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Ninti Systems | 1 Aug 2004 03:57
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RAID 5 questions

I'm about to embark on building a RAID 5 Linux software array with 4
disks (I've only ever built 2 disk RAID 1 arrays before).

Q1: Is it true that Linux can't/shouldn't boot off a RAID 5 array?

Q2: The four disks have identical manufacturer, model and size, but two
are brand new while two have had NTFS filesystems on them. Accordingly,
if I run hdparm -g on them, two disks have these specs:

geometry = 2498/255/63, sectors = 40132503, start = 0

while the other two disks have these specs:

geometry = 39813/16/63, sectors = 40132503, start = 0

Is this an issue with software RAID? 

Thanks
Michael Hall

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Jim Paris | 1 Aug 2004 05:31
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Re: RAID 5 questions

> Q1: Is it true that Linux can't/shouldn't boot off a RAID 5 array?

LILO does not support booting off of RAID 5, and I don't think GRUB
does either.  I usually make a small RAID-1 partition to boot from
(mirrored across all four drives), since LILO does handle that just
fine.

> geometry = 2498/255/63, sectors = 40132503, start = 0
..
> geometry = 39813/16/63, sectors = 40132503, start = 0
> Is this an issue with software RAID? 

No.

-jim
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Alvin Oga | 1 Aug 2004 05:50

Re: RAID 5 questions


On Sat, 31 Jul 2004, Jim Paris wrote:

> > geometry = 2498/255/63, sectors = 40132503, start = 0
> ..
> > geometry = 39813/16/63, sectors = 40132503, start = 0
> > Is this an issue with software RAID? 

no but it'd be better/faster/easier to put 1 disk per ide cable
	- its $20 or less to get a promise ide controller

	- or if you're brave, to disable the onboard raid 
	in the bios and use its connectors as regular ide

1 disk per ide cable if you can ...

c ya
alvin

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Daniel Pittman | 1 Aug 2004 05:08
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Re: RAID 5 questions

On 1 Aug 2004, Ninti Systems wrote:
> I'm about to embark on building a RAID 5 Linux software array with 4
> disks (I've only ever built 2 disk RAID 1 arrays before).
>
> Q1: Is it true that Linux can't/shouldn't boot off a RAID 5 array?

Grub certainly can't boot of a software RAID-5 array, and I don't
believe that lilo can either.  This is because of these tools not
understanding the RAID-5 disk layout, not any other limitation.

> Q2: The four disks have identical manufacturer, model and size, but two
> are brand new while two have had NTFS filesystems on them. Accordingly,
> if I run hdparm -g on them, two disks have these specs:
>
> geometry = 2498/255/63, sectors = 40132503, start = 0
>
> while the other two disks have these specs:
>
> geometry = 39813/16/63, sectors = 40132503, start = 0
>
> Is this an issue with software RAID?

No. Software RAID, unlike hardware raid, cares only that you have enough
blocks on the disk. You don't need to match *any* other characteristic
of the disks, and not even that if you don't mind wasting a few bytes.

   Daniel
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Mikael Abrahamsson | 1 Aug 2004 09:00
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Re: 1x 3ware controllers vs. 2x 3ware controllers

On Sat, 31 Jul 2004, Jurriaan wrote:

> > So why are scsi raid-5 systems so much better?
> 
> compared to other scsi raid-arrays, they are not 'better' for small
> random writes. If they are better than ide, it's probably because of the
> smaller access times for 15k rpm drives.

And also because scsi drives can do tagged queueing which makes it more 
efficient to do a lot of smaller operations. Historically the SCSI drives 
also had more cache memory which helps the situation, and the scsi 
RAID controllers probably also had more cache memory on them (I know RAID 
systems that have gigabytes of cache memory).

--

-- 
Mikael Abrahamsson    email: swmike <at> swm.pp.se

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Gordon Henderson | 1 Aug 2004 09:08
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Re: 1x 3ware controllers vs. 2x 3ware controllers

On Sun, 1 Aug 2004, Mikael Abrahamsson wrote:

> And also because scsi drives can do tagged queueing which makes it more
> efficient to do a lot of smaller operations. Historically the SCSI drives
> also had more cache memory which helps the situation, and the scsi
> RAID controllers probably also had more cache memory on them (I know RAID
> systems that have gigabytes of cache memory).

What I find amusing these days is trying to work out the "boundary" point
between a "traditional" server with an (external) RAID controller and say
a Linux server with software RAID in a purely fileserving environment (eg.
NFS/Samba, not used for local operations at all) ... Both systems as a
unit provide the same services - ie. filespace at the end of the Ether,
but what are the advantages of one over the other, and why would I ever
want a hardware RAID controller in a PCI slot in a Server PC?

