0n Mon, Sep 15, 2008 at 11:37:20PM -0700, Ben Rockwood wrote:
>I'll give you an example. I have Dell 2950's with nice LSI MegaSAS
>controllers with 256MB of Battery Backed Cache. I really want to use
>that, so I create a single RAID5 in hardware, then I create ZPool on
>it. Works well, fast. If there is a disk failure the controller deals
>with it, ZFS never knows. HOWEVER, I've have run into corrupt blocks in
>files (commonly database data files). UFS/EXT3/etc. wouldn't have even
>caught this. The MegaSAS controller isn't aware of it, but ZFS is.
>While there are redundant blocks on physical disk, ZFS can't see them
>and thus can't recover my block... this is a scenario in which I turned
>to Ditto Blocks (copies=2) to protect me.
>
>In this example (this is real) I realized that from the standpoint of
>protection, your better off giving the disks directly to ZFS than to put
>them behind a hardware RAID. If I'm going to use Ditto Blocks and
>consume double the disk, why not just use a mirror? In hardware I know
>present 2 RAID1's and mirror those in ZFS so I get write-back caching
>AND protection.
Interesting ...
-aW
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