Le 3/16/12 7:07 PM, Selcuk AYA a écrit :
On Fri, Mar 16, 2012 at 2:26 AM, Emmanuel Lécharny<elecha...@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.
MemoryMappedFile in Java are not OS dependent. It's a feature that has
been added in Java 4, and it works on top of any FS.
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.
Sure ! Just wanted to mention that, AFAIU, the current log with the
buffer could probably be switched to a MMF with some extra benefits in
the future.
There are some area that need to be checked, like the file locks and how
we should manage them.
I'll create a JIRA mentionning that this is an area which may need to be
investigated later, as it's not urgent.
We discussed a lot with Alex about using MMF instead of other kind of
file IO especially if we are to rewrite JDBM at some point.
One more question, relative to the LogBuffer : I see it's flushed in two
cases :
- when the buffer is full (obviously !)
- if a thread is calling the Log.sync() method.
My understanding is that there will be a thread somewhere which will
call this Log.sync() method periodically, right ?
One last thing : we will need to expose the configurable size so that it
can be changed. ALl our configuration is stored in the cn=config branch,
I will try to draft something on the wiki to gather all the configurable
elements plus the suggested AT/OC needed to manipulate those elements.
I'll be mostly offline this week-end, because I don't have any credit on
my 3G phone, and also forgot to bring my phone charger. I'll be back on
monday afternoon.
Have a good week-end !
--
Regards,
Cordialement,
Emmanuel Lécharny
www.iktek.com
