On Sat, 2005-06-25 at 04:01 +0200, Dag Wieers wrote:
> Hi,
> Would it be advisable to use SW-RAID using 2 different SATA controllers.
> My system happens to have both a VIA VT8237 and SIL3114 SATA controller,
> and I thought for both performance reason and redundancy using both for a
> 2 disk RAID1 array would be best.
> But it's possible that SW-RAID does not like or cannot overcome problems
> with such a setup or that one of these drivers is known to have issues.
> Any insight is welcome :)
Let's talk "driver ... issues" then.
You just hit on one of the major reasons I don't use "raw" ATA/SATA
channels. Remember, [S]ATA is "intelligent drive electronics" (IDE),
and the so-called "controllers" are little more than PCI-to-ATA bus
arbitrators. That means that the CPU and the drive itself must
communicate properly, through that arbitrator.
If the history of [S]ATA in Linux (among other, open source OSes with
their own drivers) is a testament, all sorts of controller register
settings and the IDE "intelligence" on the drives themselves _regularly_
have issues. Nothing is more enfuriating than to do a kernel upgrade
and be treated to a new slew of ATA chipset-drive timeouts, performance
issues and, in some cases, even corruption.
That's why on servers or critical desktops, I splurge and get a $125
3Ware Escalade 7006-2 or 8006-2 card. That way the kernel is far
removed from all sorts of IDE non-sense, and 3Ware can work with vendors
to ensure compatibility between its ASIC-ATA and the drives. Just like
proprietary vendor drivers for those "other" OSes. The actual IDE
implementations in [S]ATA discs are particularly nasty and sometimes,
quite often, non-ATA standards compliant in several ways.
Now let's talk "performance."
You're still pushing 2X data through your memory-I/O interconnect for
software RAID-1. If you have an "intelligent" storage controller, then
you only push 1 copy, and the card itself replicates to both channels.
Now if you're really worried about controller failure, get 2 cards and
put them on separate PCI[e/-X] busses (not just slots), then mirror
across to minimize the performance hit.
BTW, there is a table and some figures on the "performance hit" of doing
software RAID-1, RAID-0+1 and RAID-5 in the article "Dissecting ATA RAID
Options" in 2004 April Sys Admin (not available on-line).
One figure shows how the "performance hit" of doing software RAID is
_not_ the actual XOR operation. The _massive_ performance hit is when
you have to read _all_ data from the drives into memory, and then push
_all_ data to the CPU just to calculate the XOR. The "hit" is massive
in the interconnect-I/O department, and _not_ the actual XOR.
Which is why it's far, far better to put the XOR operation on the card,
where you're pushing just 1 copy of data (and it does everything
locally, away from the rest of your system).
--
Bryan J. Smith b.j.smith@xxxxxxxx
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