Re: Shrinking an array

[Wakko Warner <wakko@animx.eu.org>]

Adam Goryachev wrote:
> On 11/04/17 10:30, Wakko Warner wrote:
> >I have a question about shrinking an array.  My current array is 4x 2tb
> >disks in raid6 (md0).  The array was created on the 2nd partition of each
> >disk and spans most of the disk.  I would like to replace the 2tb disks with
> >750gb disks.  md0 is a luks container with lvm underneath.  I have less than
> >1tb actually in use.  What would the recommended procedure be for shrinking
> >this?  I've watched this list, but I don't think I've come across anyone
> >actually wanting to do this before.
> >I'm thinking of these steps already:
> >1) Shrink PV.
> >2) Shrink luks.  I'm aware that there is not size metadata, but the dm
> >mapping would need to be shrunk.
> >3) Shrink md0.  I did this once when I changed a 6 drive raid6 into a 5
> >drive raid6.  Would I use --array-size= or --size= ?  I understand the
> >difference is the size of md0 vs the individual members.
> >
> >So for number 4, if md0 is now small enough, will it accept a member that is
> >smaller?  If so, I should beable to add the member to the array and issue
> >--replace.
> >
> >Thanks.
> >
> I think the order is wrong.... or I mis-understood the layering. You
> need to shrink the highest layer first and work down the stack, for
> LVM on luks on RAID it would be something like this:
> 1) Reduce the filesystem size
> 2) Reduce the LV size
> 3) Reduce the PV size
> 4) Reduce the luks size
> 5) Reduce the RAID (mdadm) size
> 6) Replace the physical devices with smaller ones (or reduce the
> partition size/etc as needed)

Thanks, however, 1 and 2 doesn't need to happen.  I deleted several LVs.  My
highest PE in use is 771583 (each extent is 1M) and the last lv could be
moved somewhere else which would leave my highest PE 87982. 

> I generally reduce my each one with a decent margin, and then when
> I'm finished, I increase each one to fill the available space (after
> the physical device size is changed). This avoids issues with
> accidentally trimming too much and then losing data/corrupting data.
> You should also verify that all your data is accessible after each
> step, most steps are reversible if you identify the issue quickly
> enough (at least with simple stacks when changing partition size,
> LVM and/or luks might complicate that).

I'm not that concerned about the LV that is at the highest PE range.  If I
loose it, I loose it.  it's the reduce the raid size I was wondering about. 
Can you give me an example of the command to run for it?

I would be interested in doing this while the system is running (it only has
1180 days uptime =)  However, I don't have the replacement disks yet.  I
wanted ideas before I actually do it.

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