raid0: "clean" state on drive failure/removal

raid0: "clean" state on drive failure/removal

[Sushma Gurram]

Hi,

A basic question on raid0 behavior.

When a drive fails/removed in a raid0 array, “mdadm –detail” reports the array
state as clean instead of failed.

It appears that the drive is whacked out of the array, but that array slot
continues to show “active sync” state.

I/O to the failed drive errors out. However, it appears that I/O to other drives
in the raid0 array would succeed. How would the user know about the array state
and potential data loss? Should the array information be cached, drive
failure/removals be monitored and checked against the cached information?

Could anyone please explain the reason for this behavior of not reporting a
raid0 array as failed?

Thanks,
Sushma

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Re: raid0: "clean" state on drive failure/removal

[NeilBrown]

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On Fri, 27 Feb 2015 20:54:32 +0000 Sushma Gurram <Sushma.Gurram@sandisk.com>
wrote:

> Hi,
> 
> A basic question on raid0 behavior.
> 
> When a drive fails/removed in a raid0 array, “mdadm –detail” reports the array
> state as clean instead of failed.
> 
> It appears that the drive is whacked out of the array, but that array slot
> continues to show “active sync” state.
> 
> I/O to the failed drive errors out. However, it appears that I/O to other drives
> in the raid0 array would succeed. How would the user know about the array state
> and potential data loss? Should the array information be cached, drive
> failure/removals be monitored and checked against the cached information?

The user would know about potential data loss in *exactly* they same way as
if half the sectors on a single hard-drive stopped working.

i.e. the filesystem would complain.

> 
> Could anyone please explain the reason for this behavior of not reporting a
> raid0 array as failed?

RAID0 is not really RAID - there is no redundancy.
So it isn't treated like RAID.

RAID0 is simply a mapping off linear addresses onto multiple devices.
There is no sense in which the array as a whole "fails", or any value in
marking a device as "failed".

If there is something wrong, then IO requests will fail.  Not necessarily all
IO requests, but some.  Just like a normal drive that has partially failed.

NeilBrown

> 
> Thanks,
> Sushma
> 
> ________________________________
> 
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> 
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