Discuss... ;-)

Gordon
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Mikael Abrahamsson | 1 Aug 2004 11:18
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Re: 1x 3ware controllers vs. 2x 3ware controllers

On Sun, 1 Aug 2004, Gordon Henderson wrote:

> What I find amusing these days is trying to work out the "boundary" point
> between a "traditional" server with an (external) RAID controller and say
> a Linux server with software RAID in a purely fileserving environment (eg.
> NFS/Samba, not used for local operations at all) ... Both systems as a
> unit provide the same services - ie. filespace at the end of the Ether,
> but what are the advantages of one over the other, and why would I ever
> want a hardware RAID controller in a PCI slot in a Server PC?
> 
> Discuss... ;-)

Let's say you have a database server with a large RAID1+0 array. If you do
this in software, you have to transfer data over the PCI bus twice if you
do the raid in software, not so if you do it in hardware.

But I agree, it's like "what is the best car?" It all depends on your
needs and budget. A Porsche 911 is perfect for people who need to go
places fast, it's useless for people who need to haul things.

You always have to optimize everything after your needs, do you need 
highspeed sequencial access, do you need low latency small accesses, do 
you need a lot of fast low latency writes, etc. All these require 
different solutions.

--

-- 
Mikael Abrahamsson    email: swmike <at> swm.pp.se

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Luca Berra | 1 Aug 2004 11:33
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Re: RAID 5 questions

On Sun, Aug 01, 2004 at 11:27:56AM +0930, Ninti Systems wrote:
>Q1: Is it true that Linux can't/shouldn't boot off a RAID 5 array?
it cannot no boot loader can load the kernel from a striped (or raid
3/4/5/6) device
create a small raid 1 array spanning the 4 disks and use it for /boot

>Q2: The four disks have identical manufacturer, model and size, but two
>are brand new while two have had NTFS filesystems on them. Accordingly,
>if I run hdparm -g on them, two disks have these specs:
>
>geometry = 2498/255/63, sectors = 40132503, start = 0
...
>geometry = 39813/16/63, sectors = 40132503, start = 0
>
>Is this an issue with software RAID? 
no, nevertheless if you wipe the partition table on the to 'used' disks
linux will see them just like the 'new' ones.

--

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Mark Watts | 1 Aug 2004 11:51
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Re: 1x 3ware controllers vs. 2x 3ware controllers


> On Sun, 1 Aug 2004, Gordon Henderson wrote:
> > What I find amusing these days is trying to work out the "boundary" point
> > between a "traditional" server with an (external) RAID controller and say
> > a Linux server with software RAID in a purely fileserving environment
> > (eg. NFS/Samba, not used for local operations at all) ... Both systems as
> > a unit provide the same services - ie. filespace at the end of the Ether,
> > but what are the advantages of one over the other, and why would I ever
> > want a hardware RAID controller in a PCI slot in a Server PC?
> >
> > Discuss... ;-)
>
> Let's say you have a database server with a large RAID1+0 array. If you do
> this in software, you have to transfer data over the PCI bus twice if you
> do the raid in software, not so if you do it in hardware.
>
> But I agree, it's like "what is the best car?" It all depends on your
> needs and budget. A Porsche 911 is perfect for people who need to go
> places fast, it's useless for people who need to haul things.
>
> You always have to optimize everything after your needs, do you need
> highspeed sequencial access, do you need low latency small accesses, do
> you need a lot of fast low latency writes, etc. All these require
> different solutions.

Ok, I understand that for many areas, a software raid-5 will thrash the pants 
off of a hardware one.

This doesn't help explain why some of us are experiencing crap performance and 
un-responsive systems whereas others are having none of these problems  with
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Frank van Maarseveen | 1 Aug 2004 12:24
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Re: RAID 5 questions

On Sun, Aug 01, 2004 at 11:27:56AM +0930, Ninti Systems wrote:
> 
> Q1: Is it true that Linux can't/shouldn't boot off a RAID 5 array?

From software RAID it can't.

USB storage devices (memory stick) are incredibly useful for this --
I've even booted a webserver from a digital camera ;-). The only catch
may be the latency of the USB code when you don't want to use initrd:
The kernel panics (no root dev) because it takes noticeable time before
the kernel sees an USB storage device. But there are several tiny patches
for this, it's merely a nuisance.

--

-- 
Frank
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