Matt Mossholder on 28 Sep 2013 15:27:08 -0700

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Re: [PLUG] Spanning volumes with LVM (Ubuntu)

(Sorry for the partial post... gmail went all crazy on me...)

There are several issues with utilizing RAID5 for MythTV storage. The biggest is that when you write data that doesn't fill an entire stripe, you end up having to read the stripe in first, so that you can then recalculate the parity of the new stripe.Â

The ins and outs of myth and RAID are documented here.Â

These are the key statements, though:
For the system disk, reliability and IOPS are going to be favored over raw throughput, so RAID 1 or RAID 10 would be best suited. RAID 1E is another less standard alternative which can be done using Linux MDRAID in 'f2' mode.

For recording disks, MythTV will be storing bulk files. Normally RAID 5 or RAID 6 would be ideal for such a scenario, however MythTV may be recording multiple files simultaneously in small chunks, and the write behavior of parity sets will result in very poor performance in any storage system not using a non-volatile cache. The recommended method would actually be to not use RAID at all, and instead define the drives independently usingÂStorage Groups.

For bulk storage of non-recorded media, such as music, pictures, and videos, the usage will be nearly all read only. RAID 5 or 6 would be a good trade off between redundancy and space efficiency. RAID 5 can only handle a single drive failure before data loss, so for larger arrays (6-drives or larger), RAID 6 would be a better option.

I noticed when I was previously recording to RAID5 that I would get occasional corruption in the data. This disappeared completely when I rebuilt to use stand-alone recording drives.

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