Re: MDADM RAID1 of external USB 3.0 Drives
On Sat, Sep 13, 2014 at 08:06:59PM -0400, Dan Ritter wrote:
> 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.
>
+1 - once upon a time, I needed a large array to build a local Debian
mirror so slung together a few 500GB external drives - the largest easily available
drives to make 1.5TB RAID5. It would last a couple of weeks, then something
would unmount one or more drives, then I'd take a week to get everything back in
order ...
Using large external drives is fine for copying around large files and using rsync, making
backups and then disconnecting the large drive. Good quality SATA drives are really, really cheap and building
a machine as a storage server will cost little more than the largest external drives.
Goodness, an HP Microserver fully made and ready for four drives costs about twice the cost
of a 4TB internal drive.
If you have the luxury of a datacentre, then machines become cheap, relatively speaking, with small amounts of
storage. Adding 36TB of storage becomes feasible in a 4U box - it's expensive, but cheap compared to the cost
of running a datacentre ... the hard part is when you decide that you at home want greater than about 12TB
and don't want a server farm at home :)
> 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.
>
AndyC
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