Re: [linux-lvm] Building up a RAID5 LVM home server (long) [0x05B52F13]

Re: [linux-lvm] Building up a RAID5 LVM home server (long) [0x05B52F13]

[Sam Vilain]

Andy Boyko wrote:
>>The underlying principle, is keeping the MD layer as simple as possible,
>>so that you've got nice little resilient building blocks for your Volume
>>Group.  Never use concatenation, striping or RAID5 at the MD layer if
>>you've got LVM, it's simply in the wrong place!
> Could you by any chance explain this?  Assuming, say, five identical
> disks, is it really wrong to make an MD RAID-5 out of them, and then
> put an LVM LV atop that, for later extension?   I'm feeling pretty 
> dopey, 'cause I'm not immediately grasping what would be an alternative 
> MD/LVM/EVMS/whatever approach, assuming you do fundamentally want N-1 
> worth of available space from those disks.  What am I missing?

Sorry, I shouldn't have said "never".  All I'm saying is that most of
the time, you think you want this but in balance you probably want to
simply buy a few more disks, or bigger disks, instead of worrying
about an extra X% of space, which invariably will just increase
backup requirements anyway.

If you do have an equally sized (ideally also homogenous brand/model)
set of disks, *and you don't overly care about write performance or
excessive performance degradation in the case of a single failure*,
then sure, RAID 5 setup as you say is the right solution.

This is just an old saying repeated.

   RAID - Fast, Cheap, Reliable.  Pick any two.

fast+cheap = raid 0/striping
fast+reliable = raid 1 or 10
cheap+reliable = raid 5

Sam.

Re: [linux-lvm] Building up a RAID5 LVM home server (long)[0x05B52F13]

[Ron Watkins]

Remember, this is for a HOME SERVER, not a corporate database.  Assuming he 
doesn't have Gigabit Ethernet, and assuming the CPU in his server is 
reasonably beefy (at least a gigahertz), the extra speed in RAID 1 or 10 
will be entirely wasted.   For almost any home application, RAID5 is going 
to be exactly what folks want.... cheap, reliable, and more than fast enough 
to saturate the wire.

Doing RAID 10 properly would cost a great deal for disks, a lot for 
motherboards with proper, non-PCI choked Gigabit ports (need them on both 
server and client, mind you), and would take a significant amount of 
learning on the part of the home admin to get really running properly.... 
how many people want to learn about jumbo frames and cat5e wiring to set up 
a video server that will only be serving one or two videos at a time?

Yes, your solution would run faster in some circumstances, but it would be a 
LOT more expensive.   After investing all that money, he'd see it run 
somewhat faster on writes, and the server wouldn't be speed-impaired during 
a rebuild.   But he's a HOME USER.  It doesn't MATTER if the server is slow 
for a night or two.  Everything will be sluggish for a couple nights, once a 
year or so.  So what?  Since when is this is worth spending a couple grand 
to avoid?

It would often make sense for a business to do that, but from a home user 
perspective, it seems like following this specific advice would be a 
gigantic waste of money.

For nearly all home users with reasonably modern hardware, RAID5 is the 
right way to go for servers, at least for now.

RE: [linux-lvm] Building up a RAID5 LVM home server (long)[0x05B52F13]

[Erik Ohrnberger]

Ron,
    All your conclusions are very true.  

    The machine is a 2 GHz AMD Althorn with 512 MB RAM running SuSE 9.1.
The local network is very small.  Only 5 or 6 devices (laptop) at most, and
is serviced by two 100 Mb hubs quite well.  Hardly ever see collisions, and
then only when there is a large file transfer going on.

    At some point in time, I may be fooling around with MythTV or KnoppMyth,
which would generate a lot of data, and I figure that I could NFS mount the
large disk space across the network.  In addition, all the stuff that I
download from the net is on there too, patches for programs (mostly MS) and
other software install sets.

    Has LVM2 been upgraded to handle an LVM size larger than before?  Last I
recall, it was limited to 256 PVs, and 250 GB or so usable space.

    Thanks for all your ideas and contributions.

    Erik.

> -----Original Message-----
> From: linux-lvm-bounces@redhat.com 
> [mailto:linux-lvm-bounces@redhat.com] On Behalf Of Ron Watkins
> Sent: Thursday, March 10, 2005 10:56 AM
> To: LVM general discussion and development
> Subject: Re: [linux-lvm] Building up a RAID5 LVM home server 
> (long)[0x05B52F13]
> 
> 
> Remember, this is for a HOME SERVER, not a corporate 
> database.  Assuming he 
> doesn't have Gigabit Ethernet, and assuming the CPU in his server is 
> reasonably beefy (at least a gigahertz), the extra speed in 
> RAID 1 or 10 
> will be entirely wasted.   For almost any home application, 
> RAID5 is going 
> to be exactly what folks want.... cheap, reliable, and more 
> than fast enough 
> to saturate the wire.
> 
> Doing RAID 10 properly would cost a great deal for disks, a lot for 
> motherboards with proper, non-PCI choked Gigabit ports (need 
> them on both 
> server and client, mind you), and would take a significant amount of 
> learning on the part of the home admin to get really running 
> properly.... 
> how many people want to learn about jumbo frames and cat5e 
> wiring to set up 
> a video server that will only be serving one or two videos at a time?
> 
> Yes, your solution would run faster in some circumstances, 
> but it would be a 
> LOT more expensive.   After investing all that money, he'd see it run 
> somewhat faster on writes, and the server wouldn't be 
> speed-impaired during 
> a rebuild.   But he's a HOME USER.  It doesn't MATTER if the 
> server is slow 
> for a night or two.  Everything will be sluggish for a couple 
> nights, once a 
> year or so.  So what?  Since when is this is worth spending a 
> couple grand 
> to avoid?
> 
> It would often make sense for a business to do that, but from 
> a home user 
> perspective, it seems like following this specific advice would be a 
> gigantic waste of money.
> 
> For nearly all home users with reasonably modern hardware, 
> RAID5 is the 
> right way to go for servers, at least for now.
> 
> 
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