[linux-lvm] LVM2 striping or md raid0

[linux-lvm] LVM2 striping or md raid0

[Kelly Byrd]

Two questions really:

I'm going to use lvm to partition up 4 equal size partitions.
I can either use the md driver to RAID0 them the make /dev/md0
my PV, or just use the 4 partitions as PVs and stripe across
them.

How does lvm2 striping compare with md's RAID0 as far as speed?
I don't quite get how it stripes on chunk sizes smaller than
a PE, but a 4MB stripe size seems crazy.

Finally, what's the tuning advice for LVM2 striping? I usually
create my RAID0 or RAID5 with 128k chunk sizes, is it similar
for LVM2 striping?

Re: [linux-lvm] LVM2 striping or md raid0

[Kelly Byrd]

On Sat, 22 Sep 2007 18:36:41 -0700, Kelly Byrd <kbyrd-lvm@memcpy.com>
wrote:
> 
> Two questions really:
> 
> I'm going to use lvm to partition up 4 equal size partitions.
> I can either use the md driver to RAID0 them the make /dev/md0
> my PV, or just use the 4 partitions as PVs and stripe across
> them.
> 
> How does lvm2 striping compare with md's RAID0 as far as speed?
> I don't quite get how it stripes on chunk sizes smaller than
> a PE, but a 4MB stripe size seems crazy.
> 
> Finally, what's the tuning advice for LVM2 striping? I usually
> create my RAID0 or RAID5 with 128k chunk sizes, is it similar
> for LVM2 striping?
> 
> 

anyone?

Re: [linux-lvm] LVM2 striping or md raid0

[Chris Cox]

Kelly Byrd wrote:
> 
> 
> On Sat, 22 Sep 2007 18:36:41 -0700, Kelly Byrd <kbyrd-lvm@memcpy.com>
> wrote:
>> Two questions really:
>>
>> I'm going to use lvm to partition up 4 equal size partitions.
>> I can either use the md driver to RAID0 them the make /dev/md0
>> my PV, or just use the 4 partitions as PVs and stripe across
>> them.
>>
>> How does lvm2 striping compare with md's RAID0 as far as speed?
>> I don't quite get how it stripes on chunk sizes smaller than
>> a PE, but a 4MB stripe size seems crazy.
>>
>> Finally, what's the tuning advice for LVM2 striping? I usually
>> create my RAID0 or RAID5 with 128k chunk sizes, is it similar
>> for LVM2 striping?
>>
>>
> 
> anyone?

This is merely my own opinion, but md was designed to be
THE software raid mechanism.  So, my preference would be
to establish a RAID md device and make that into a PV.

Of course, I don't believe in non-RAID (i.e. RAID0) anyhow.
Maybe someone else will chime in on this one.


-- 
Chris Cox
Sr. Unix Sys Admin

Re: [linux-lvm] LVM2 striping or md raid0

[Kelly Byrd]

On Tue, 25 Sep 2007 14:21:44 -0700, Kelly Byrd <kbyrd@memcpy.com> wrote:
> 
> 
> On Tue, 25 Sep 2007 16:26:30 -0500, Chris Cox <chris_cox@stercomm.com>
> wrote:
>> Kelly Byrd wrote:
>>>
>>>
>>> On Sat, 22 Sep 2007 18:36:41 -0700, Kelly Byrd <kbyrd-lvm@memcpy.com>
>>> wrote:
>>>> Two questions really:
>>>>
>>>> I'm going to use lvm to partition up 4 equal size partitions.
>>>> I can either use the md driver to RAID0 them the make /dev/md0
>>>> my PV, or just use the 4 partitions as PVs and stripe across
>>>> them.
>>>>
>>>> How does lvm2 striping compare with md's RAID0 as far as speed?
>>>> I don't quite get how it stripes on chunk sizes smaller than
>>>> a PE, but a 4MB stripe size seems crazy.
>>>>
>>>> Finally, what's the tuning advice for LVM2 striping? I usually
>>>> create my RAID0 or RAID5 with 128k chunk sizes, is it similar
>>>> for LVM2 striping?
>>>>
>>>>
>>>
>>> anyone?
>>
>> This is merely my own opinion, but md was designed to be
>> THE software raid mechanism.  So, my preference would be
>> to establish a RAID md device and make that into a PV.
>>
>> Of course, I don't believe in non-RAID (i.e. RAID0) anyhow.
>> Maybe someone else will chime in on this one.
>>
>>
> 

I had the same thought, but there's also this instinct that
'one less layer between my files and this physical disk is
better'. I'm really hoping someone will say something like:
'LVM2 striping uses the same code as md raid0' or some other
silly thing.

Re: [linux-lvm] LVM2 striping or md raid0

[Stuart D. Gathman]

On Tue, 25 Sep 2007, Chris Cox wrote:

> This is merely my own opinion, but md was designed to be
> THE software raid mechanism.  So, my preference would be
> to establish a RAID md device and make that into a PV.

This is FOSS, and choice is king.  There is no "THE" desktop, you
can choose from KDE, Gnome, or a host of desktop-like window managers.
It is entirely in character to have 2 different RAID mechanisms.

That said, LVM raid will likely always be restricted to RAID0 and RAID1,
because these are simple and efficient to implement as part of what
LVM does anyway.  For RAID5, etc, your only choice in linux will be
hardware or the md driver.

> Of course, I don't believe in non-RAID (i.e. RAID0) anyhow.
> Maybe someone else will chime in on this one.

RAID0 on top of RAID1 or (RAID1 on RAID0) gives you mirroring plus high
bandwidth.  For RAID5, I'd go with hardware.

-- 
	      Stuart D. Gathman <stuart@bmsi.com>
    Business Management Systems Inc.  Phone: 703 591-0911 Fax: 703 591-6154
"Confutatis maledictis, flammis acribus addictis" - background song for
a Microsoft sponsored "Where do you want to go from here?" commercial.

Re: [linux-lvm] LVM2 striping or md raid0

[Stuart D. Gathman]

On Tue, 25 Sep 2007, Chris Cox wrote:

> This is merely my own opinion, but md was designed to be
> THE software raid mechanism.  So, my preference would be
> to establish a RAID md device and make that into a PV.

I prefer RAID1 integrated with LVM - it can handle mixed disk sizes 
with ease.  On a particular system with AIX LVM (where it has been integrated
for some time), We started with 3 512M disks, then added a 1G, then 2 4Gs, then
an 18G.  The filesystems were RAID1 at all times, and were expanded and PEs
moved between devices all with the filesystems mounted and active.
Linux LVM can do all this now, it is just a little kludgier at present.

With md, you have to arrange matching partition sizes by hand.  It *is*
possible, however, to move live/active md partitions between devices 
manually if you are careful (allocate partition on new device and add
as mirror, sync, set faulty and remove old partition).

Changing the partition table requires a reboot (or deactivating/unmounting all
partitions on the drive), so that limits what you can do live with md - unless
you have the foresight to divide your disks into roughly equal sized
partitions.  But that, of course, is what LVM does for you automatically.

-- 
	      Stuart D. Gathman <stuart@bmsi.com>
    Business Management Systems Inc.  Phone: 703 591-0911 Fax: 703 591-6154
"Confutatis maledictis, flammis acribus addictis" - background song for
a Microsoft sponsored "Where do you want to go from here?" commercial.