Re: Implementing virtual memory on cassette tape

From	Bernd Paysan <bernd.paysan@gmx.de>
Newsgroups	comp.lang.forth
Subject	Re: Implementing virtual memory on cassette tape
Date	2012-08-10 01:27 +0200
Organization	1&1 Internet AG
Message-ID	<1642819.e4UGJbCkPf@sunwukong.fritz.box> (permalink)
References	(12 earlier) <882dd387-f2ab-4d08-8b12-fde5a656157f@a9g2000vbn.googlegroups.com> <5095812.K4UvBJtjZl@sunwukong.fritz.box> <25899bec-a795-4f1f-b84c-32f3da20b0ad@b10g2000vbj.googlegroups.com> <2845685.VmIIC3HCAb@sunwukong.fritz.box> <269f51bb-8d8b-4433-9bda-35aba932e31f@n13g2000vby.googlegroups.com>

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Alex McDonald wrote:
> There are enterprise class SATA drives too, btw, something that may
> not be apparent from a casual inspection of a disk mfrs website. Both
> SATA and SAS enterprise drives have a much lower bit error rate (a
> factor of 10), a lower AFR and higher MTBF than the corresponding
> desktop variety.

Actually, most of the desktop varieties have bit error rate, AFR and 
MTBF unspecified, and the enterprice class disks have them specified.  
This is IMHO, because they are actually identical, maybe except firmware 
issues, as below.  What is certainly possible is that the desktop 
varieties haven't been tested and contain severe bugs.

> The firmware is not the same either, since the
> assumption is made that the drives are part of a RAID group, and the
> hardware or software upstream can handle the errors. Desktop drives go
> to extraordinary lengths to read data to the point of "spasm" for what
> might be several minutes, since they assume that this is your only
> copy.

Yes, and the RAID controller assumes that when exceeding a certain 
timeout, the disk is due to replacement.  Which is true.  I'm completely 
unconvinced by what e.g Western Digital says about this topic: If your 
drive in a RAID array has problems reading data, replace it *now*.  
Crash early, as we Forthers say.  This long spasm is the right reaction 
in both environments: The desktop user gets his precious data back, and 
the RAID controller throws the bad disk out.  Which he should.

IMHO, they got the complete protocol wrong, this shouldn't be hidden.  
The correct way to deal with these problems should be:

Say "Oops, read error" when you encounter a read error.  The host then 
can respond with "retry", "retry harder", and "attempt to repair", if it 
feels like it.  For a RAID system, a read error is no problem, there is 
enough redundancy to deliver the data, anyways.  It's much more 
important that you say "Oops" quickly.  Even on a mirrored system where 
you only access one disk for one request to improve throughput (they can 
serve twice as many read requests in that mode), retrying on the other 
trive is faster than the thorrough "retry harder".

We had that 20 years ago with floppy disk drivers, and the 
"Abort/Ignore/Retry" message from DOS.  The wrong thing was to present 
this message to the user; internally, the protocol was perfectly ok.  
Say something when you don't feel ok, say it quickly.

I've a similar thing in my net2o protocol.  TCP tries to retransmit 
packets which have been dropped.  net2o tries to re-request packets 
which didn't arrive.  This turns the situation around, the client is 
responsible for correct transmission, not the server.  Which allows the 
client to use more intelligent strategies - e.g. when copying identical 
files from several peers, you can ask any of them to transmit that lost 
block.  Or when you stream real-time low-latency audio data, just 
interpolate the lost block.  Assuming that you have to deliver 100% 
quality of service all the time can be wrong.  Deliver what you can, and 
say when you can't.  If that's not acceptable, the other side will 
complain, and then you can try harder.

> And so on. They are not the same. The price is not hugely different
> from a desktop drive; around twice the price.

Yes, I know, and I'm quite convinced that this is not worth it, and that 
this comes from ill-percieved risk assessment.  Or ill-perceived ways to 
save costs or something - it *is* cheaper to remove the vibrations of 
hard-disks than to make vibration-resistant ones, which probably are 
more vibration-resistant on paper than in reality.

Always remember: For twice the price, you can get twice the cheap disks.  
Usually, you don't need that many to reduce the risk to the same level 
you paid twice the price for.  Or put differently: Flying business class 
is no more secure than flying economy class.  But when paying a higher 
price makes you feel better, you should fly business class.

