> You can't add devices to an existing raidz1 or raidz2 "stripe". And the
> reason you can't is precisely because of full-stripe -- but variable width --
> writes that RAID-Z uses to avoid the RAID-5 write hole. Let's say you have
> a 4+2 raidz2 configuration and you're doing a bunch of 1 sector writes:
>
> | P | P | D | P | P | D |
> | P | P | D | P | P | D |
> ...
>
> There may be several parity sectors per row so adding another column doesn't
> work.
But presumably it would be possible to use additional columns for future
writes?
new new
|1P |1P |1D |2P |2P |2D | 0 | 0 |
|3P |3P |3D |4P |4P |4D | 0 | 0 |
|5P |5P |5D |6P |6P |6D |7P |7P |
|7D |8P |8P |8D |9P |9P |9D | 0 |
The fs blocks retain their own parity, so there's no need to zero the
disks when adding. Presumably if you did this only when the earlier
disks were completely almost full, you might not have enough columns to
complete writes, but before that point you should be okay. Over time,
freeing existing blocks would allow more space for expanded writes. (or
perhaps a background process to rewrite them).
That's just a thought. I have no idea if such a scheme would be
technically feasable given the existing code.
--
Darren Dunham [EMAIL PROTECTED]
Senior Technical Consultant TAOS http://www.taos.com/
Got some Dr Pepper? San Francisco, CA bay area
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