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Re: Advice sought on moving to AMD64



On Mon, Jun 20, 2005 at 10:39:37PM +0200, Mattias Wadenstein wrote:
> Ah, but SAS was mostly non-conflicting in the storage world. Remember that 
> SSA is both the architecture: Serial Storage Architecture (IBM SSA*) and a 
> brand acronym for the Sparc Storage Array.

Well yes SSA would have been worse in the storage area.  I forgot about
those ones.

> When it comes to the SATA vs SCSI argument, the question is if you are 
> seek-limited in your load. If you are, you want more RPMs on your drive, 
> which leads to SCSI. Also, SCSI parts usually less troublesome. But then, 
> if you have many drives, you have to plan for failure anyway.

Well hopefully NCQ on SATA which is starting to become normal now will
help with the seek issue, although not as much as a very high speed
drive.  Too bad you can't get high speed and high capacity at the same
time. :)

> Note that due to the workings of linux io scheduling, multiple (how many 
> depends on tuning) pure-bandwidth streams turn into a seek-limited io 
> load.

Maybe someone will fix that some day.

> If you need huge ammounts of storage or only a few simultaneous 
> high-performance streams, SATA is the way to go.

I certainly don't understand personally, why people put 15k rpm drives
in a SAN and then connect a dozen machines to it using 1Gbit fibre
channel.  I suspect the software in the FC switch, and the SAN probably
take longer to figure out where to seek to than the drive does to make
the seek, making me wonder if the 15k rpm drive really makes it much
faster, and if using SATA wouldn't have given 10 times the storage for
less than half the cost with about the same performance in the SAN.

> Btw, SCA has been a standard connector for many years now. It's the 
> standard for scsi hotswap slots.

Sure.  It also looks like all SATA drives have the same connector
placement so they can be hotplugged as well.  Certainly SATA was
designed with hotplug in mind even if most controllers don't support it.

> *) Btw, ftp.se.debian.org would enjoy a donation of IBM SSA, if anyone 
> happens to have some 18G+ drives around. Say a couple of racks or so? :)

Len Sorensen



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