[CentOS] 3ware disk failure -> hang -- SUMMARY: Failed drives/hot-swap recommendations

Sat Jan 7 00:11:42 UTC 2006
Bryan J. Smith <thebs413 at earthlink.net>

"Bryan J. Smith" <thebs413 at earthlink.net> wrote:
> Hold on a second ...
> Are you using a SCSI backplane?
> If so, that's the difference right there!  ;->
> SCSI backplanes and host adapters work very, very different
> on transient (or failure for that matter) than _any_ ATA or
> regular SCSI (without a backplane).
> They are still formulating similarly for SATA, and there
> are some SCSI adopted standards for SATA backplanes.
> But with SAS, much of that is becoming moot.
> Okay, now things make far more sense.  ;->

Okay, let me put this summary out ...

1)  Software RAID with SCSI

If you want reliable software RAID for failed drives or
hot-swap drives, you want to get a host adapter _and_ a SCSI
backplane that work together.  The card must then have full
SCSI-2 support via their driver for the SCSI subsystem to
enable such disconnect and hot-swap features, which is then
paired with the backplane hardware.

2)  Hardware RAID with ATA

3Ware Escalade, Areca ARC and other ATA RAID controllers use
_true_ hardware RAID by the way of ASIC or microcontrollers
that _never_ let the OS see the "raw" disc.  When the discs
are managed into arrays, the on-board intelligence can handle
failures and hot-swaps.

If the discs are not managed as arrays, they report the discs
as they are to the OS, which means if they fail or are
removed, you'll _lose_ the device.  Although these card's
drivers might load via the SCSI subsystem, they are _not_
SCSI cards, and do _not_ have a full SCSI-2 feature set.

3)  Software RAID with ATA, SCSI (non-backplane) or JBOD
modes

You're on your own here.  If there is not a full SCSI-2
driver for your controller, with associated hardware to
handle loss or transient, then you're likely going to get a
panic.  The "new option" in kernel 2.6 is allegedly hotswap,
but I have never configured a storage device for it -- other
than USB, FireWire, CompactFlash, etc...

Now does this make more sense?  ;->


-- 
Bryan J. Smith     Professional, Technical Annoyance                      b.j.smith at ieee.org      http://thebs413.blogspot.com
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*** Speed doesn't kill, difference in speed does ***