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

[Joseph Fry]

On Fri, Sep 26, 2014 at 9:09 AM, Eric Sharkey <eric at lisaneric.org> wrote:

> 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.
>

Thank you for backing me up.

When I built this I only had 4 sata ports, one of which was connected to an
e-sata port so I could do backups to an external hard disk that gets locked
up in a firesafe.  That left 3 for recording drives, unless I wanted to
waste an entire 750GB drive on the OS.

As you said, the performance impact is negligible.  Yes, writes to the
database are mirrored, so there is an impact to writes... but with three
recording drives and only 5 physical tuners, I rarely have more than one
recording occurring on any one drive.  Essentially, it would have been
wasteful to add spindles for the OS/DB... the overhead for them is quite
low.

And with a 3 drive mirror, read performance is excellent from the raided
drives... and mythtv does a lot more reading than writing on the array.
For example, from the details below you can see that I have read (column 3)
about 42x more sectors from the array than I have written (column 5) since
my last reboot a couple of days ago.  So the minor hit to writes is more
than made up for by distributed reads.

jfry at mythserver:~$ cat /sys/block/md2/stat
  478611        0 11073882        0   261767        0  7165720        0
   0        0        0
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