RAID array [LWN.net]

RAID array

Posted Mar 20, 2008 19:35 UTC (Thu) by dlang (subscriber, #313)
In reply to: RAID array by clugstj
Parent article: On the vger.kernel.org outage

there are many ways that an electrical failure of a drive can knock out a computer. think of
what would happen if it shorts out the 5v or 12 power going into the drive for an obvious
example.

in practice it's normally not that bad, but especially fro a SCSI array you don't normally
have a seperate cable for each drive, you have them connected to a bus. if a drive is bad it
can cause the bus to lock up when the drive address is probed.

I would also be interested in the specifics of the hardware, but this sort of failure mode is
not uncommon, and the number of systems that actually implement protection against all of them
is extremely small.

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RAID array

Posted Mar 20, 2008 20:28 UTC (Thu) by hmh (subscriber, #3838) [Link]

At least for RAID1 and RAID10, you should use two channels (and depending on what is doing the
mirroring, two controllers), and two enclosures.  Each half of a mirror pair is on a separate
enclosure and channel.

An electrical failure can still knock out the entire system, as the buses are not optical, but
the usual failure modes for drives nowadays have them at maximum locking up the SCSI bus (and
usually they don't get that bad).  Shorts are not common IME.

Anyway, with SAS and SATA, which are point-to-point links, the use of separate enclosures (due
to the power feed) and controllers becomes the real point.

One point of notice: Linux is not good for reliability here, unless you can remove the "IRQ
handler killer" from the picture (e.g by using extremely good controllers that don't do stupid
things, and only one device per IRQ line).  You have been warned.  A lot of failure modes
cause weirdness on the controllers, and may cause spurious IRQs.  If the kernel kicks the IRQ
line away, there goes everything tied to that IRQ line.  This is specially bad on SATA.