[mythtv-users] Recording and Archive Storage Approaches

Steve Christall mythtv at mctubster.com
Tue Jul 11 04:04:24 UTC 2006


Some Raid5 comments for Joel


I have been using RAID5 in Linux for four years.  There are three kinds
of RAID5 that I have come across.

1 / Real RAID5, where the parity information is generated on the card.
You can boot from this (3ware, LSILogic, other server raid chipsets)

2 / Fake RAID5, where the parity information is generated by the CPU.
The card has bios extensions so you can boot from it (apparently, never
tried) (Promise, Highpoint etc).  Driver is generally proprietary.

3 / Software RAID5, parity is generated by the CPU, you def can't boot
from it.  Raid code is in the Linux kernel.

At work I have used a lot of real RAID5, generally in Dell servers.
Works well, is fast, monitoring is OK

At home I started trying to use fake raid, because I couldn't afford /
justify $1000 for a real raid card.  A lot of the proprietary drivers
are locked to a particular kernel version, so upgrading is a nightmare.
  Sometimes you have to recreated the array, just to upgrade.
Monitoring is, well, please stay clear

Now I am using mdadm, to control true software RAID5 in Linux.  It is
fast (Myth is ideal for RAID5, large sequential writes, hence you can
have a large block size), mdadm has great monitoring, and you can
upgrade your kernel without issues.  Can't boot from it (I use 2x80GB
RAID1), which give me space for mythburn etc.

I am using a fake raid card, highpoint 404, as my four channel ATA
controller (but using the Linux simple ata driver for it).

There is one thing to watch out for with software RAID5.  It doesn't
like power outages.  It is very good at scrambling the superblock on one
or more drives if a power outage occurs on write.  You can recover, but
it is not pretty, and leaves you wondering where there is inconsistent
data on the array.  I would recommend an UPS, and auto shutdown.

HTHs
Steve






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