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Re: [zfs-discuss] Direct I/O ability with zfs?

Subject: Re: [zfs-discuss] Direct I/O ability with zfs?
From: "Rayson Ho"
Date: Wed, 3 Oct 2007 13:34:05 -0400
On 10/3/07, Roch - PAE <Roch.Bourbonnais@xxxxxxx> wrote:
> We do not retain 2 copies of the same data.
>
> If the DB cache is made large enough to consume most of memory,
> the ZFS copy will quickly be evicted to stage other I/Os on
> their way to the DB cache.
>
> What problem does that pose ?

Hi Roch,

1) The memory copy operations are expensive... I think the following
is a good intro to this problem:

"Copying data in memory can be a serious bottleneck in DBMS software
today. This fact is often a surprise to database students, who assume
that main-memory operations are "free" compared to disk I/O. But in
practice, a welltuned database installation is typically not
I/O-bound."  (section 3.2)

http://mitpress.mit.edu/books/chapters/0262693143chapm2.pdf

(Ch 2: Anatomy of a Database System, Readings in Database Systems, 4th Ed)


2) If you look at the TPC-C disclosure reports, you will see vendors
using thousands of disks for the top 10 systems. With that many disks
working in parallel, the I/O latencies are not as big as of a problem
as systems with fewer disks.


3) Also interesting is Concurrent I/O, which was introduced in AIX 5.2:

"Improving Database Performance With AIX Concurrent I/O"
http://www-03.ibm.com/systems/p/os/aix/whitepapers/db_perf_aix.html

"Improve database performance on file system containers in IBM DB2 UDB
V8.2 using Concurrent I/O on AIX"
http://www-128.ibm.com/developerworks/db2/library/techarticle/dm-0408lee/

Rayson



>
> -r
>
>  >
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>  > - Ryan
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