Hello, I am seeing some unexpected (to me, anyway) behavior after reshaping an array. This is with kernel 3.10.0-862.9.1.el7.x86_64 (Centos 7.5.1804) with mdadm-4.0-13.el7.x86_64. All of the disks I'm discussing here are are exactly 5GiB in size (I'm testing on a VM). Basically, if I create a 4-disk RAID5 with those disks, I get a component size of 5237760 and an overall array size of 16090398720. If I try and grow --size=max on that array, it (reasonably) has no effect. If I create a 3-disk RAID5 with those disks and then add a 4th disk, after the reshape completes, I again have a component size of 5237760, and overall array size of 16090398720, matching that of the original 4-disk RAID5 But, now, if I try and grow --size=max on that array, it *does* have an effect, and the overall array size goes up by 4.5 MiB. My question is why did the --grow manage to increase the component size in the 3+1 case? Shouldn't it have already been set as high as it could go after the reshape? Here's the details: Create a RAID 5 over 4 devices: # mdadm --create /dev/md2 --level 5 --raid-devices 4 /dev/sdb /dev/sdc /dev/sdd /dev/sde mdadm: Defaulting to version 1.2 metadata mdadm: array /dev/md2 started. The component size is for this array is: # cat /sys/block/md2/md/component_size 5237760 Trying to grow it further has no effect: # mdadm --grow /dev/md2 --size=max mdadm: component size of /dev/md2 unchanged at 5237760K The overall size of the array is: # blockdev --getsize64 /dev/md2 16090398720 Now the 3-disk+1 case: Create a 3-disk RAID5: # mdadm --create /dev/md2 --level 5 --raid-devices 3 /dev/sdb /dev/sdc /dev/sdd mdadm: Defaulting to version 1.2 metadata mdadm: array /dev/md2 started. The component size: # cat /sys/block/md2/md/component_size 5237760 And like before, trying to grow it further has no effect: # mdadm --grow /dev/md2 --size=max mdadm: component size of /dev/md2 unchanged at 5237760K Now, I add another disk to the array: # mdadm --grow /dev/md2 --raid-devices 4 --add /dev/sde mdadm: added /dev/sde The component size is, as expected, unchanged: # cat /sys/block/md2/md/component_size 5237760 The overall size of the newly 4-disk array now matches that of the array created with 4 disks from the start: # blockdev --getsize64 /dev/md2 16090398720 So everything is as expected thus far. The component and overall size of the 4-disk array and the 3-disk array reshaped into a 4-disk array match exactly. Now the unexpected bit. I try and resize this new 3-disk-resized-to-4-disk array, and unlike the original 4-disk array, this one *grows*: # mdadm --grow /dev/md2 --size=max mdadm: component size of /dev/md2 has been set to 5239296K The component size went up by 1536K. # blockdev --getsize64 /dev/md2 16095117312 The overall array size went up by 4.5 MiB (because 3 * 1536 ?) Any light anyone can shed on this would be very appreciated! David
