On Thu, Nov 30 2017, Duane wrote: > Why is the data offset set so big? I created a 3x4TB RAID5 array and the > data offset was 128MB. Chunk size was the default 512kB. It is less than 0.1% of the device... > > I cannot see why such a large offset is used. I would think the data > offset need only be at most the chunk size plus the space (1 sector) for > the superblock and bitmap. It is insurance. If you want to change the chunksize later, having a lot of head-room will allow the reshape to go much faster. > > When reshaping the array, I am prompted to use an external file, so I > don't see that mdadm ever uses the space. Citation needed.... what version of mdadm, what kernel? What reshape command? > > > I tried making some test arrays and got much smaller sizes. A 3x1GB > RAID5 array with 64k chunks had a 1MB data offset. > > > If I make a 7x4TB RAID5 array with 64kB chunks, is there a problem with > setting the data offset to around 2MB? Only that it might reduce your options in the future, though probably not by much. NeilBrown
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