On Wed, 13 Apr 2011 00:22:38 +0600 Roman Mamedov <rm@xxxxxxxxxx> wrote: > On Tue, 12 Apr 2011 18:21:13 +0100 > Mathias Burén <mathias.buren@xxxxxxxxx> wrote: > > > If I use --layout=preserve , what impact will that have? > > If I preserve the layout, what is the final result of the array > > compared to not preserving it? > > Neil wrote about this on his blog: > "It is a very similar process that can now be used to convert a RAID5 to a > RAID6. We first change the RAID5 to RAID6 with a non-standard layout that has > the parity blocks distributed as normal, but the Q blocks all on the last > device (a new device). So this is RAID6 using the RAID6 driver, but with a > non-RAID6 layout. So we "simply" change the layout and the job is done." > http://neil.brown.name/blog/20090817000931 > > Admittedly it is not completely clear to me what are the long-term downsides of > this layout. As I understand it does fully provide the RAID6-level redundancy. > Perhaps just the performance will suffer a bit? Maybe someone can explain this > more. If you specify --layout=preserve, then all the 'Q' blocks will be on one disk. As every write needs to update a Q block, every write will write to that disk. With our current RAID6 implementation that probably isn't a big cost - for any write, we need to either read from or write to each disk anyway. Anyway: the only possible problem would be a performance problem, and I really don't know what performance impact there is - if any. > > If anything, I think it is safe to use this layout for a while, e.g. in case > you don't want to rebuild 'right now'. You can always change the layout to the > traditional one later, by issuing "--grow --layout=normalise". Or perhaps if > you plan to add another disk soon, you can normalise it on that occasion, and > still gain the benefit of only one full reshape. Note that doing a normalise by itself later will be much slower than not doing a preserve now. Doing the normalise later when growing the the device again would be just as fast as no doing the preserve now. NeilBrown > > > Will the array have redundancy during the rebuild of the new drive? > > If you choose --layout=preserve, your array immediately becomes a RAID6 with > one rebuilding drive. So this is the kind of redundancy you will have during > that rebuild - tolerance of up to one more (among the "old" drives) failure, > in other words, identical to what you currently have with RAID5. >
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