Rayson Ho writes:
> 1) Modern DBMSs cache database pages in their own buffer pool because
> it is less expensive than to access data from the OS. (IIRC, MySQL's
> MyISAM is the only one that relies on the FS cache, but a lot of MySQL
> sites use INNODB which has its own buffer pool)
>
The DB can and should cache data whether or not directio is used.
> 2) Also, direct I/O is faster because it avoid double buffering.
>
A piece of data can be in one buffer, 2 buffers, 3
buffers. That says nothing about performance. More below.
So I guess you mean DIO is faster because it avoids the
extra copy: dma straight to user buffer rather than DMA to
kernel buffer then copy to user buffer. If an I/O is 5ms an
8K copy is about 10 usec. Is avoiding the copy really the
most urgent thing to work on ?
> Rayson
>
>
>
>
> On 10/2/07, eric kustarz <[EMAIL PROTECTED]> wrote:
> > Not yet, see:
> > 6429855 Need way to tell ZFS that caching is a lost cause
> >
> > Is there a specific reason why you need to do the caching at the DB
> > level instead of the file system? I'm really curious as i've got
> > conflicting data on why people do this. If i get more data on real
> > reasons on why we shouldn't cache at the file system, then this could
> > get bumped up in my priority queue.
> >
I can't answer this although can well imagine that the DB is
the most efficent place to cache it's own data all organised
and formatted to respond to queries.
But once the DB has signified to the FS that it doesn't
require the FS to cache data then the benefit from this RFE
is that the memory used to stage the data can be quickly
recycled by ZFS for subsequent operations. It means ZFS
memory footprint is more likely to contain useful ZFS
metadata and not cached data block we know are not likely to
be used again anytime soon.
We also would operated better in mixed DIO/non-DIO workloads.
See also:
http://blogs.sun.com/roch/entry/zfs_and_directio
-r
> > eric
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> > http://mail.opensolaris.org/mailman/listinfo/zfs-discuss
> >
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