Re: Replacing SW RAID-1 with SSD RAID-1

[Warren Young <warren@xxxxxxxxxxx>]

On Nov 24, 2020, at 10:05 AM, Simon Matter <simon.matter@xxxxxxxxx> wrote:
> 
> Why is a layered approach
> worse than a fully included solution like ZFS?

Just one reason is that you lose visibility of lower-level elements from the top level.

You gave the example of a bad block in a RAID.  What current RHEL type systems can’t tell you when that happens is which file is affected.

ZFS not only can tell you that, deleting or replacing the file will fix the array. That’s the bottom-most layer (disk surface) telling the top-most layer (userspace) there’s a problem, and user-space fixing it by telling the bottom-most layer to check again.

Because ZFS is CoW, this isn’t forcing the drive to rewrite that sector, it’s a new set of sectors being brought into use, and the old ones released.  The sector isn’t retried until the filesystem reassigns those sectors.

Red Hat is attempting to fix all this with Stratis, but it’s looking to take years and years for them to get there.  ZFS is ready today.

> The tools differ but some
> complexity always remains.

In my experience, ZFS hides a lot of complexity, and it is exceedingly rare to need to take a peek behind the curtains.

(And if you do, there’s the zdb command.)

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