Jef Pearlman wrote:
Perhaps I'm not asking my question clearly. I've already experimented a fair amount
with zfs, including creating and destroying a number of pools with and without
redundancy, replacing vdevs, etc. Maybe asking by example will clarify what I'm
looking for or where I've missed the boat. The key is that I want a grow-as-you-go
heterogenous set of disks in my pool:
The short answer:
zpool add -- add a top-level vdev as a dynamic stripe column
+ available space is increased
zpool attach -- add a mirror to an existing vdev
+ only works when the new mirror is the same size or larger than
the existing vdev
+ available space is unchanged
+ redundancy (RAS) is increased
zpool detach -- remove a mirror from an existing vdev
+ available space increases if removed mirror is smaller than
vdev
+ redundancy (RAS) is decreased
zpool replace -- functionally equivalent to attach followed by detach
Let's say I start with a 40g drive and a 60g drive. I create a non-redundant pool
(which will be 100g). At some later point, I run across an unused 30g drive, which
I add to the pool. Now my pool is 130g. At some point after that, the 40g drive
fails, either by producing read errors or my failing to spin up at all. What happens
to my pool? Can I mount and access it at all (for the data not on or striped across
the 40g drive)? Can I "zfs replace" the 40g drive with another drive and have it
attempt to copy as much data over as it can? Or am I just out of luck? zfs seems like
a great way to use old/unutilized drives to expand capacity, but sooner or later one
of those drives will fail, and if it takes out the whole pool (which it might
reasonably do), then it doesn't work out in the end.
For non-redundant zpools, a device failure *may* cause the zpool to be
unavailable.
The actual availability depends on the nature of the failure.
A more common scenario might be to add a 400 GByte drive, which you can use to
replace the older drives, or keep online for redundancy.
The zfs copies feature is a little bit harder to grok. It is difficult to
predict how the system will be affected if you have copies=2 in your above
scenario, because it depends on how the space is allocated. For more info,
see my notes at:
http://blogs.sun.com/relling/entry/zfs_copies_and_data_protection
-- richard
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