[gentoo-user] Software RAID Advice Needed

[gentoo-user] Software RAID Advice Needed

[Doug Brown]

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My mobo's chipset (nvidia nf 4) doesn't support  raid real well, and I have read that Linux Software raid is very  good.  I am getting ready to install Gentoo 2005.1 64bit real soon  (I am new to Gentoo), and I was wondering what  types of raid it  supports.  I know it supports 0 and 1, but I am more interested in  raid 0+1 and 1+0.   
  
  Thanks   
  

			
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Re: [gentoo-user] Software RAID Advice Needed

[Mike Williams]

On Thursday 15 December 2005 21:55, Doug Brown wrote:
> My mobo's chipset (nvidia nf 4) doesn't support  raid real well, and I have
> read that Linux Software raid is very  good.  I am getting ready to install
> Gentoo 2005.1 64bit real soon  (I am new to Gentoo), and I was wondering
> what  types of raid it  supports.  I know it supports 0 and 1, but I am
> more interested in  raid 0+1 and 1+0.  

If the linux kernel supports it, gentoo supports it.

-- 
Mike Williams

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Re: [gentoo-user] Software RAID Advice Needed

[kashani]

Doug Brown wrote:
> My mobo's chipset (nvidia nf 4) doesn't support raid real well, and I 
> have read that Linux Software raid is very good.  I am getting ready to 
> install Gentoo 2005.1 64bit real soon (I am new to Gentoo), and I was 
> wondering what  types of raid it supports.  I know it supports 0 and 1, 
> but I am more interested in raid 0+1 and 1+0.  
> 

	Linux software raid is capable of doing all the usual stuff as well as 
RAID 0+1 or 1+0. I'd want a RAID 0 stripe of mirrored RAID 1 sets rather 
than mirroring two RAID 0 sets, but my requirements may not be yours.

	Assuming this is a small home system I'd go with RAID 5 with maybe a 
hot spare if I have more than four drives in a normal server setting 
where reads happen more often than writes. That's more space with 
comparable performance for anything you're likely to be doing. If you 
really need the performance spend the money on a real RAID card with 
local cache. The difference is night and day.

kashani
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Re: [gentoo-user] Software RAID Advice Needed

[Ognjen Bezanov]

kashani wrote:

> Doug Brown wrote:
>
>> My mobo's chipset (nvidia nf 4) doesn't support raid real well, and I 
>> have read that Linux Software raid is very good.  I am getting ready 
>> to install Gentoo 2005.1 64bit real soon (I am new to Gentoo), and I 
>> was wondering what  types of raid it supports.  I know it supports 0 
>> and 1, but I am more interested in raid 0+1 and 1+0. 
>
>
>     Linux software raid is capable of doing all the usual stuff as 
> well as RAID 0+1 or 1+0. I'd want a RAID 0 stripe of mirrored RAID 1 
> sets rather than mirroring two RAID 0 sets, but my requirements may 
> not be yours.
>
>     Assuming this is a small home system I'd go with RAID 5 with maybe 
> a hot spare if I have more than four drives in a normal server setting 
> where reads happen more often than writes. That's more space with 
> comparable performance for anything you're likely to be doing. If you 
> really need the performance spend the money on a real RAID card with 
> local cache. The difference is night and day.
>
> kashani

RAID 5 support in Linux is good. I have been using RAID-5 for my home 
fileserver (4x40GB IDE disks) and it has worked flawlessly, and has been 
a lifesaver when one of my drives failed.

I have a page devoted to setting up RAID on linux  quickly (gentoo and 
debian) , you can find it at http://ziva-vatra.dnsalias.com/~ognen/ 
under "Software RAID5 Project".  And if you want more info look at the 
"RAID Overview" Section.

Linux kernel 2.6 has added support for new RAID levels (including 
RAID6)  but some people are saying that other RAID Implementations (such 
as RAID-5) have better performance on the 2.4 kernels.

I have found Linux Software RAID very useful and reliable. While 
probably being beaten in the performance  area by hardware 
implementations, I have to say it does do the job well, and I have no 
issues using it both in my home server and in Commercial implementations 
(have used RAID-5 and RAID-1 software with 5 SCSI drives on a dual PIII 
Gentoo LTSP server, it worked well, but there were issues regarding the 
SCSI hardware (like no hotplug support for the disks) ).

Google about, and look at the gentoo-wiki site.

Essentially it depends on what you are looking from your RAID setup. For 
me it was re-using componets I already had and price (it cost me a total 
of £8 to build my RAID Setup). Performance was not an issue because my 
two 10mbit networks (one wireless @ 11mbit) were unlikely to push the 
RAID performance to its limits.



