[Pc_Support] Re: Using OS software RAID with FRAID organization, and when not to use it (in case I wasn't clear) ...

[Bryan J. Smith]

On 8/28/06, Bryan J. Smith <thebs413 at gmail.com> wrote:
> First off, there's been 0 change of heart.  FRAID still _sucks_ for
> RAID-10, 3 and 5...
> Again, narrowing the focus to _only_ 2 discs in RAID-1 for a _desktop_ system ...
> I had heard about DeviceMapper supporting the Intel ICH5-7 FRAID
> organization and the nVidia MCP4/4x0 FRAID organization more recently,

This is for a 2-disc SATA RAID-1 volume, using the mainboard FRAID for
firmware/boot, then relying on the DeviceMapper in Linux to match the
FRAID driver in Windows.  Test before you attempt.

If you even attempt to use RAID-0, RAID-10, RAID-5, etc... you will
probably run into stripe size issues/assumptions between DeviceMapper
in Linux and the FRAID driver in Windows.  I would not trust it for a
moment because the FRAID vendor _might_ change things at any time in
any firmware/BIOS update or Windows driver update.

But for RAID-1, it's typically _not_ striped -- just "dumb" mirroring.
 At the most, the FRAID logic might reserve cylinder 0 for its own
housekeeping, and then cylinder 1 actually becomes the Master Boot
Record (MBR).  Or, more likely, it's the last cylinder used for
housekeeping, which doesn't do anything but reduce the total capacity
by 1 cylinder.

(I personally need to look at the low-level of the disk to find out
what nVidia is doing, Intel if I have more time).

> I still would _not_ use this on a server.  I would _not_ trust it to
> handle single sector errors/remapping and notifications well either.

This is _key_ here.  You do _not_ get sector remapping or fully
transparent error handling.  You now rely on the mapping to get such.
In other words, be prepared for your system to _crash_ as a result of
a device disconnect.  That is, and has always been, the #1 issue with
software RAID -- _period_ -- even if you are using native Linux MD or
Windows LDM volume management.

As such, this is regulated to uses where you want a _desktop_ system
to maintain a mirror of its data.  There is _no_ guarantee of
consistency or "uptime" -- just that bytes on one disk are being
mirrored to another.  It's really "software RAID" with boot-time
support via the FRAID firmware/BIOS -- and it's really limited to an
application of 2 SATA disks in RAID-1 -- *NO* striping (that means no
RAID-1e or RAID-10 with 2 discs either).

If you even attempt RAID-5 with FRAID, not only don't I expect it to
_not_ work, but you've basically turned your high-speed DMA transfer
into sub-20MBps Programmed I/O (PIO) operations -- killing everything
in your system.  The benchmarks don't lie, do *NOT* use FRAID-5 on the
Intel ICH7+ or nVidia MCP51 (4x0) _period_ -- not even under Windows.

> You _want_ the "piece of mind" out of a 3Ware Escalade 8506-2LP for
> that.

On a server, the extra $100 for a mainboard with PCI-X and another
$100 for a 3Ware Escalade 8506-2LP will save you time and headaches
(as well as I/O waste -- although for 2-disc RAID-1, it's minor when
your SATA is on its own PCIe x1).