[Beowulf] hardware RAID versus mdadm versus LVM-striping
landman at scalableinformatics.com
Sun Jan 17 19:36:43 PST 2010
Rahul Nabar wrote:
> If I have a option between doing Hardware RAID versus having software
> raid via mdadm is there a clear winner in terms of performance? Or is
Depends upon workload, writes vs reads, streaming vs random IO, number
of simultaneous readers/writers. There is no real clear answer.
> the answer only resolvable by actual testing? I have a fairly fast
> machine (Nehalem 2.26 GHz 8 cores) and 48 gigs of RAM.
Testing is a good thing. Sadly, too many people test *after* they
purchased something (only to discover what is meant by the term
"marketing benchmark numbers").
> Should I be using the vendor's hardware RAID or mdadm? In case a
Ohhh ... it depends. Some of the "vendors" hardware raid ... heck ...
most of it ... is rebadged LSI gear. Usually their lower end stuff
which is sometimes fake-raid. Use fake-raid only if no other options
More in a moment.
> generic answer is not possible, what might be a good way to test the
> two options? Any other implications that I should be thinking about?
Benchmark your load using a load generator like fio.
> Finally, there;s always hybrid approaches. I could have several small
> RAID5's at the hardware level (RIAD5 seems ok since I have smaller
> disks ~300 GB so not really in the domain where the RAID6 arguments
> kick in, I think) Then using LVM I can integrate storage while asking
RAID6 kicks in purely from the second correlated disk failure scenario.
This is size independent. It happens, and you need to be prepared.
> LVM to stripe across these RAID5's. Thus I'd get striping at two
> levels: LVM (software) and RAID5 (hardware).
LVM is not a performance tool. Use it to help you manage things, not
Our own testing puts our 24 bay DV4 unit at a bit more than 1GB/s
sustained read (large block sequential) in RAID6, with writes in the
400-500 MB/s region (large block sequential). This is MD RAID based.
Our "equivalent" JR4 system clocks in at nearly double the read speed,
and about 3+ x the write speed. This is a hardware RAID system.
Your mileage will vary ... tremendously ... as a function of your IO
My own suggestion is to test before you buy. After you buy, well, its a
bit harder to change your mind.
Joseph Landman, Ph.D
Founder and CEO
Scalable Informatics, Inc.
email: landman at scalableinformatics.com
web : http://scalableinformatics.com
phone: +1 734 786 8423 x121
fax : +1 866 888 3112
cell : +1 734 612 4615
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