Re: PATCH: Enhance queueing/scsi-midlayer to handle kiobufs. [Re: Request splits]

["Gérard Roudier" <groudier@club-internet.fr>]

On Fri, 19 May 2000, Brian Pomerantz wrote:

> On Fri, May 19, 2000 at 08:11:10PM +0200, Gerard Roudier wrote:
> > 
> > Low-level drivers have limits on number of scatter entries. They can do
> > large transfers if scatter entries point to large data area. Rather than
> > hacking low-level drivers that are very critical piece of code that
> > require specific knowledge and documentation about the hardware, I
> > recommend you to hack the peripheral driver used for the Ciprico and let
> > it use large contiguous buffers (If obviously you want to spend your time
> > for this device that should go to compost, IMO).
> > 
> > Wanting to provide best support for shitty designed hardware does not
> > encourage hardware vendors to provide us with well designed hardware. In
> > others words, the more we want to support crap, the more we will have to
> > support crap.
> > 
> 
> I really don't want to get into a pissing match but it is obvious to
> me that you haven't had any experience with high performance external
> RAID solutions.  You will see this sort of performance characteristic
> with a lot of these devices.  They are used to put together very large
> storage systems (we are looking at building petabtye systems within
> two years).
> 
> There is no way I'm going to hack on the proprietary RAID controller
> in this system and even if I wanted to or could, I'm quite certain
> there is a reason for needing the large transaction size.  When you
> take into account the maximum transaction unit on each drive (usually
> 64KB), the fact that there are 8 data drives, then you have parity
> calculation, latency of the transfer, and various points at which data
> is cached and queued up before there is a complete transaction, then
> you come up with the magic number.  These things were designed for
> large streaming data and they do it VERY well.
> 
> If you have a hardware RAID 3 or RAID 5 solution that will give me
> this kind of performance for the price point and size that these
> Ciprico units have, then I would LOVE to hear it because I'm in the
> market for buying several of them.  If you can find me a way of
> getting >= 150MB/s streaming I/O on a single I/O server for my cluster
> and fill an order for 2 I/O servers for under $100K, then I may
> consider something other than the Ciprico 7000.  They deliver this
> performance for a very attractive price.  And in a year, I'll come
> back and buy a fifty more.

The SCSI BUS is transaction based and shared. I donnot care of affordable
stuff that moves needless burden to other parts. If they need specific
support for their hardware they must pay for that, and in this situation
these products would probably not really be so affordable.

Note that the same pathology happens to PCI technology. Some PCI devices,
notably Video boards, IDE controllers, brigdes, Network boards, have
abused a LOT of this BUS too. These hardware are/were also probably for an
interesting price but made shit in that place too.

The 36 bit adress extension from Intel is of the same idiomania that
costed a lot for a pathetic result, it seems. It adds complexity to VM
handling on Intel 32 bit systems when 64 bit have been proven to be
feasible 10 years ago and works fine.

I am only interested in technical issues but, in my opinion, it is the
ones that induce costs that must pay for these costs and not others. A
device that requires special handling because it abuses of technologies it
uses should be discarded unless vendor want to pay for the additionnal
effort that such offending stuff requires. But, if such vendors have
interested customers that are ready to pay for the effort or to spend time
implementing specific stuff for these products, I donnot see any problems.

A low-latency BUS does not require huge transactions in order to allow to
use efficiently its bandwitch. If a device requires so, then it can only
be badly designed. Given you description of the Capricio, it seems that it
is based on some kind of stupid batch mode that looks extremally poor
design to me.

Gerard.

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