-- 
Bernd Paysan
"If you want it done right, you have to do it yourself"
http://bernd-paysan.de/

Thread

Implementing virtual memory on cassette tape chitselb <chitselb@gmail.com> - 2012-08-07 06:21 -0700
  Re: Implementing virtual memory on cassette tape Andrew Haley <andrew29@littlepinkcloud.invalid> - 2012-08-07 08:44 -0500
  Re: Implementing virtual memory on cassette tape anton@mips.complang.tuwien.ac.at (Anton Ertl) - 2012-08-07 14:01 +0000
  Re: Implementing virtual memory on cassette tape Mark Wills <markrobertwills@yahoo.co.uk> - 2012-08-07 07:24 -0700
  Re: Implementing virtual memory on cassette tape Stan Barr <plan.b@dsl.pipex.com> - 2012-08-07 15:30 +0000
    Re: Implementing virtual memory on cassette tape Stan Barr <plan.b@dsl.pipex.com> - 2012-08-07 17:36 +0000
  Re: Implementing virtual memory on cassette tape Jason Damisch <jasondamisch@yahoo.com> - 2012-08-07 11:52 -0700
    Re: Implementing virtual memory on cassette tape Paul Rubin <no.email@nospam.invalid> - 2012-08-07 12:39 -0700
      Re: Implementing virtual memory on cassette tape Paul Rubin <no.email@nospam.invalid> - 2012-08-07 12:55 -0700
        Re: Implementing virtual memory on cassette tape Bernd Paysan <bernd.paysan@gmx.de> - 2012-08-07 22:00 +0200
          Re: Implementing virtual memory on cassette tape Paul Rubin <no.email@nospam.invalid> - 2012-08-08 00:27 -0700
            Re: Implementing virtual memory on cassette tape Mark Wills <markrobertwills@yahoo.co.uk> - 2012-08-08 01:26 -0700
              Re: Implementing virtual memory on cassette tape Alex McDonald <blog@rivadpm.com> - 2012-08-08 02:31 -0700
              Re: Implementing virtual memory on cassette tape Paul Rubin <no.email@nospam.invalid> - 2012-08-08 02:46 -0700
            Re: Implementing virtual memory on cassette tape Alex McDonald <blog@rivadpm.com> - 2012-08-08 02:23 -0700
              Re: Implementing virtual memory on cassette tape anton@mips.complang.tuwien.ac.at (Anton Ertl) - 2012-08-08 10:57 +0000
                Re: Implementing virtual memory on cassette tape Alex McDonald <blog@rivadpm.com> - 2012-08-08 04:59 -0700
                Re: Implementing virtual memory on cassette tape anton@mips.complang.tuwien.ac.at (Anton Ertl) - 2012-08-08 12:24 +0000
                Re: Implementing virtual memory on cassette tape Alex McDonald <blog@rivadpm.com> - 2012-08-08 11:10 -0700
                Re: Implementing virtual memory on cassette tape Bernd Paysan <bernd.paysan@gmx.de> - 2012-08-09 00:13 +0200
                Re: Implementing virtual memory on cassette tape Alex McDonald <blog@rivadpm.com> - 2012-08-08 16:05 -0700
                Re: Implementing virtual memory on cassette tape Paul Rubin <no.email@nospam.invalid> - 2012-08-08 17:30 -0700
                Re: Implementing virtual memory on cassette tape Bernd Paysan <bernd.paysan@gmx.de> - 2012-08-09 03:26 +0200
                Re: Implementing virtual memory on cassette tape Alex McDonald <blog@rivadpm.com> - 2012-08-09 05:30 -0700
                Re: Implementing virtual memory on cassette tape Bernd Paysan <bernd.paysan@gmx.de> - 2012-08-09 19:21 +0200
                Re: Implementing virtual memory on cassette tape Alex McDonald <blog@rivadpm.com> - 2012-08-09 13:30 -0700
                Re: Implementing virtual memory on cassette tape Bernd Paysan <bernd.paysan@gmx.de> - 2012-08-10 01:27 +0200
                Re: Implementing virtual memory on cassette tape vandys@vsta.org - 2012-08-09 00:32 +0000
                Re: Implementing virtual memory on cassette tape Bernd Paysan <bernd.paysan@gmx.de> - 2012-08-09 03:33 +0200
                Re: Implementing virtual memory on cassette tape anton@mips.complang.tuwien.ac.at (Anton Ertl) - 2012-08-09 06:00 +0000
                Re: Implementing virtual memory on cassette tape Alex McDonald <blog@rivadpm.com> - 2012-08-09 05:26 -0700
