Shrinking mdadm w/XFS to over-provision SSD

December 3rd, 2013, 03:54 AM

I emailed this to a friend, but thought I would post it here as well as I need some help.

Big Mike

Given now that two drives out of four have failed in a year, I really need to re-provision both of these servers so that they are only using 80% of the SSD space (over-provision). But I cannot figure out a good way to do so.

I don�t know of any utility that would let me shrink a mdadm array with an XFS filesystem. So this means I have to back everything up, then do a secure erase on the SSD to free up the blocks, then re-partition the drives so only 80% is used by mdadm raid, and then restore the backup to that now smaller XFS partition.

But I don�t know how to do this really. I am not sure if a simple tar backup is good enough with all the symlinks everywhere, plus is there data on the file level that describes the total size of the partitions or mdadm array which upon restore would be incorrect?

Any ideas? I definitely do not want to re-create the servers. I just want to do a backup and restore at most. I am very uncomfortable trusting tar because I�ve had so many problems with it in the past with symlinks, and I�ve never tried to tar the entire / and expect it to restore correctly. I am more comfortable using something like CloneZilla to clone an entire partition, assuming that CloneZilla will let you restore to a smaller partition.

After I emailed him, I looked around some more and found some people using GParted and rsync, but this is without mdadm which won't work for me.

Utechu: Cloning Ubuntu to a smaller disk

I also found an article on CloneZilla about restoring a cloned partition to a smaller disk:

Clonezilla restore to smaller hard drive - Spiceworks

Clonezilla - Advanced Mode

The consensus was generally to use the -icds option, and it sounds like it would work. But since these are live production servers I have a lot of testing to do in a VM before I attempt it. Even if it would work, it's still a lot of work and I am unsure how mdadm will complicate things.

If anyone has advice let me know.

If you are unaware of SSD over provisioning, google it. Short version: I need to shrink mdadm and XFS (impossible, so will take backup+restore of some sort) so that only 80% of the drive is addressable by the OS, leaving 20% over-provisioned. To make things more interesting, the only way to gain this on an existing formatted drive would to be to do an SSD Secure Erase after the backup, so that each block is "owned" by the SSD and not addressable by the OS, freeing it up. So that means I had better have a good backup!

If anyone has direct experience with this or cares to do some VM simulations for me let me know. Debian 7 wheezy on kernel 3.12.0, mdadm 3.2.5, (2) disk RAID 1:

Code

root@phoenix:~# cat /proc/mdstat
Personalities : [raid1]
md0 : active raid1 sda1[0] sdb[2]
      249926464 blocks super 1.2 [2/2] [UU]

Keep in mind that during all of this futures.io (formerly BMT) would be off-line, so I am looking for a compact/quick method. I can use a local server (NFS or SSH) to store the image at 1Gb link, so that will help.

Mike

December 3rd, 2013, 07:32 AM

Saving this for later.

Technology of Today - Thoughts & Exploration: Using Clonezilla to Clone From a Larger Hard Drive to a Smaller Drive

Also I don't think I mentioned that the filesystem is XFS. Everything I am reading so far is only talking about raw file systems. The issue is when you add in mdadm it seems, not sure on this one...

Mike

December 3rd, 2013, 11:49 AM

@sam028 recommended

Quoting

I would use xfsdump, that will dump all the file-system.
In the worst case, a simple dd if=/dev/... can do the job.
All these can be sent to stdout, which can be send on another host using ssh.

And I had looked at xfsdump even before I started this thread, but just afraid to use it on something like / but I guess I will take a look and see if I can make it work in a VM.

Mike

Shrinking mdadm w/XFS to over-provision SSD

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Shrinking mdadm w/XFS to over-provision SSD