Rayson Ho wrote: > On 10/3/07, Roch - PAE <[EMAIL PROTECTED]> 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)
... just the ones people are complaining about ;-) Indeed it seems rare that a DB performance escalation does not involve I/O tuning :-( > 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 This is a pretty decent paper and some of the issues are the same with UFS. To wit, direct I/O is not always a win (qv. Bob Sneed's blog) It also describes what we call the single writer lock problem, which IBM solves with Concurrent I/O. See also: http://www.solarisinternals.com/wiki/index.php/Direct_I/O ZFS doesn't have the single writer lock problem. See also: http://blogs.sun.com/roch/entry/zfs_to_ufs_performance_comparison Slightly off-topic, in looking at some field data this morning (looking for something completely unrelated) I notice that the use of directio on UFS is declining over time. I'm not sure what that means... hopefully not more performance escalations... -- richard _______________________________________________ zfs-discuss mailing list zfs-discuss@opensolaris.org http://mail.opensolaris.org/mailman/listinfo/zfs-discuss
