On 12 April 2011 22:15, NeilBrown <neilb@xxxxxxx> wrote: > On Wed, 13 Apr 2011 00:22:38 +0600 Roman Mamedov <rm@xxxxxxxxxx> wrote: > >> On Tue, 12 Apr 2011 18:21:13 +0100 >> Mathias BurÃn <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: Âthe 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 > > >> >> > ÂWill 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. >> > > Right, so using --preserve seems like a sane and good option. Thanks for the info, I'll let you know what happens, HDD should arrive the next few days. // Mathias -- To unsubscribe from this list: send the line "unsubscribe linux-raid" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