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Re: [gentoo-user] Software RAID Advice Needed

[Richard Fish]

On 12/15/05, Ognjen Bezanov <ognjen@mailshack.com> wrote:
> I have found Linux Software RAID very useful and reliable. While
> probably being beaten in the performance  area by hardware
> implementations,

I just want to point out that when we are talking hardware here, we
mean real hardware RAID...made by companies like 3-ware.  The
'hardware' RAID in the NForce4 chipset (like just about all MB chips,
and a lot of the cheap add-in cards) is just a BIOS helper...all of
the actual RAID functions are expected to be implemented by the driver
running on the CPU.

-Richard

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Re: [gentoo-user] Software RAID Advice Needed

[Jim Burwell]

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Richard Fish wrote:

>On 12/15/05, Ognjen Bezanov <ognjen@mailshack.com> wrote:
>  
>
>>I have found Linux Software RAID very useful and reliable. While
>>probably being beaten in the performance  area by hardware
>>implementations,
>>    
>>
>
>I just want to point out that when we are talking hardware here, we
>mean real hardware RAID...made by companies like 3-ware.  The
>'hardware' RAID in the NForce4 chipset (like just about all MB chips,
>and a lot of the cheap add-in cards) is just a BIOS helper...all of
>the actual RAID functions are expected to be implemented by the driver
>running on the CPU.
>
>  
>
Don't you hate how the hardware and mobo manufacturers have muddied the 
hardware RAID waters by marketing this sort of thing has hardware RAID 
(or at least implying it) ?

Another thing to check out, seeing that he has a mobo with built in 
ghetto-RAID (TM), is dmraid 
<http://people.redhat.com/%7Eheinzm/sw/dmraid/readme>.  This is a device 
mapper implementation of RAID which makes use of various fake hardware 
RAID metadata to support them under Linux.  Someone's also done up a 
Gentoo LiveCD <http://tienstra4.flatnet.tudelft.nl/%7Egerte/gen2dmraid/> 
with dmraid support on it too (who knows, perhaps the latest liveCDs 
have it also).  The only advantage of using this I can see is the 
ability to to make use of the BIOS RAID helpers to create and manage 
your arrays, and deal with the inherent boot time issues.  I'm not sure 
about the stability or reliability of this though, as I havn't used it, 
and the readme doesn't really give me courage :-).  Anyone using this 
successfuly ?  It seems interesting.

I just put together a little home server which uses both Linux RAID (md) 
and LVM2 on an old Abit KG7-RAID motherboard.  Even though it has a 
built in Highpoint HPT37X "RAID chip" (a ghetto-RAID BIOS helper), I 
decided to go with good old "md".  I've tested it by pulling power on 
drives, and it even boots up when the 'primary' drive doesn't exist 
(boot blocks on both mirrored drives of course).  Seems to work very 
well.  I have /boot mirrored (md0), and root and swap on LVM2 partitions 
which live on another mirrored partition (md1). 

For any wanting to do similar, you just need to set up GRUB on both 
drives, and make sure your have initramfs support for starting up md and 
LVM2.  Generkernel will produce a kernel with this if you compile the md 
drivers into the kernel, and include --lvm2 in the genkernel flags.  
Make sure you include "dovlm2" and lvmraid=/dev/mdX lines for each of of 
your RAID devices on the boot line, which tells the linuxrc scripts to 
start up your RAID devices in the initramfs so it can mount your LVM2 
root partition.

- Jim

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Re: [gentoo-user] Software RAID Advice Needed

[Richard Fish]

On 12/15/05, kashani <kashani-list@badapple.net> wrote:
>         Assuming this is a small home system I'd go with RAID 5 with maybe a
> hot spare if I have more than four drives in a normal server setting
> where reads happen more often than writes. That's more space with
> comparable performance for anything you're likely to be doing.

I would say the choice between RAID 5 and RAID 0+1 would be based upon
what you are doing.  Assuming a 4-disk array, RAID5 will require 2
reads and 2 writes for writing a single block of data, while the
RAID0+1 array would only require 2 writes.  Read performance with
RAID5 should be at least 75% of the RAID0+1 setup, possibly equal
depending upon the bandwidth of the PCIe bus.

So if you are doing something like video streaming, RAID0+1 would be a
better choice.  Web browsing, email, compiling, and typical desktop
use would be well suited to RAID5.

