Re: Linux LVM software raid, and SAN question.

On Jan 22, 10:27 pm, Faeandar <mr_casta...@xxxxxxxxx> wrote:
On Tue, 22 Jan 2008 15:30:05 -0800 (PST), Kyle Schmitt



<kyleaschm...@xxxxxxxxx> wrote:
In the beefy enterprise-level linux database installs, the ones where
you need massive redundancy supplied by two SAN devices, what is the
current ideal for setting them up?

Software raid to mirror the luns from both sans, and use static
partitions?
Software raid, then LVM ontop of it?
LVM to mirror and handle the whole damned thing?
PowerPath for multipathing, or using the multipathing the kernel
provides?

The particular instance I'm dealing with is a database server that
will have two HBAs, each connected to a different SAN, which is
mirrored from the linux side.  While there are several ways I know how
to set it up, I'm unsure as to which will really be the best way (not
necessarily easiest) to do it.

Thanks
--Kyle

Someone posted the inherent flaws in your statement about beefy and
such so I'll skip that (but they're right you know...).

What you're asking about is two fabrics.  One hba goes to a switch
which goes to a storage port on the array.  The other hba goes to a
different switch and a storage port on the same array as the other
switch.
Nothing ties these two switches together.

This is fairly common practice when availability is a top priority.

Forget LVM based raid, it's almost pointless when the hardware based
raid is in the array (I assume you have an array though you do not
state that explicitly).
However make sure the LVM is in place.  Such things make backend
migrations much simpler.

Do not cross the streams Ray....
Do not create any connection betwee the two fabrics.  This allows for
independent maintenance to each fabric with zero impact on the
application.

As for multi-pathing, I thought MPIO was suported on Linux.  If so,
use it.  It works well and is free.  Avoid PowerPath like the plague.
I'm not even sure they sell it anymore.  I think Hitachi has EOL'd
their version of pay-for multipathing as well.

Hmm, perhaps I'm missing a piece here.  Are you asking about having
two completely seperate SANs and not just fabrics?  So two arrays as
well?  If so, that's less common but not unheard of.
Same practice applies only you would use LVM to mirror.  Or at least I
would.

~F


I probably didn't state it clearly enough then (and lets avoid the
flame war that Linux vs Solaris vs AIX for enterprise level databases
goes, I can only assume neither of you were silly enough to suggest
windows should ever be used ;)

The switches won't be attached in any direct way, wasn't suggesting
that.

The box has two HBAs:
HBA1 attaches to Switch1, which attaches n* ways SAN1 (which is floor
standing cabinet full of discs, using hardware raid of course).
HBA2 attaches to Switch2, which attaches n ways to SAN2.
*I think n is 4 in this case

We're dealing with two physical sans.

The idea is the box will handle the mirroring of Disk1 (mounted on
SAN1 via HBA1), onto Disk2 (mounted on SAN2 via HBA2).

Does that make more sense pertaining to: software raid 1 vs lvm
mirroring.

And I'll look into the builtin MPIO (it exists and is considered
stable IIRC, though I've never used it). I didn't realize powerpath
raised such ire in people. Is it really that bad? Using it for the
past half day it hasn't left a good impression so far, that's for
sure.

Thanks,
Kyle
.

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