Re: Solaris ZFS on Linux [Was: Re: the " 'official' point of view"expressed by kernelnewbies.org regarding reiser4 inclusion]

[Hans Reiser <reiser@namesys.com>]

Theodore Tso wrote:

>On Mon, Jul 31, 2006 at 09:41:02PM -0700, David Lang wrote:
>  
>
>>just becouse you have redundancy doesn't mean that your data is idle enough 
>>for you to run a repacker with your spare cycles. to run a repacker you 
>>need a time when the chunk of the filesystem that you are repacking is not 
>>being accessed or written to. it doesn't matter if that data lives on one 
>>disk or 9 disks all mirroring the same data, you can't just break off 1 of 
>>the copies and repack that becouse by the time you finish it won't match 
>>the live drives anymore.
>>
>>database servers have a repacker (vaccum), and they are under tremendous 
>>preasure from their users to avoid having to use it becouse of the 
>>performance hit that it generates. (the theory in the past is exactly what 
>>was presented in this thread, make things run faster most of the time and 
>>accept the performance hit when you repack). the trend seems to be for a 
>>repacker thread that runs continuously, causing a small impact all the time 
>>(that can be calculated into the capacity planning) instead of a large 
>>impact once in a while.
>>    
>>
>
>Ah, but as soon as the repacker thread runs continuously, then you
>lose all or most of the claimed advantage of "wandering logs".
>  
>
Wandering logs is a term specific to reiser4, and I think you are making
a more general remark.

You are missing the implications of the oft-cited statistic that 80% of
files never or rarely move.   You are also missing the implications of
the repacker being able to do larger IOs than occur for a random tiny IO
workload which is impacting a filesystem that is performing allocations
on the fly.

>Specifically, the claim of the "wandering log" is that you don't have
>to write your data twice --- once to the log, and once to the final
>location on disk (whereas with ext3 you end up having to do double
>writes).  But if the repacker is running continuously, you end up
>doing double writes anyway, as the repacker moves things from a
>location that is convenient for the log, to a location which is
>efficient for reading.  Worse yet, if the repacker is moving disk
>blocks or objects which are no longer in cache, it may end up having
>to read objects in before writing them to a final location on disk.
>So instead of a write-write overhead, you end up with a
>write-read-write overhead.
>
>But of course, people tend to disable the repacker when doing
>benchmarks because they're trying to play the "my filesystem/database
>has bigger performance numbers than yours" game....
>  
>
When the repacker is done, we will just for you run one of our
benchmarks the morning after the repacker is run (and reference this
email);-)....  that was what you wanted us to do to address your
concern, yes?;-)

>					- Ted
>
>
>  
>

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