Dear all, First I'd like to thank you for the great work you've done with mdadm. It's flexible, powerful and reasonably easy to use. I have a question that seems both important for the redundancy of my RAID6 devices and too sharp for me, my friends and the newsgroup fr.comp.stockage . I think your understanding of the inner workings of mdadm should allow you to already know the answer. A 12-disks machine was recently assembled with disks coming from several sources (Linux Ubuntu server, kernel 2.6.15, RAID6 array). All 12 disks are S-ATA with 250 GB capacity. I understand that partitions should have the same size, meaning the same number of bytes. All 12 disks have exactly 250059350016 bytes, but the number of blocks that can be allocated are not the same from one disk to another, ranging from 244196001 to 244198384. Small difference, but significant nonetheless if this means that data can be lost. So I looked for a number of blocks that would be the same for all disks. But the number of blocks cannot be imposed when creating a partition, only the number of cylinders. On the disks with 244196001 blocks, 'fdisk -l' says: Units = cylinders of 16065 * 512 = 8225280 bytes For the other half, it says: Units = cylinders of 2048 * 512 = 1048576 bytes Since 16065 and 2048 have no common divisor, the smallest multiple common to 8225280 and 1048576 is 8225280 × 1048576 / 512 = 16845373440. Thus trying to have exactly the same number of bytes in each partition means choosing a partition size that is a multiple of roughly 16 GB. Now the total size (250059350016) divided by this step (16845373440) is roughly 14.84. I can only have 14 steps, leading to a partition size of 235835228160 and a loss of \approx 15 GB per disk or 150 GB total. That's a lot. Now I imagine that there might be embedded in mdadm a correction mechanism. Since each partition is divided into stripes, the number of stripes for each disk could be the number of stripes on the smallest partition of the array. Thus I could create partitions as big as each disk supports, and 'mdadm' would take care of the small differences. I'd have my 150 GB back. Could you tell me if such a mechanism exists in mdadm? Or should I accept the "loss" of the 150 GB? Thanks a lot for your help! Sébastien. - 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
