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korkman
Asked: 2011-10-21 11:28:52 +0800 CST

lvm2 restriping?

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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).

linux lvm

2 Answers

  1. Best Answer

    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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  2. 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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