Storage Area Network as a tlug meeting topic

[Lennart Sorensen]

On Wed, Aug 05, 2009 at 12:58:40PM -0400, Mark Lane wrote:
> The article does not give support for the claim by Oliver Ratzesberger  
> that 15K drives are 10x faster than 7200RPM drives. Maybe they are but  
> it doesn't really matter when dealing with a lot of drives. You only get  
> an advantage of a faster disk when you are running single or small  
> arrays of disks. If you have a large array, the bandwidth of your bus or  
> storage network will be the limiting factor and not the drives  
> themselves. When I was designing NAS boxes, we found that it would only  
> take 4 7200RPM SATA 1 drives to max out a PCI-X bus using RAID 5 and  
> 3ware card. So if I had 4 15RPM drives you might get better seek times  
> but the overall transfer rate will not be any better than the 4 7200RPM  
> drives. I guess if you have an application that requires seeking a lot  
> of small files and not a lot of bandwidth, the 15K RPM will still be  
> faster but for most applications it won't be.

It is not about bandwidth.  It is about access time.

A 15000rpm drive takes half as long to rotate to a given piece of data
as a 7200rpm drive.  At the same time, it is a samller platter so the
maximum seek time is reduced too.  Of course you loose 85% of the capacity
the 7200rpm drive gives you but you can get to a given piece of data in
less than half the time of the 7200 rpm drive.  You combine lots of those
15k drives to increase bandwidth and storage space back to what you need.

> Also the article talked about cheap drives being optimized for power  
> consumption and not performance and reliability. While this is true for  
> desktop drives, the article fails to mention that for not a lot of more  
> money you can get RAID edition SATA drives from WD and Seagate that are  
> actually rated for performance and reliability.
>
> Ultimately, the decision will come down to the executives. They assume  
> that SCSI, Fiber Channel are enterprise class hardware and SATA isn't. I  
> was at a Western Digital presentation once where they were showing of  
> drives that were equivalent in performance and reliability to SCSI  
> offering yet the still refrained from referring to them as enterprise  
> class. Yet Google has no problem using commodity hard drives.

SCSI (parallel) is dead.  SAS is what you use these days.  No single
point of failure (scsi bus) anymore, and dual ported (so you can connect
every drive to redundant controllers).

> If you need a ton of storage, cheaper SATA drives are definitely much  
> more cost effective. You can even get them with 5 year warranties.

If you need cheap storage or raw throughput for large files being streamed
then a 7200rpm sata drive is a good choice.  If you need low latency
random access, then a 15k SAS drive is your choice or better yet, you
go to solid state drives instead since they have no seek or rotational
latency at all.

> Fiber Channel based SANs are not even the fastest storage networks out  
> there. They are just very well established in the enterprise. Running  
> iSCSI over 10Gig Ethernet or Myrinet will give you a much fastest  
> Storage Network.

Certainly.  FC is getting a bit old these days.

-- 
Len Sorensen
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