My 'mdadm' (v1.6.0) man page includes: # -z, --size= # Amount (in Kibibytes) of space to use from each drive in # RAID1/4/5/6. This must be a multiple of the chunk size, and # must leave about 128Kb of space at the end of the drive for the # RAID superblock. If this is not specified (as it normally is # not) the smallest drive (or partition) sets the size, though if # there is a variance among the drives of greater than 1%, a # warning is issued. There are several problems when trying to interpret the phrase "This must be a multiple of the chunk size, and must leave about 128Kb of space at the end of the drive for the RAID superblock." (1) Someone resizing an array may not know the chunk size, and it's unclear if assuming the default of 64 is OK, or dangerous. (My experiments show that 'mdadm' will accept a size value that is not a multiple of 64, and will update the array size as shown by --detail to this non-multiple size, at least for RAID1. Does this risk disaster?) (2) "Kb" is technically the abbrieviation of "kilobits", not "kibibytes". I'm assuming "128Kb" means "128 kibibytes" from the preceding context. I suggest avoiding the abbrieviation entirely to avoid confusion. (3) To "leave" space is ambiguous in what it means for the value specified. Should the we take the amount of space needed for our filesystem and add 128K to get the 'size' value to specify? Or specify exactly what's needed for the filesystem, and be aware that RAID will actually use 128K more on the consitutent devices than what was specified? (I *think* the second is meant, but I'm not sure.) (4) The imprecise "about 128Kb" raises the question: is more than 128K sometimes needed? If I "leave" exactly 128K, is that a recipe for eventual disaster when someday the superblock goes over this allotment? Any clarifications appreciated. Thanks! - Gordon @ Bitzi - 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
