[mythtv-users] Replace backend 3.5 system disk with 2 2.5 raid1?

[Eric Sharkey]

On Fri, Sep 26, 2014 at 5:13 AM, Simon Hobson <linux at thehobsons.co.uk> wrote:
> Joseph Fry <joe at thefrys.com> wrote:
>
>> You do realize that you can raid partitions, not just drives?
>>
>> My system has its OS/database mirrored across the first partition of all 3 of my recording drives.  Essentially I created a 50GB partition on each drive, mirrored them with mdadm, and mounted it as /.  The remainder of the drives is not raided. There are various guides on how to put your /boot on a mirrored array, I have actually tested disconnecting a drive, and other than the missing recordings, the system booted and functioned normally.
>
> You do realise that from the performance PoV that is possibly the worst configuration possible !

I only use USB disks which I route through a USB -> RS232 -> USB conversion.

> It means that *every* write to the system filesystem has to contend with accesses to all the recording filesystems

The OS drive usage is dominated by reads.

> - and every read from it has to contend with at least one of the recording filesystems.

But only if the system is really busy.  With raid 1, the OS doesn't
issue a read to a drive randomly, it generally picks the least busy
drive (or drives for large reads).  If active recordings are busy
writing to 2/3rds of the disks, OS reads will generally favor the
currently unused mirror.  If the goal is high availability, it's
really not that bad of a set up.


> An alternative would be to run off a single drive for the OS/database, and periodically use rsync (or whatever your preferred tool is) to update a clone on one of the physical disks. That way you get the performance benefits, but you have the ability to recover from a failure by booting from the cloned system.

Now you're talking backups, not high availability.  Mike Rice's
initial request was for a system which "would let me wait until the
weekend to replace a failing drive".  For that, you need raid.

Eric