Date: Wed, 04 Mar 1998 11:49:35 -0800 (PST) From: Simon Shapiro <shimon@simon-shapiro.org> To: sbabkin@dcn.att.com Cc: wilko@yedi.iaf.nl, tlambert@primenet.com, jdn@acp.qiv.com, blkirk@float.eli.net, hackers@FreeBSD.ORG, grog@lemis.com, karl@mcs.net Subject: RE: SCSI Bus redundancy... Message-ID: <XFMail.980304114935.shimon@simon-shapiro.org> In-Reply-To: <C50B6FBA632FD111AF0F0000C0AD71EE4132D4@dcn71.dcn.att.com>
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On 04-Mar-98 sbabkin@dcn.att.com wrote:
...
> Databases are not regular filesystems. They have little number of
> big files.
Wrong.
First, databases can have many small tables (let's stick with RDBMS
for simplicity). What you refer to is what you can see from the utside.
Inside these few big files, is a complex organization that takes closely
related blocks of data and names them. This is what a filesystem is.
So, inside the big files, which reside in a Unix filesystem, there is
another filesystem. It is called a ``storage manager'' or something like
that, but it is a filesystem.
Second, A Unix filesystem is nothing more than a heirarchial database, with
a single key index, variable length records, with one record per table.
Then, at the application level, you assign meanings to the contents of that
record by breaking it into sub records. For example, most Unix utilities
will read this one record sequentially, and everytine they see a '\n', they
will declare it an end-of-record.
...
> Save it to tape as an image of logical disk. Restore it in the same
> way. With things like OnlineJFS you can even do save it online.
I have this new, imaginary Super-DAT tape that does 1 gigabyte/Sec. Itis
attached to a Super-Duper-Ultra-SCSI thatt= has 1GB/Sec throughtput, and
a small, 100GB database. The application is sinmple, for every telephone
call the switch wants to make, we need to find the customer's telephone
number, his long-distance company code, is his account valid, etc. then we
add a CDR (Call Detail Record) that said that Simon called George on
such-and-such date and talked for so many minutes. This is a highly,
overly simplified application. Now, these database inquiries come at the
rate of only 200/Sec. For simplicity sake, each CDR is exactly one half of
a sector on the disk (forget databases. We are in the raw here :-). The
database never shuts down, as you hate it when there is no dialtone when
you pick up the phone. Now go and backup this database:
a. You cannot shut it down, because the PUC said so and you hate prisons.
b. By the time you back it up (100 seconds, there have been 20,000
modifications to the database.
c. If you do a hot backup of the files, you will have approximately 10,000
changes that are not in your backup.
d. If this was on a Unix filesystem, your files are now corrupt. Totall.
you tell me why.
This is a simplistic examples. Life is nastier than that. Can it be
solved? Of course. With Unix? Yes, what do you think a 5ESS switch runs?
With FreeBSD? Yes. As is today? No....
----------
Sincerely Yours,
Simon Shapiro
Shimon@Simon-Shapiro.ORG Voice: 503.799.2313
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