top of the RAID6 / raidz[1-3] overhead.
What about using the free space in the pool to cover for the failed drive?
With bp rewrite, would it be possible to rebuild the vdev from parity
and simultaneously rewrite those blocks to a healthy device? In other
words, when there is free space, remove the failed device from the
zpool, resizing (shrinking) it on the fly and restoring full parity
protection for your data. If online shrinking doesn't work, create a
phantom file that accounts for all the space lost by the removal of
the device until an export / import.
It's not something I'd want to do with less than raidz2 protection,
and I imagine that replacing the failed device and expanding the
stripe width back to the original would have some negative performance
implications that would not occur otherwise. I also imagine it would
take a lot longer to rebuild / resilver at both device failure and
device replacement. You wouldn't be able to share a spare among many
vdevs either, but you wouldn't always need to if you leave some space
free on the zpool.
Provided that bp rewrite is committed, and vdev & zpool shrinks are
functional, could this work? It seems like a feature most applicable
to SOHO users, but I'm sure some enterprise users could find an
application for nearline storage where available space trumps
performance.
-B
What you describe makes no sense for single-parity vdevs, since it
actually increases the likelihood for data loss. In multi-parity vdevs,
even with the loss of one drive, you still have full parity protection,
so why would you go for all that extra effort, since it gains you what?
From a global perspective, multi-disk parity (e.g. raidz2 or raidz3) is
the way to go instead of hot spares.
Hot spares are useful for adding protection to a number of vdevs, not a
single vdev.
--
Erik Trimble
Java System Support
Mailstop: usca22-123
Phone: x17195
Santa Clara, CA
Timezone: US/Pacific (GMT-0800)
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