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Old 11-20-2007, 01:39 AM   #1
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Disk IO Performance Drops Off

We have recently built a new computer that works greate except for one small thing, when writing disk to the data the amount of RAM used counts up and the Disk IO Performance Drops down ot a crawl, less then 1Meg a second. The computer is 2 quad core xeons with 12 gigs of ram. The drives are sata II with a MegaRAID® SATA (Serial ATA) 300-8XLP. The card has 8 500g drives attatched to it. The volumes are something like /boot 120m / 20g /opt 20g /vol1 1.7tb /vol2 1.1tb. The raid controller is configured to handle the drives with raid 5.

My question is does anyone know why when the system tries to write to the disks the performance gets slower and slower until it crawls when the system runs out of ram, why is the ram usage going way up really fast when data is being written. The used ram is never really released and disk io performance only goes up slightly, after a while, after the writes are complete. The read performance of the volumes does not seem to be effected.

Any help or pointers would be greatly appreciated.

Old 11-20-2007, 10:13 AM   #2
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Welcome to LQ.

The first question I'd have is what distro are you using and which kernel? That would be helpful to know. In any case, your system is far beyond anything I've used myself, but if you are seeing poor disk performance it potentially could be because DMA has not been enabled. This article gives a good overview.

With respect to memory "never releasing", that's not a problem, and it's how Linux manages memory


Edit: D'oh, hdparm is for IDE drives, but sdparm is the equivalent for SATA

Last edited by J.W.; 11-20-2007 at 10:18 AM.


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