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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?

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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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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
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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