RHEL3 and SAS5/iR: very bad performance

Eric Rostetter rostetter at mail.utexas.edu
Wed Jan 24 12:34:01 CST 2007


Quoting Giulio Orsero <giulioo at gmail.com>:

> The SAS5/iR has no battery back-up.

Well, then, you're SOL.  You can put a UPS on the whole system, and have
it trigger a safe shutdown on power fail, and be _fairly_ safe, but I'd
feel kind of worried doing that myself.  I only buy raid cards with a
battery backup for a machine which needs reliable disk storage...

> My problem is that a PE840 with SAS5/ir costs more than one with just
> a single disk, but the one with a single disk is by far more
> performant.

Well, it isn't really a fair comparison.  You need to compare the cost
of the machine with the HW RAID-1 two disk setup to the cost of the machine
with cost of the machine with a SW RAID-1 two disk setup...  Same for
performance.

> Then I have to enable the write-cache, and I was  hoping I could tell
> the end-user that by doing so I would not put him at more risk than if
> he were to use a single disk.

You are _not_ putting him at more risk than a single drive.  You are putting
him at more risk perhaps than a SW RAID-1 set, depending on the SW RAID-1
setup.

In other words, your HW RAID-1 is still safer, even with the cache not be
backed up, than a single disk, since you can recover from a failed single
disk in the RAID set, but not from a a failed disk in the single disk
setup.

You will lose a lot (whole lot) more data if you have only one disk and
it fails, then if you have a RAID-1 and one of the disks fail.

Now, you said you have a UPS, so you're even better off.  Setup monitoring
to make sure the UPS is being used (best you can), setup the UPS so it
does a clean shutdown on power failure, and you're basically set.  That
would be _much_ superior to a single drive system as far as reliability
goes.

> --
> giulioo at pobox.com

-- 
Eric Rostetter
The Department of Physics
The University of Texas at Austin

Go Longhorns!



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