On 10/07/2015 11:42 PM, Neil Brown wrote:
Heinz Mauelshagen <heinzm@xxxxxxxxxx> writes:
On 10/01/2015 12:20 AM, Neil Brown wrote:
Heinz Mauelshagen <heinzm@xxxxxxxxxx> writes:
BTW:
When you create a raid1/4/5/6/10 LVs _and_ never read what you have not
written,
"--nosync" can be used anyway in order to avoid the initial
resynchronization load
on the devices. Any data written in that case will update all
mirrors/raid redundancy data.
While this is true for RAID1 and RAID10, and (I think) for the current
implementation of RAID6, it is definitely not true for RAID4/5.
Thanks for the clarification.
I find that to be really bad situation.
For RAID4/5 a single-block write will be handled by reading
old-data/parity, subtracting the old data from the parity and adding the
new data, then writing out new data/parity.
Obviously for optimization reasons.
So if the parity was wrong before, it will be wrong afterwards.
So even overwriting complete stripes in raid4/5/(6)
would not ensure correct parity, thus always requiring
initial sync.
No, over-writing complete stripes will result in correct parity.
Even writing more than half of the data in a stripe will result in
correct parity.
Useless, as you say, because we can never be sure, that
any filesystem/dbms/... upstack will guarantee >= half stripe
writes initially; even more so with many devices and large chunk sizes...
So if you have a filesystem which only ever writes full stripes, then
there is no need to sync at the start. But I don't know any filesysetms
which promise that.
If you don't sync at creation time, then you may be perfectly safe when
a device fails, but I can't promise that. And without guarantees, RAID
is fairly pointless.
Indeed.
We should think about a solution to avoid it in lieu
of growing disk/array sizes.
With spinning-rust devices you need to read the entire array ("scrub")
every few weeks just to make sure the media isn't degrading. When you
do that it is useful to check that the parity is still correct - as a
potential warning sign of problems.
If you don't sync first, then checking the parity doesn't tell you
anything.
Yes, aware of this.
My point was avoiding superfluous mass io whenever possible.
E.g. keep track of the 'new' state of the array and initialize
parity/syndrome on first access to any given stripe with
the given performance optimization thereafter.
Metadata kept to housekeep this could be organized in a b-tree
(e.g. via dm-persistent-data), thus storing just one node
defining the whole array as 'new' and splitting the tree up
as we go and have a size threshold to not allow to grow
such metadata too big.
Heinz
And as you have to process the entire array occasionally anyway, you
make as well do it at creation time.
NeilBrown
Heinz
If the device that new data was written to then fails, the data on it is
lost.
So do this for RAID1/10 if you like, but not for other levels.
NeilBrown
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