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After some search I assume lvm2 restriping, as in take all extents of an active LV and redistribute them evenly across all current PVs in the VG, is not possible?

For example:

  • 1 Volume Group with 6 Physical Volumes HDD-based and 1 Physical Volume SSD-based.
  • A new LV is created exclusively on the SSD PV, using lvcreate options, to cope with high write IOPS.
  • Later, IOPS decline and the LV hogs valuable SSD space, so it shall be moved to the 6 Physical Volumes, striped to keep up with medium write IOPS.

(having a mix of SSD and HDD in the same VG is basically a fun thing to do, because pvmove can move LVs online from SSD to HDD and back, but unfortunately it can't stripe while moving)

The offline case is straight forward: create a new striped LV across the HDDs, dd all content from old to new, rename, online again. But is there a way to do this online?

Closest thing I could come up with is write a bash script running pvmove to scatter the extents, but that wouldn't be the same (since extent size is a whopping 4 MiB opposed to typical stripe size of 64 KiB).

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How many disks at which size are at your disposal? Do you want to do this online? How big is your maintenance window, how big is your current data? – Nils Oct 20 '11 at 21:28
I already moved the LV in question offline, so this is more a hypothetical question. Just added an example to clear things up. – korkman Oct 21 '11 at 11:03
So see my answer as hypothetical, too. ;-) How long were you offline for your move? – Nils Oct 21 '11 at 21:48
50 GB management volume, non-production anyways. I'm just curious :-) – korkman Oct 22 '11 at 10:33
up vote 1 down vote accepted

Since you clarified your setup I would once again have gone through the md-layer.

  1. Create a degraded Raid 1 with mdadm which consists of your original LV (on SSD)
  2. Remount that md-device instead of the LV
  3. Add the LV consisting of the 6 HDs to that raid, let it settle
  4. remove the SSD-LV from the md-device (thus degrading the md raid 1 again)
  5. Optional: umount md-device, mount HD-LV instead.

    1. and 5. will propably lead to a short downtime (if remount does not work online).

But: md-raid1 mirroring is way faster than pvmove - and will lead to the new layout almost without service interruption.

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Interesting approach. I wonder if a similar one realized with LVM mirroring, or even more low-level dm-mirror would work without interruption? Anyways, I take this as a "no" to my original question: no re-striping without workarounds. – korkman Oct 22 '11 at 10:27
BTW I have drbd in place on top of LVM, so in fact I can re-stripe without downtime. Only that one volume didn't have drbd. – korkman Oct 22 '11 at 10:29
How did you do that with drbd? Did you change the backend device online? Or did you change the secondary side and then switched over? – Nils Oct 23 '11 at 21:03
Come to speak of it, I didn't try that yet. I'd go for "drbdadm down /dev/drbdX" on the secondary, create a new LV, let primary overwrite secondary, put primary in diskless mode, then sync back secondary to primary. – korkman Oct 23 '11 at 21:13
BTW - you could propably do the above steps online if you assign the md-device to your VG and then pvmove to that md. – Nils Oct 24 '11 at 20:54

I'm afraid your best approach would be to do a full dump of the LVs within the VG; destroy the VG; recreate the VG with the new striping parameters;recreate the LVs; restore the LVs from the dump.

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VG recreation is not necessary for new striping parameters, only LV recreation, provided the VG contains all PVs. – korkman Oct 22 '11 at 11:49

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