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Asked: 2011-12-18 16:14:25 +0800 CST

lvm over two raid1's - one slow, one fast. What will happen?

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Two software RAID1's,

0: 2x 15K SAS drives, 73GB

1: 2x WD Caviar Green, 3TB

So the first RAID is a faster one, the second a slower data grave. What happens when I add both to a LVM logical volume so they form one large partition?

I don't expect lvm to intelligently move more often used data to the quicker hardware... right? Any suggestions for simple multi-tiered storage with these 4 drives?

Unanswered so far: Will the lv slow down, as in "the weakest link"? Or will it randomly slow down, with some blocks coming from fast, some from slow disks?

lvm storage raid1

2 Answers

  1. Right. LVM is not that smart. I'm afraid that best bet at this is ZFS. But I have no idea what's the ZFS on linux code status...

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  2. No, LVM will not magically relocate 'hot blocks' to the faster disks. As far as I know, there is no standard and/or open-source method to do this (ZFS comes close).

    You could (manually) decide on a per filesystem (or directory-tree) basis to host it on the SAS or SATA disks.

    I myself have a somewhat equal setup, with the OS and databases on fast disks, and backups, logging and other less time-critical bulk data on slower sata disks.

    I think there are a few experimental filesystems coming around that will do more in this area, but nothing production-ready.

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