On Thursday June 22, davidsen@xxxxxxx wrote: > Neil Brown wrote: > > >On Monday June 19, Paul.Davidson@xxxxxxxxxx wrote: > > > > > >>Hi, > >> > >>I'd like to shrink the size of a RAID5 array - is this > >>possible? My first attempt shrinking 1.4Tb to 600Gb, > >> > >>mdadm --grow /dev/md5 --size=629145600 > >> > >>gives > >> > >>mdadm: Cannot set device size/shape for /dev/md5: No space left on device > >> > >> > > > >Yep. > >The '--size' option refers to: > > 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. > >(from the man page). > > > >So you were telling md to use the first 600GB of each device in the > >array, and it told you there wasn't that much room. > >If your array has N drives, you need to divide the target array size > >by N-1 to find the target device size. > >So if you have a 5 drive array, then you want > > --size=157286400 > > > > May I say in all honesty that making people do that math instead of the > computer is a really bad user interface? Good, consider it said. An > means to just set the target size of the resulting raid device would be > a LOT less likely to cause bad user input, and while I'm complaining it > should inderstand suffices 'k', 'm', and 'g'. Let me put another perspective on this. Why would you ever want to reduce the size of a raid5 in this way? The only reason that I can think of is that you want to repartition each device to use a smaller partition for the raid5, and free up some space for something else. If that is what you are doing, you will have already done the math and you will know what size you want your final partitions to be, so setting the device size is just as easy as setting the array size. If you really are interested in array size and have no interest in recouping the wasted space on the drives, then there would be no point in shrinking the array (that I can think of). Just 'mkfs' a filesystem to the desired size and ignore the rest of the array. In short, reducing a raid5 to a particular size isn't something that really makes sense to me. Reducing the amount of each device that is used does - though I would much more expect people to want to increase that size. If Paul really has a reason to reduce the array to a particular size then fine. I'm mildly curious, but it's his business and I'm happy for mdadm to support it, though indirectly. But I strongly suspect that most people who want to resize their array will be thinking in terms of the amount of each device that is used, so that is how mdadm works. > > Far easier to use for the case where you need, for instance, 10G of > storage for a database, tell mdadm what devices to use and what you need > (and the level of course) and let the computer figure out the details, > rounding up, leaving 128k, and phase of the moon if you decide to use it. > mdadm is not intended to be a tool that manages your storage for you. If you want that, then I suspect EVMS is what you want (though I am only guessing - I've never used it). mdadm it a tool that enables YOU to manage your storage. NeilBrown > Sorry, I think the current approach is baaad human interface. > > -- > bill davidsen <davidsen@xxxxxxx> > CTO TMR Associates, Inc > Doing interesting things with small computers since 1979 - 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
