Re: [HACKERS] The dangers of "-F" - Mailing list pgsql-hackers

From Don Baccus
Subject Re: [HACKERS] The dangers of "-F"
Date
Msg-id 3.0.1.32.19990622224345.00deb3cc@mail.pacifier.com
Whole thread Raw
In response to Re: [HACKERS] The dangers of "-F"  (Bruce Momjian <maillist@candle.pha.pa.us>)
List pgsql-hackers
At 08:36 PM 6/22/99 -0400, Bruce Momjian wrote:
>> At 06:38 PM 6/22/99 -0400, Bruce Momjian wrote:
>> 
>> >No Fsync is only dangerous if your OS or hardware crashes without
>> >flushing the disk. Anything else is unaffected, and is just as reliable.
>> 
>> Yes, this much I realize...
>> 
>> >The database could be inconsistent, in the sense that partial
>> >transactions are recorded as completed.
>> 
>> With recovery possible without a rebuild?  Or is rebuilding
>> from dumps required?  (I dump nightly and copy the results
>> to a second machine for additional safety, and soon will
>> be ftp'ing dump files to the east coast for even more
>> safety).  
>
>
>> 
>> Perhaps fsync'ing then is only LESS dangerous, since
>> a system can crash while blocks are being written even
>> when fsync is enabled.  The window of evil opportunity
>> for a system crash is much smaller than if the data's sitting
>> around for a lengthy time in the Linux FS cache, of course,
>> but not absent.
>
>Yes, this is true, but much less likely because the ordering of the
>flushing is done before the transaction is marked as completed.
>
>> 
>> Or does the fact that the backend loses control over the
>> order in which stuff is written (in other words, blocks
>> are written whenever and in what order Linux choses rather
>> than fsync'd a file at a time) mean that the kind of 
>> inconsistency that might result is different?  I.E.
>> log file written before datablocks are, that kind of
>> thing.
>
>Yes.  It is not a problem that a give transaction aborts while it is
>being done because it couldn't have been marked as completed, but the
>previous transaction was marked as completed, and only some blocks could
>be on the disk.
>
>
>> 
>> >I think it is a major issue too.
>> 
>> Is there any estimate of the difficulty of fixing it?
>> >From previous discussions, it sounded as though new
>> bookkeeping would be needed to determine which queries
>> actually result in a change in data.
>
>I hope for every release.  I tried to propose some solutions, but
>couldn't code it.
>
>-- 
>  Bruce Momjian                        |  http://www.op.net/~candle
>  maillist@candle.pha.pa.us            |  (610) 853-3000
>  +  If your life is a hard drive,     |  830 Blythe Avenue
>  +  Christ can be your backup.        |  Drexel Hill, Pennsylvania 19026
>
>


- Don Baccus, Portland OR <dhogaza@pacifier.com> Nature photos, on-line guides, and other goodies at
http://donb.photo.net


pgsql-hackers by date:

Previous
From: Don Baccus
Date:
Subject: Re: [HACKERS] The dangers of "-F"
Next
From: "Collin F. Lynch"
Date:
Subject: Images