[Wylug-discuss] Help needed with 'failed' Linux software RAID 10

[Dave Fisher]

Dear all,

Thanks to everyone who has contributed to this thread. Most suggestions have
pointed to options that I have previously considered, but the discussion has
really helped me focus and prioritise.

Although I've yet to make any definite decisions I'm tending towards the
following solutions to my data storage and video editing/encoding needs.

1. Buy/build 3 dedicated systems

   1.1. A Desktop PC with a single 4-6 core CPU
   1.2. A 'modest' 4-5 TB File server
   1.3. A near-identical backup mirror of the file server

2. The Desktop

   2.1. Will be used primarily for video editing and rendering
   2.2. Have limited local storage
   2.3. Storing only system files, working copies of data, and recently
        captured multimedia files

3. The File Server (and its Mirror)

   3.1. Will probably be built around something like this:
        http://www.tranquilpc-shop.co.uk/acatalog/BAREBONE_SERVER_Series_2.html
   3.2. Will re-use discs that I already own
   3.3. Will have separate file systems for active data and archive data
   3.4. Will store most active files under version control (presumably not
        video/databases)
   3.5. The desktop machine will checkout/update working copies from the active
        store, and have read-only access to the archive

Given the limits on how far one can re-configure RAID and backup systems after
the fact, I'd appreciate comments on my ideas before I start laying out the
file server. N.B. assume RAID = Linux RAID, aka Software RAID / mdadm.

In the spirit of Aaron's suggestion, my archived video collection will be
permanently capped at 2/3 of the file server's total storage capacity.

Following John H's suggestion, I was thinking of partitioning the discs so that
each disc contributes to several RAID arrays ... to limit the disruption caused
by degradation, recovery or re-build of any single array.

Since I have 4x2TB and 8x1TB discs, I was thinking of putting 2x2TB discs and
3x1TB drives in each barebones server ... and dividing every disc into 100GB
partitions.

If I'm not mistaken, that would allow me to construct 10x100GB RAID-5 arrays
and 10x100GB RAID-1 arrays in the 5-bay Tranquil PC BBS2 enclosure.

Which would yeild 4TB of RAID-5 storage and 1TB of RAID-1 storage, with all 20
arrays able to survive (at least) one disc failure.

Given the relative performance, I'm guessing that active data should go on the
RAID-1 arrays and archive data on the RAID-5 arrays.

Having reflected on John H's defence of LVM, I think I might use it for
snapshotting active data and systems files. However, I am not sure what
benefits it might offer for managing archive data.

Although 100GB partitions 'feel' a little tight for video work (especially
'raw' DV/HD), I picked this number with the following in mind:

  a. I own a 1x140GB 2.5 inch USB HDD

  b. I was already planning to buy a 128GB pendrive (using <20GB for bootable
     systems)

  c. Copying/synching more than 100GB can be a bit of a pain, even over gigabit
     ethernet ... and definitely over the internet.

  f. Most really big video files will be 'incoming', i.e. split/compressed
     before they get to the file server.

  e. Although the RAID-5 recovery time would be no better than could be
     achieved with 200GB partitions, I'm guessing that it won't be much worse
     (assuming parity is distributed 'round-robin').

  f. Although recovery of a RAID-1 mirror from complete disc failure will still
     be time-consuming, I'm guessing that recovery from badblock/metadata
     errors within just one 100GB partition should be a lot quicker (assuming
     that you can fail a partition-based array member, without failing the
     entire physical disc).

I'd appreciate further comments and won't be in the least bit upset if someone
makes a constructive suggestion alongside any observation to the effect that my
ideas are 'barking mad'.

As you can see, I have barely begun to think about monitoring, beyond the
default 'email on error' stuff that mdadm provides.  If anyone does feel
inclined to make suggestions on that topic, please remember that I'm only
managing a small home network and probably don't have the time or the
inclination to support an 'enterprise solution'.

Dave