Re: raid1: cannot add disk to replace faulty because can only mount fs as read-only.

[Adam Borowski <kilobyte@angband.pl>]

On Fri, Jan 27, 2017 at 03:03:18PM -0500, Austin S. Hemmelgarn wrote:
> On 2017-01-27 11:47, Hans Deragon wrote:
> > However, as a user, I am seeking for an easy, no maintenance raid
> > solution.  I wish that if a drive fails, the btrfs filesystem still
> > mounts rw and leaves the OS running, but warns the user of the failing
> > disk and easily allow the addition of a new drive to reintroduce
> > redundancy.

> Before I make any suggestions regarding this, I should point out that
> mounting read-write when a device is missing is what caused this issue in
> the first place.  Doing so is extremely dangerous in any RAID setup,
> regardless of your software stack.  The filesystem is expected to store
> things reliably when a write succeeds, and if you've got a broken RAID
> array, claiming that you can store things reliably is generally a lie. MD
> and LVM both have things in place to mitigate most of the risk, but even
> there it's still risky.  Yes, it's not convenient to have to deal with a
> system that won't boot, but it's at least a whole lot easier from Linux than
> it is in most other operating systems.

Now, now.  Other RAID implementations already have this feature that you're
clamoring for!  When it is degraded, they will continue without a hitch, and
perform their duties not even bothering the user.  Then a couple years
later, the other disk will fail.  Obviously, there are no backups -- "we
have RAID".  This is when I get a call.

> The second is proper monitoring.  A well set up monitoring system will let
> you know when the disk is failing before it gets to the point of just
> disappearing from the system most of the time.

No problem, the second busted disk I mentioned above will include a full
mbox with a mail from mdadm for every single day.  They were either unread,
or read by an admin who ignored them and perhaps even wrote a filter to send
them to /dev/null.  Because the system still works, what's the hurry?


Meow!
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