Re: Raid 1 Performance Impact
- From: Arno <me@xxxxxxxxxxx>
- Date: 15 May 2010 23:11:54 GMT
David Brown <david.brown@xxxxxxxxxxxxxxxxxxxxxxxxxx> wrote:
Rod Speed wrote:[...]
And you really need to keep a spare of what is the most expensive
approach too.
That depends on why you are using raid - if it is for improved uptime,
you might well want a spare card on-hand. If it is for improved speed,
it is probably not necessary.
Agreed.
Generally speaking, I would say that if you are concerned enough about
uptime to want a spare hardware raid controller, you are better off
setting up a redundant system or a hot spare of the entire system.
Not really. The controller is the only non-generic part. Everything
else you can replace with generic spares you can get fast and locally.
Or you can re-purose a different machine. For the controller alone,
this does not work.
After all, the raid card is not a weak point in the system - things like
power supplies are much more likely to fail, as is the motherboard. But
you will at least want to make sure replacement parts are easily available.
Indeed. The RAID conroller is typically the only part not easily
avaliable.
Fakeraid gives you the worst of both worlds.
Thats overstating it, particularly with price.
The price is good, certainly - but not any better than the price of
software raid.
And the flexibility and problem resilience is worse.
There is no separate processor, so everything is handled by the
host - either by BIOS routines, or drivers loaded by the OS.
But as you say, thats not a problem with modern systems and the
simpler forms of RAID.
It's not a problem - certainly not in terms of performance. But it
means you have the same vulnerabilities to power failures or system
crashes as you do with software raid.
Actually, the system crash risk you get in addition with software
RAID as compared to hardware RAID is pretty low or non-existent.
As for power-failures, that is not a RAID risk at all, but a filesystem
risk. RAID does not make it worse, unless you do stupid things like
using a hardware RAID controller with a large buffer an no battery backup.
The way to deal with power-failures is UPS and journalling file
system deswigned to be power-failure resistant. Of course the
relevant applications also need to be resilient, for example
databases need to use a recovery log and text editors need to
do automatic, regular backups while you write.
You don't get the full flexibility of proper software raid, but
only the limited functionality provided by the fakeraid. And
access to the disk is limited to the chipset used in the
motherboard - if the motherboard dies (more likely than a hardware
raid controller dying),
That last is very arguable.
Motherboards are a higher risk component than a raid card. They are
high power, high speed, and dense boards, leaving them very vulnerable
to overheating if you have problems with your fans or cooling systems.
They are connected to all sorts of external devices, making them
vulnerable to external wrong connections or misuse (depending on the
usage environment, of course). And the heavy competition in the market
leads to low-cost shortcuts sometimes being taken in the design and
manufacture of motherboards. Raid controller cards are simpler,
dedicated to a specific job, and aimed for a market which places value
on reliability and stability.
Well, fakeRAID cards are still in the low-cost segment. Don't depend
on them too much. Having a recovery strategy for RAID failure saves
one from loosing all data after the last backup. The same is true
for proper RAID.
Of course, you can still argue about it - and you can probably find
motherboards with a better track record than some hardware raid cards,
so it's worth doing some research before buying.
you could lose access to your disks.
Not necessarily forever and a spare motherboard is going to cost less
than the spare controller with fancy hardware raid too.
It's certainly true that a good hardware raid card will cost more than a
typical motherboard, so if you are buying spares, the motherboard is
cheaper. But if you are looking at this over a long term, motherboard
designs (and in particular, chipsets and bios versions) come and go a
lot faster than raid card designs. And even if your particular model of
raid card is no longer available, newer models from the same company
will often be compatible with the older drives - motherboard fakeraid
gives no such promises.
Actually the fakeRAID compatibility might be higher, provided you
stay with the same controller, whether card or mainboard. For example
Silicon Image seems to have a single fakeRAID implementation for all
their chips. However I recently ran into another nice problem: I
was unable to reeflash a fakeRAID card with a non RAID BIOS, because
the SiL flasher does not support the flash chip used on the card.
Overall, I tink fakeRAID has severe limits that most people do
not understand and that are likely to bite them.
Of course, software raid is a better choice if this is an important
issue, since the same OS will work on a wide range of hardware.
Indeed. My recovery strategy for my linux arrays is to attach them
in any way (even USB enclosures attached to a laptop would work) to
a different Linux box. And I have done this just to make sure it works.
Now I only with Win7 had something compatible with Linux and
remotely as flexible and powerful. No such luck. If virtualization
gets at any time t the pont it supports games well (my main Win
application), I will restric Wndows to run that way and use
Linux as basis system. Not too surprising that Vmware ESXi
basically does that with a custom Linux basis.
Where fakeraid wins is if you are using a limited OS like windows,
and want to install the whole system on a raid drive but don't want
to pay for a hardware raid card.
So it isnt actually the worst of both worlds.
As far as I know, you can't install windows on a windows software
raid drive
Yes you can.
- you can only use such drives for non-system partitions.
Thats just plain wrong.
OK - I was careful to prefix that statement with "as far as I know".
It's not something I've tried, and the quick google I did about windows
software raid turned up plenty of information about making dynamic disk
raid sets on extra drives and partitions, but the only article I found
about windows software raid and windows system partitions said that it
couldn't be done. But you know more about such setups than I do, so
I'll take your word for it that it's possible.
I am also not sure. But I think you can install on non-raided
and then turn your systempartition into a dynamic disk afterwards.
Arno
--
Arno Wagner, Dr. sc. techn., Dipl. Inform., CISSP -- Email: arno@xxxxxxxxxxx
GnuPG: ID: 1E25338F FP: 0C30 5782 9D93 F785 E79C 0296 797F 6B50 1E25 338F
----
Cuddly UI's are the manifestation of wishful thinking. -- Dylan Evans
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