Also, consider that you can mix-and-match RAID levels with different
partitions.  You can create a 4-partition RAID0 array for swap, a
4-partition RAID0+1 array for filesystems that experience a lot of
writes (/var, /tmp, and maybe /usr/src, for example), and a
4-partition RAID5 setup for /root, /home, et al.  If a disk fails,
your system would likely crash (due to the swap device), but would
reboot in a degraded mode (no swap, slow performance, etc).

I wouldn't overdo the complexity here, but the above becomes quite a
bit easier to manage if you combine RAID with LVM or EVMS to manage
your filesystems.

-Richard

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Re: [gentoo-user] Software RAID Advice Needed

[Neil Bothwick]

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On Thu, 15 Dec 2005 18:21:20 -0700, Richard Fish wrote:

> Also, consider that you can mix-and-match RAID levels with different
> partitions.  You can create a 4-partition RAID0 array for swap, a
> 4-partition RAID0+1 array for filesystems that experience a lot of
> writes (/var, /tmp, and maybe /usr/src, for example), and a
> 4-partition RAID5 setup for /root, /home, et al.  If a disk fails,
> your system would likely crash (due to the swap device), but would
> reboot in a degraded mode (no swap, slow performance, etc).

You could avoid that by not using RAID for swap. Instead, use four
separate swap partitions, one on each drive. As long as they all have the
same priority, the kernel will share swap duties between them equally.
There's no real benefit to using RAID for swap, unless you are limited on
RAM and use swap a lot, when RAID0 may help.


-- 
Neil Bothwick

Excuse me for butting in, but I'm interrupt-driven.

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Re: [gentoo-user] Software RAID Advice Needed

[jarry]

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>> If a disk fails,
>> your system would likely crash (due to the swap device), but would
>> reboot in a degraded mode (no swap, slow performance, etc).
> 
> You could avoid that by not using RAID for swap. Instead, use four
> separate swap partitions, one on each drive. As long as they all have
> the same priority, the kernel will share swap duties between them
> equally.
 
If you make more swap partitions on more physical drives with the same
priority, it is the same as swap on raid0: system strips swap across
drives. And if some drive crashes and swap partition on that drive has
been used, very probably system crashes too. But then reboots at least
with remaining swap partitions...

> There's no real benefit to using RAID for swap, unless you are
> limited on RAM and use swap a lot, when RAID0 may help.

There is some benefit, if you use raid1 for swap. In such a case
even drive failure does not cause system crash, because swap space
is mirrored too. But raid1 slightly degrades swap performance...

Jarry

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Re: [gentoo-user] Software RAID Advice Needed

[Jim Burwell]

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jarry@gmx.net wrote:

>>>If a disk fails,
>>>your system would likely crash (due to the swap device), but would
>>>reboot in a degraded mode (no swap, slow performance, etc).
>>>      
>>>
>>You could avoid that by not using RAID for swap. Instead, use four
>>separate swap partitions, one on each drive. As long as they all have
>>the same priority, the kernel will share swap duties between them
>>equally.
>>    
>>
> 
>If you make more swap partitions on more physical drives with the same
>priority, it is the same as swap on raid0: system strips swap across
>drives. And if some drive crashes and swap partition on that drive has
>been used, very probably system crashes too. But then reboots at least
>with remaining swap partitions...
>  
>

Yes, although there's a posibility of an 'endless crash/reboot' scenereo 
here, if the errors are 'soft' (e.g. not drive just vanishing).  For 
instance, a few bad sectors develop on one of your swap partitions, the 
kernel can't read them, and panic/reboots.  The system comes back up, 
the same swaps are used, and it happens again, over and over until you 
edit the bad partition out of the fstab.

In a redundant RAID situation, I'm presuming that a bad sector or two 
would result in the RAID driver detaching the bad drive, and chugging 
along in degraded mode, where if this happened in a distrubuted swap 
situation, it's already 'too late', since the sectors are lost and the 
kernel would probably panic.

>>There's no real benefit to using RAID for swap, unless you are
>>limited on RAM and use swap a lot, when RAID0 may help.
>>    
>>
>
>There is some benefit, if you use raid1 for swap. In such a case
>even drive failure does not cause system crash, because swap space
>is mirrored too. But raid1 slightly degrades swap performance...
>  
>

This is exactly why I'm doing RAID1 on swap.  If one drive goes poof, my system stays up.  Based on what this server is going to do, it should rarely use much swap, so swap performance isn't a priority for me.  Plus, as you say, I believe the performance hit on swap writes (reads should actually be faster) should wind up being only a bit slower than if you were swapping to a single drive.


- Jim

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+---------------------------------------------------------------------------+
|         Jim Burwell - Sr. Systems/Network/Security Engineer, JSBC         |
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