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

[Simon Hobson]

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 !
It means that *every* write to the system filesystem has to contend with accesses to all the recording filesystems - and every read from it has to contend with at least one of the recording filesystems. While it's hard to quantify, there is a definite performance advantage to not sharing spindles* between recordings and OS/database (especially database).

There is something to be said for having a separate drive for the system/OS, and mirroring it with a partition on one of the recording drives. With proper DB tuning you can vastly reduce disk writes for the DB, and with raid tuning you can configure the OS to mostly read from the dedicated drive (especially if it's an SSD). With these two tunings, you can gain the redundancy of a mirrored disk set, with hardly any of the performance penalties.

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.


* Yes, there's another term borrowed from days of yore.

As an aside, the last system I set up at work has 6 drives. The boot partition is mirrored across all 6 drives, the root partition is raid6 across them.