Re: MDADM RAID1 of external USB 3.0 Drives
On Thu, Sep 11, 2014 at 06:17:00PM +0200, Linux-Fan wrote:
> some time ago, I bought two external Seagate 2 TB USB 3.0 HDDs in order
> to expand my local storage (all internal slots are already in use).
> Having created a RAID1 with MDADM just as normal, it all seemed to work,
> until at one system startup MDADM told me via local mail that the Array
> was degraded. In fact, one of the devices had not been recognized by the
> system. Reconnecting the drive however, did not allow me to simply
> continue as normal -- I had to add the device back into the RAID and
> thereby triggered a resync. This did not seem a problem at the
> beginning, but as the problem occurred a second time, I thought I should
> search for a solution (which did not bring up anything interesting or
> related) and then ask:
>
> Is there any means to configure MDADM (or such) to make sure that all
> devices are recognized before attempting to start the array so that I
> could manually reconnect the missing disk and then start the array
> without any resync?
>
> If not, might it be a good idea to write a script to check if the
> devices are available and only then enable that RAID?
>
> I want to avoid doing superflous resyncs as this always takes a lot of
> time and seems to be an unnecessary load for the drives.
What's actually happening here is that mdadm is rejecting one or
the other disk because of a problem reading or writing to that.
It's almost certainly a real problem, and in my experience it is
not the disk itself which is bad, but something in the path (the
USB port, the USB cable, the USB-SATA interface) or the power
supply for the disk.
You will continue to have these problems if you persist in doing
this, up until the day that one disk actually fails. Time to do
something else. If you can change to ESATA or invest in a SAS
controller and external SAS multi-disk chassis, you can get
reliable data storage again.
In the meantime, you can:
- add a bitmap file to the RAID, which will speed up rebuilds.
- use the --no-degraded flag, to prevent assembly of a RAID that
is lacking a disk.
-dsr-
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