Re: Queuing of dm-raid1 resyncs to the same underlying block devices

[Neil Brown <neilb@suse.de>]

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Heinz Mauelshagen <heinzm@redhat.com> 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.

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.
So if the parity was wrong before, it will be wrong afterwards.

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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