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?


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...

  • For the root partition, ZFS is not really natively avail. Could be it might be smarter to make the fast/slow decisions myself, separating RAIDs, the fast one (root) an md raid, the slow one, a ZFS RAID. Good idea? – isync Dec 18 '11 at 0:32
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    Do you really need this kind of data management for root partition? If you need such speed from root, then what you describe seems as a good idea ... but then, there's no point in using ZFS. It all depends on what do you want to store on those drives and/or how do you use the computer. – Fox Dec 18 '11 at 0:42
  • It's just a shame to "waste" a few gigs after I've got the root system and my dbs on the fast spindles. Manually allocating stuff seems the only option (I hate when it's not automagic! ;) ). – isync Dec 18 '11 at 1:44

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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