Neil Brown wrote:
On Thursday April 9, steven@xxxxxxxxxxxxxxx wrote:
Given I have a pair of 1TB drives Raid1 I'd prefer to reduce any recovery
sync time. Would an internal bitmap help dramatically, and are there any
other benefits.
Bryan answered some of this but...
- if your machine crashes, then resync will be much faster if you
have a bitmap.
- If one drive becomes disconnected, and then can be reconnected,
recovery will be much faster.
- if one drive fails and has to be replaced, a bitmap makes no
difference(*).
- there might be performance hit - it is very dependant on your
workload.
- You can add or remove a bitmap at any time, so you can try to
measure the impact on your particular workload fairly easily.
(*) I've been wondering about adding another bitmap which would record
which sections of the array have valid data. Initially nothing would
be valid and so wouldn't need recovery. Every time we write to a new
section we add that section to the 'valid' sections and make sure that
section is in-sync.
When a device was replaced, we would only need to recover the parts of
the array that are known to be invalid.
As filesystem start using the new "invalidate" command for block
devices, we could clear bits for sections that the filesystem says are
not needed any more...
But currently it is just a vague idea.
It's obvious that this idea would provide a speedup, and might be useful
in terms of doing some physical dump software which would just save the
"used" portions of the array. Only you have an idea of how much effort
this would take, although my thought is "very little" for the stable
case and "bunches" for the case of an array size change.
I have been trying making a COW copy of an entire drive with qemu-img,
then booting it under KCM, and besides giving an interesting slant to
the term "dual boot," I can back up the changes files (a sparse file)
quickly and into small space with a backup which knows about sparse
files. There is lots of room to imagine uses for this if we had it.
--
bill davidsen <davidsen@xxxxxxx>
CTO TMR Associates, Inc
"You are disgraced professional losers. And by the way, give us our money back."
- Representative Earl Pomeroy, Democrat of North Dakota
on the A.I.G. executives who were paid bonuses after a federal bailout.
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