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From Mike Matrigali <mikem_...@sbcglobal.net>
Subject Re: [jira] Commented: (DERBY-1) Can't create a new db on OS X
Date Tue, 28 Sep 2004 17:02:10 GMT
Did you get the below information from jdk interface documentation
somewhere, or from reading a specific jdk implmentation?  If the cross
platform documentation had the information you describe below then I
would agree with the change, the limited jdk documentation I have seen
has nothing about this.

Please note that a workaround was posted such that with no patch one
could get the system up and running on OSX.  I will work to get that
information into the jira report.


Joseph Grace wrote:

> On Sep 24, 2004, at 4:48 PM, Jan Hlavatý wrote:
> 
>> Let me warn you though - we got suspiciously high throughput when
>> using "rwd", so we suspect it may not do what it is supposed to do,
>> and crash recovery may not work as good as expected.
> 
> 
> Very important, reliability is a top priority.
> 
> However, from what I gathered, "rwd" is preferred if you want to
> guarantee the data reliability and are willing to delay (risk) the
> (non-essential) metadata reliability.
> 
> As an aside, metadata appears to be the directory entry information (at
> least) associated with the database file.
> "rwd" only updates essential metadata.  Essential directory information
> would be the additional inode (unix) record keeping required as a db
> grew in size.  This metadata is required to ensure all database content
> is fully recoverable.
> "rws" updates all metadata (even metadata non-essential to recovering
> the database content).  Non-essential directory information would be
> updating the last-modified-time in the directory entry for the file
> containing the db.  IOW, an "old" last-modified-time metadata is
> non-essential since it is unnecessary to recover even the latest updates
> to the database.
> 
> From a performance perspective, "rwd" (relative to "rws") is very
> compelling.  In theory it could be almost twice as fast as "rws" (I
> think) under certain situations (many database modification
> transactions).  Basically, "rws" must modify not only the db file but
> also its directory entry (e.g., the last-modified-time) for all database
> transactions.  In contrast, "rwd" just updates the db file (content) and
> is done.  So "rwd" updates just one file whereas "rws" must update two
> files per transaction.  In theory, "rwd" could be roughly twice as fast
> as "rws" (I believe).
> 
> That said, I think it's important to judge "rwd" support on a platform
> by platform basis to verify it works as expected where desired.  Also, I
> wonder if it works at least as well as "rws" everywhere?  I gather the
> "serious" (e.g., Oracle) databases would use "rwd" instead of "rws" (for
> performance reasons) so one indication of a platform's "rwd" reliability
> may be whether Oracle runs on it (guessing).
> 
> Another question for Derby is whether it has the luxury (as does, e.g.,
> Oracle and perhaps Cloudscape) of limiting its use to certain
> platforms.  How does that work with open source databases, or Derby in
> particular?  IOW, requiring proper "rwd"/"rws" support may be too
> stringent, but it does beg the question of whether there's any platform
> of sufficient interest which does not properly support "rwd".
> 
> -=-
> 
> On a more grounded note, my "rwd" patch is a simple enough patch, that
> it could be configurable as well to flip from "rwd" to "rws" as
> desired.  However, I'm not sure platforms without "rwd" would
> necessarily be any better at supporting "rws" (since "rwd" is really
> less demanding and more interesting than "rws").  I suspect the
> commercial approach was simply not to support platforms without full
> "rwd" support, but how does that translate to Apache Derby?
> 
> I'm not sure what happens as file systems get more sophisticated, i.e.,
> with more metadata than just directory entries to update or even
> create.  I'm suspicious of this with OSX since it has a variety of
> metadata files which are unique to OSX.  I want to test what's happening
> there and if that's what's causing the schizophrenic
> file-exists/file-not-found issue under certain database open
> circumstances using "rws".  However, this issue may be moot (at least on
> OSX) if "rwd" is really the preferred method anyway _and_ has proper
> support where desired (hmm, this "rwd" feature could make a good
> candidate for a low-level unit test to verify "rwd"/"rws" database
> integrity on target platforms!).
> 
> All that said, I'd appreciate if those with more experience in db's and
> file system's could chime in if they have any corrections, additions, etc..
> 
> Thanks,
> 
> = Joe =
> 
> 

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