hardware raid vs. software raid

On Thu, 2006-11-30 at 17:20 +0100, Johannes Wiedersich wrote:
> My concern is that we are going to have only one server. So if there was
> a hardware problem, with software raid I could just temporarily move the
> disks to an ordinary workstation and serve the data from there. With
> hardware raid, I suppose I would have to wait for replacement parts.

If you do the following:

1. Take a snapshot of the system after installing and configuring
everything.

2. Make good backups of /home and (for Samba /etc/samba)

then you can get back up and running en very little time.

I take periodic snapshots of servers when I get a chance to restart them
(like after a kernel update scheduled for after hours. Takes me a
couple of minutes to make a tarball of the / partition and regular
bakcups take care of /home and databases. Thanks to the kind of
unstable power around here that can blow a UPS and everything on it in
one go, I've had to recover whole servers a few times. Above recipy
works every time.

As for software v.s. hardware, for a simple raid-1 I find software good
enough but only if cost is really a huge factor. If I do Raid-5 I
insist on a raid card. Adaptec 2410SA PCI SATA (and the SCSI
equivalent) has served me very well.

Hans

--

> Dave Ewart wrote:
> > Although it's worth pointing out that software RAID-*1* (one of the
> > options under consideration) has almost no CPU overhead, and is often a
> > good low-cost option.
> >
> > Part of your decision must rest on what exactly the machine will be
> > doing. Different RAID setups are best suited for different usage
> > patterns, e.g. RAID-5 is often a good general-purpose storage server
> > option, RAID-10 is usually recommended for database servers, etc.

On 30.11.06 17:20, Johannes Wiedersich wrote:
> It's a 'general purpose' data server for a group of ca. 20 people,
> serving linux-homes and as a samba domain controller where all the data go.
>
> At the moment we have two servers for different kind of data, but the
> idea is to have just one server with all data. The hardware has
> integrated Raid-1, Raid 5 costs extra.
>
> My concern is that we are going to have only one server. So if there was
> a hardware problem, with software raid I could just temporarily move the
> disks to an ordinary workstation and serve the data from there. With
> hardware raid, I suppose I would have to wait for replacement parts.

In such case, you should search for your RAID documentation, if it's real HW
RAID at all.

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On Thu, Nov 30, 2006 at 10:53:26PM +0100, Matus UHLAR - fantomas wrote:
>
> I did not want to mention this, because:
>
> - software RAID-1 has bus overhead (the same data have to be transferred
> multiple times to multiple drives, so writing may be twice as slow)
>
This is only true if you use IDE and also put the mirrored pair on the
same channel (which would be incredibly foolish). Under every other
conceivable scenario, the system is smart enough to do both writes
simultaneously since they will be going difference places.

> - software RAID-1 has high CPU and bus overhead in case of resyncing after
> disk outage (hw raid does that itself)
>

Huh? I have built lots of RAID one systems and seen many of them
resync. They do have high I/O overhead (naturally, since it is a
resync). However, I have never seen one produce high CPU overload. I
suppose if you forgot to enable DMA on your drives, or somehow managed
to disable it (it is enabled by default), then you might see high CPU
load during resync. But then, you would see high CPU load for anything
that does any significant amount of I/O.

Regards,

-Roberto
--
Roberto C. Sanchez
http://people.connexer.com/~roberto
http://www.connexer.com

On Thursday, 30.11.2006 at 22:53 +0100, Matus UHLAR - fantomas wrote:

> > > On 30.11.06 15:44, Johannes Wiedersich wrote:
> > > > What is debian's opinion about hardware/software raid?
>
> > On Thursday, 30.11.2006 at 16:56 +0100, Matus UHLAR - fantomas wrote:
> > > it highly depends on the hardware raid. The true hardware RAID with
> > > hotspare, hotswap etc. support and with proper drivers (so you won't have to
> > > boot into its bios to reconfigure it) will be much more reliable.
> > >
> > > > On the other hand, performance will probably be better with a dedicated
> > > > hardware raid controller.
> > >
> > > it highly depends on the hardware raid. The true hardware RAID with XOR
> > > counting and mirroring support will be much faster.
>
> On 30.11.06 16:01, Dave Ewart wrote:
> > Although it's worth pointing out that software RAID-*1* (one of the
> > options under consideration) has almost no CPU overhead, and is often a
> > good low-cost option.
>
> I did not want to mention this, because:
>
> - software RAID-1 has bus overhead (the same data have to be transferred
> multiple times to multiple drives, so writing may be twice as slow)

That's why I also said "Part of your decision must rest on what exactly
the machine will be doing" since one may or may not care about the extra
disk I/O in some environments. Equally, it could be completely
inappropriate in other environments.

As always, "it depends" :-)

Dave.

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