[rescue] So, I'm bringing up my E3000....

Skeezics Boondoggle skeezics at q7.com
Thu Apr 8 16:22:04 CDT 2004

On Thu, 8 Apr 2004, Dan Duncan wrote:

> On Thu, 8 Apr 2004, Charles Shannon Hendrix wrote:
> > I've always wondered... how does performance work out when a drive is
> > partially mirrored, with swap interleaved?
> I've never had a problem with it.  I've also managed systems
> that had swap mirrored rather than interleaved so that a
> drive failure wouldn't take the system down.

Right.  I've done it both ways, depending on the circumstances.  For
highest availability, mirroring swap affords some extra protection.  But
swapping is expensive enough without having to do each write to both
drives and through the extra software layer... and you get twice as much
swap space too. :-)  Being able to add and remove swap devices on the fly
in Solaris is a nice feature...

Boot disk failures have been, in my experience, rare enough that this
method is generally safe enough.  I've had maybe 10 drives die in the last
6-7 years in various systems, and in only *one* of those failures did the
disk actually croak and take it self off the bus and allow Disksuite to do
the right thing.  In every other case, the drive doesn't just die - it
gets stupid, and stays around timing out every bloody operation so that
the kernel keeps trying and trying and trying... so the machine stays up
and just gets v e r y  s l o w.  In that case you almost always have to 
reboot anyway, even if you hot-swap out the bad drive.

(I had a friend who had a strange knack for actually blowing up Seagate
drives.  Like, he literally made two of them catch fire in the couple of
years I worked with him.  Fortunately, he worked in another group and
didn't touch our machines! :-)

I'd probably look at 3-way mirroring or multipathing or other more 
difficult, more expensive solutions if I were running a 911 call center or 
air traffic control or some heart monitor machine... but for a web farm?  
Naw. :-)

> > I've thought of mirroring part of some of my drives too, but always felt
> > like it was wrong to mirror partitions instead of entire drives.
> An advantage of mirroring partitions instead of entire drives is
> that you can mirror only desired partitions which in the case of
> software RAID might improve performance.  (such as interleaving
> instead of mirroring swap above)  Your drives can also be of different
> sizes.  You can also mirror partitions on one drive to partitions spread
> across two other drives that might also be on different SCSI channels.
> Me, I like hardware mirroring of entire drives, but it's spendy.

Yeah.  Like most things, you have to find the balance between complexity, 
cost and performance that's right for your application.

-- Chris

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