                Re: Implementing virtual memory on cassette tape anton@mips.complang.tuwien.ac.at (Anton Ertl) - 2012-08-09 13:44 +0000
                Re: Implementing virtual memory on cassette tape Alex McDonald <blog@rivadpm.com> - 2012-08-09 10:21 -0700
                Re: Implementing virtual memory on cassette tape Bernd Paysan <bernd.paysan@gmx.de> - 2012-08-09 19:50 +0200
                Re: Implementing virtual memory on cassette tape Alex McDonald <blog@rivadpm.com> - 2012-08-09 12:32 -0700
                Re: Implementing virtual memory on cassette tape Bernd Paysan <bernd.paysan@gmx.de> - 2012-08-09 22:07 +0200
                Re: Implementing virtual memory on cassette tape Alex McDonald <blog@rivadpm.com> - 2012-08-09 13:58 -0700
                Re: Implementing virtual memory on cassette tape Paul Rubin <no.email@nospam.invalid> - 2012-08-09 17:36 -0700
                Re: Implementing virtual memory on cassette tape Alex McDonald <blog@rivadpm.com> - 2012-08-10 04:13 -0700
                Re: Implementing virtual memory on cassette tape Paul Rubin <no.email@nospam.invalid> - 2012-08-11 20:27 -0700
                Re: Implementing virtual memory on cassette tape anton@mips.complang.tuwien.ac.at (Anton Ertl) - 2012-08-10 15:57 +0000
                Re: Implementing virtual memory on cassette tape Alex McDonald <blog@rivadpm.com> - 2012-08-13 05:23 -0700
                Re: Implementing virtual memory on cassette tape anton@mips.complang.tuwien.ac.at (Anton Ertl) - 2012-08-15 15:13 +0000
                Re: Implementing virtual memory on cassette tape Alex McDonald <blog@rivadpm.com> - 2012-08-15 11:57 -0700
      Re: Implementing virtual memory on cassette tape anton@mips.complang.tuwien.ac.at (Anton Ertl) - 2012-08-08 07:08 +0000
        Re: Implementing virtual memory on cassette tape chitselb <chitselb@gmail.com> - 2012-08-08 06:25 -0700
      Re: Implementing virtual memory on cassette tape Mark Wills <markrobertwills@yahoo.co.uk> - 2012-08-08 01:23 -0700
      Re: Implementing virtual memory on cassette tape kenney@cix.compulink.co.uk - 2012-08-08 05:06 -0500
  Re: Implementing virtual memory on cassette tape Percy <percival.andrews@gmail.com> - 2012-08-08 21:11 -0700
    Re: Implementing virtual memory on cassette tape chitselb <chitselb@gmail.com> - 2012-08-08 21:30 -0700
      Re: Implementing virtual memory on cassette tape percival.andrews@gmail.com - 2012-08-08 23:50 -0700
        Re: Implementing virtual memory on cassette tape chitselb <chitselb@gmail.com> - 2012-08-09 03:54 -0700
          Re: Implementing virtual memory on cassette tape Paul Rubin <no.email@nospam.invalid> - 2012-08-09 09:07 -0700
            Re: Implementing virtual memory on cassette tape chitselb <chitselb@gmail.com> - 2012-08-09 12:20 -0700
  Re: Implementing virtual memory on cassette tape Mat <dambere@web.de> - 2012-08-10 13:41 -0700
    Re: Implementing virtual memory on cassette tape Coos Haak <chforth@hccnet.nl> - 2012-08-10 23:54 +0200
      Re: Implementing virtual memory on cassette tape dambere@web.de - 2012-08-10 15:41 -0700
        Re: Implementing virtual memory on cassette tape Coos Haak <chforth@hccnet.nl> - 2012-08-11 01:47 +0200
    Re: Implementing virtual memory on cassette tape Andrew Haley <andrew29@littlepinkcloud.invalid> - 2012-08-11 03:50 -0500
      Re: Implementing virtual memory on cassette tape anton@mips.complang.tuwien.ac.at (Anton Ertl) - 2012-08-11 09:03 +0000
        Re: Implementing virtual memory on cassette tape Andrew Haley <andrew29@littlepinkcloud.invalid> - 2012-08-11 16:08 -0500
    Re: Implementing virtual memory on cassette tape "Elizabeth D. Rather" <erather@forth.com> - 2012-08-11 21:51 -1000

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