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From Selcuk AYA <ayasel...@gmail.com>
Subject Re: Question about the Log file
Date Fri, 16 Mar 2012 18:07:40 GMT
On Fri, Mar 16, 2012 at 2:26 AM, Emmanuel L├ęcharny <elecharny@gmail.com> wrote:
> Hi,
>
> AFAICS, the og file contains a buffer which stores UserLogRecord until we
> write them on disk. This allows the Log system to be fast, and also allow a
> dedicated thread to flush the data on disk regularly.
>
> So far, so good.
>
> But I'm wondering if it would'nt be simpler to use a MemoryMappedFile
> instead, as the Log file size is fixed. We will then let the underlying OS
> to deal with flushes, not needing a dedicated thread to handle it. Also
> MemoryMappedFile are faster than RandomFile, as it's working on pages, which
> are managed by the OS (their size is depending on the OS tuning). Last, not
> least, we won't need to dedicate a 4Mb buffer to store temporary
> userRecords, as MemoryMappedFiles aren't using the JVM memory.

the current implementation is flexible to work with any underlying
file system rather than being tied to a single implementation.
Currently it is a random access file system. But memory mapped file
implementation should work as well. I am also thinking of using HDFS
files in the future.A couple of notes about some concerns:

*I dont see 4MB being a big concern. This size can be tuned. It can
also be made zero if the underlying implementation is good to deal
with writes.
*I dont think a dedicated background thread is a problem either. If we
want, we can make user threads to do the log sync as they log the
records.

Right now this is very low priority given the things we need to
implement to get the txns to work.

>
> The only modification would be to check that the file is not full, and if
> so, we will just have to create a new one.
>
> Concurrent access to the file are also protected by the MemoryMappedFile, we
> don't need to do anything but to declare the kind of access we want on the
> portion of the file we wanyt to access.
>
> thoughts ?
>
> --
> Regards,
> Cordialement,
> Emmanuel L├ęcharny
> www.iktek.com
>

thanks
Selcuk

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