On 07/08/2017 02:44 PM, Andreas Klauer wrote:
On Sat, Jul 08, 2017 at 01:12:11PM -0500, Ram Ramesh wrote:
1. My MD uses partitions sd{b,c,d,e,f,g}1 instead of full disks. So
I can create partitions on the drive instead of on the MD.
2. This means I need to shrink my current md device to smaller size
(say 12TB-14TB) - need to check my current
active ext4 data size. It is definitely less than 16TB.
3. Repartition the disks to create sd{b-g}2 for the reamining unused
6xnTB area.
4. Created md1 with sd{b-g]2 to get md1.
5. Mount and use md1.
Should work; you have to shrink the filesystem first, (or stick to
whatever size it has now), then the md, then the partition.
What makes this risky is that you have to pick the correct sizes.
When growing you can't go wrong. When shrinking, you have to be
careful not to shrink too much. Filesystems don't like it at all
if their end is missing, and md doesn't like it if the block device
is smaller than what it says in the metadata.
So you have to determine the exact filesystem size (tune2fs -l),
and take mdadm data offsets into account.
Not necessary if you know what you're doing but if in doubt,
you can leave a safety margin with each of these steps.
Regards
Andreas Klauer
I already shrunk ext4 and md as they can be done without having to boot
to a rescue disk. I like to do the last step of partitioning disk using
a rescue disk.
Here is my partition table on one disk (they all are very similar or
identical - each is a 6TB enterprise disk)
zym [rramesh] 431 > sudo gdisk -l /dev/sdb
GPT fdisk (gdisk) version 0.8.8
Partition table scan:
MBR: protective
BSD: not present
APM: not present
GPT: present
Found valid GPT with protective MBR; using GPT.
Disk /dev/sdb: 11721045168 sectors, 5.5 TiB
Logical sector size: 512 bytes
Disk identifier (GUID): D5C9B768-D2E5-4DEE-8D89-73A7B631FE28
Partition table holds up to 128 entries
First usable sector is 34, last usable sector is 11721045134
Partitions will be aligned on 2048-sector boundaries
Total free space is 2014 sectors (1007.0 KiB)
Number Start (sector) End (sector) Size Code Name
1 2048 11721045134 5.5 TiB FD00 Linux RAID
Here is mdadm -E on the same disk
zym [rramesh] 430 > sudo mdadm -E /dev/sdb1
/dev/sdb1:
Magic : a92b4efc
Version : 1.2
Feature Map : 0x1
Array UUID : 0e9f76b5:4a89171a:a930bccd:78749144
Name : zym:0 (local to host zym)
Creation Time : Mon Apr 22 00:08:12 2013
Raid Level : raid6
Raid Devices : 6
Avail Dev Size : 11720780943 (5588.90 GiB 6001.04 GB)
Array Size : 12348030976 (11776.00 GiB 12644.38 GB)
Used Dev Size : 6174015488 (2944.00 GiB 3161.10 GB)
Data Offset : 262144 sectors
Super Offset : 8 sectors
State : active
Device UUID : 2a844ae2:4b028cc3:36095185:7b09f7cc
Internal Bitmap : 8 sectors from superblock
Update Time : Sun Jul 9 17:55:51 2017
Checksum : 74d5464 - correct
Events : 290897
Layout : left-symmetric
Chunk Size : 64K
Device Role : Active device 4
Array State : AAAAAA ('A' == active, '.' == missing)
I am guessing anything more than 6174015488+2048 as the end sector for
/dev/sdb1 should be ok with mdadm. Let me know if I am not calculating
correctly.
I plan to set /dev/sd{b,c,d,e,f,g}1 to 3TB even (start = 2048, end =
6442452991 size = 6442450944 sectors = 3072GiB)
to get a 12TB md0.
Ramesh
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