On Jul 6, 2008, at 4:58 AM, Matthew Seaman wrote:

John Almberg wrote:
Luckily, I have a pretty powerful machine sitting right next to my main webserver that I mainly use for backup. The two servers are directly connected to each other with a twisted ethernet cable, using extra NIC cards in the machines, so they have a fast, dedicated 'LAN' to share. Desperate, I moved mysqld to this other machine, so basically this second machine became a dedicated database server. The improvement this change made seems out of proportion. Both machines are now cruising with extremely low load averages and the WCPU for the mysqld instance on the new machine is practically zero. I'm not complaining. Problem solved. But I am scratching my head over how mysql could be getting crushed on the first, 8 core/8G machine, but running cool as a cucumber on the second, 2-core/2G machine???

Database performance is critically dependant on the underlying Disk IO sub-

There's two things to consider here:

 * What's the difference in disk hardware between the two machines?

 * How well does the database compete with apache for disk IO when
   they are sharing the same drives?

In the first case, you'll see very big improvements in MySQL performance if you use disks with plenty of IO cache memory on them, plus disks that work with controllers that support tagged queuing (SAS and higher end SATA
generally.)  DB disk IO consists primarily of a large number of small
transactions randomly distributed across the drive. Reducing the latency of each of those transactions gets you the best improvement in performance. RAID levels are important: single disks are not bad if dedicated to the DB. RAID1 mirrors are fine. RAID10 striped mirrors are excellent. RAID5 not so good. If you're willing to spend money, hardware RAID controllers help. Hardware RAID controllers with Battery Backup Units (so you can safely turn
on write caching) help a very great deal.

The first box has 2 independent raid arrays, with an Intel hardware raid controller with battery backup. But apache and mysql were probably on the same array. I have to confirm this.

The second box has 2 drives with raid 1, from an integrated controller (i.e, software raid).

So the second box is much less powerful, but running mysqld is all it's doing. I guess that is the primary difference.

My main worry about the second box is that it only has 1G of RAM (not 2 as I mis-remembered before checking specs.) Easily expanded, but it's something to watch.

Bottom line, I might do better by moving mysqld back to the first machine, but putting the data on the second, raid 10 array (which is more or less empty at the moment.)

I appreciate your insight. This is starting to make sense to me.

Brgds: JOhn

Websites for On-line Collectible Dealers
Identry, LLC
John Almberg
(631) 546-5079

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