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Re: lvm mirror or mdadm mirror or raid 5

On Sun, Sep 02, 2007 at 08:44:21AM +1000, Alex Samad wrote:
> On Sat, Sep 01, 2007 at 09:48:38AM -0400, Douglas A. Tutty wrote:
> > On Sat, Sep 01, 2007 at 12:16:00PM +1000, Alex Samad wrote:
> > > raid 1 / raid 1 /boot raid 5 - pv
> > > 
> > > I have been looking at why not to raid 5 and I would go to raid 10
> > > but I can only put 3 drives in my machine.
> > > 
> > > Currently have 3x250G and am looking at getting 3x500G, if I go to
> > > mdadm raid1, then I have 500G safe 500G unsafe which is not really
> > > what I want.
> > > 1 big vgroup and lots of lv's some with mirrors and some without ?
> > 
> > I don't see a mirror option in Etch's LV create.
> Okay I have lenny but I thought mirroring had been in for a while,
> from my man lvcreate 
>  -m, --mirrors Mirrors
> I haven't looked at this previously so I might be wrong.

Before Etch came out, when I knew that its installer would be doing LVM,
I decided that when I moved from Sarge that I'd go with LVM.  On Sarge's
lvcreate, there was a -m --mirrors option.  When Etch came out it wasn't
there.  I haven't looked into why not but its not there.  To me, its the
one big thing missing from LMV.  I suppose though that if we all start
mirroring in LVM instead of setting up md, then we'll need an LV version
of mdadm to email us when a mirror degrades from a failing drive.

> > 
> > Here's how I would do it:
> > 
> > Partition 1: 3-way raid1 64 MB for /boot
> I usually go for 500M, leaves me lots of room to leave stuff not sure
> why 3 way though, the spare space on the 3rd drive if i go 2 way i use
> for tmp or swap or ...

If you don't have swap protected, then a failed drive could crash the
system.  Put the swap on LVM.  You could also put / on LVM but I didn't
suggest that since you seemed to want to keep it to a non-LV.  

I have two drives, 70 GB each.  3 partitions:  md0 for /boot, md1 raid1
for LVM VG-mirror for the system stuff and then the two third partitions
for VG-cat for non-mirror protected stuff.  /home is on a regular LV,
/var/tmp is on a striped LV.  /tmp is on tmpfs.  I'm running amd64 and
have a 300 M LV for / with ext3 gives 291 MB total and I have 170 MB
used with 107 free with two kernels and their modules installed.

I suggested that you go with 3-way raid1 since you have three huge
drives anyway, you wouldn't miss 300 MB.  Since its a small box with
three drives in it, cooling is tight.  I would guess that you may have
an increased risk of drive failure because of it.  With 3-way raid1 for
/boot and either three-way raid1 or a mirrord LV if told to mirror to
all three drives, your whole system is protected from one drive failure
and your / and /boot are protected from two drive failures.

> > Partition 2: 3-way raid1 300 MB for /
> again not sure why the 3 way, currently i give 10G for this

300 MB for / is pleanty if you put all the usual mount points on
mirrored LVs.

> nice idea to break up into little pv
> > 
> > Of course, performance won't be as good as if you had 4 drives so
> > that you didn't have more than one busy md per drive, but this is a
> > good compromise.
> > 
> > Why is it that you can't fit in a 4th drive?  Are there only 3 bays?

> its a shuttle case, it only has bays for 2 hard drives, I suppose I

How do you fit in three drives plus a DVD then?

> could remove the dvd drive, but I use it a lot, so 3 is the magic
> number, and it worked well for me, whilst I was a raid 5 guy, but it I
> am thinking more back to raid1, but I would like to make the decision
> to mirror or not at lvcreate time not underneath
> been playing with some HP eva's and I like being able to set the raid
> level at the lun level
> > What about an external bay that operates at SATA speeds; look at
> > addonics.


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