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Julian
Asked: 2012-04-15 04:19:13 +0800 CST

What's the difference between "mirror" and "raid1" in LVM?

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On the lvconvert(8) man page it says:

--type SegmentType
       Used to convert a logical volume to another segment type  or  to
       explicitly  state  the  desired  RAID1 segment type ("mirror" or
       "raid1") when converting a linear logical  volume  to  a  mirror
       with the '-m' argument.

But what exactly is the difference between "mirror" and "raid1"?

lvm mirroring raid1

2 Answers

  1. The practical difference is that the 'raid1' mirror segment type always stores its logs (in fact, metadata subvolumes) on-disk on the same PVs as the lv being mirrored. You no-longer need a third pv for the log, or to store the log in-memory, and consequently the --corelog and --mirrorlog disk/core/mirrored options to lvconvert are not applicable to the raid1 mirror segment type.

    Another practical difference is that you need an additional 1 PE on both PVs (original and mirror) in order to store the log(s), allocated when the mirror is created. If you see an error like 1 extents needed, but only 0 available when trying to create a raid1 type mirror with lvconvert, this is probably a failure to allow 1 PE additional space for the log on each of the PVs.

    Since about September 2013, raid1 has been the default mirror segment type in lvm2.

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  2. Best Answer

    I have not yet tried the new LVM segment types, but the overview is that they are support for the Linux MD RAID personalities in LVM. That is, they are RAID levels 1, 5, 6 etc. using the MD code with the eventual goal of removing the duplicate functionality of LVM's mirroring and having both MD and LVM use the same code.

    This is very new stuff so may not be appropriate for a production setup yet.

    For example it is still considered a technology preview in RHEL 6.2:

    http://docs.redhat.com/docs/en-US/Red_Hat_Enterprise_Linux/6/html/6.2_Release_Notes/storage.html

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