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High iowait -- always caused by slow disks?
| Posted by mrzippy, 09-25-2012, 07:12 AM |
| Hello,
I am seeing very high iowait on one of our servers:
CPU is a Xeon E31270 @ 3.40GHz (8 cores) with 8gb of RAM.
There are 2 x SATA II disks in a RAID 1 configuration.
Normal load is about 1.5 or so, but it goes very high like this for about 2 hours every morning, causing all sites on the server to either crawl or timeout...
Any suggestions?
(We've already tried looking into cron to see if there are any high-disk-usage processes running during this time. There are no backups, maldet, or anything obvious being run.
When I use "top", it just shows php and mysql processes.
Would putting in a better RAID system help?
Thanks for any help.
Last edited by mrzippy; 09-25-2012 at 07:15 AM.
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| Posted by ovais, 09-25-2012, 07:20 AM |
| The better raid would definitely help but the best would be to find out what is causing high io. it could be a bad script hammering MySQL or any other thing.
Install iotop and run when having high load to see individual applications io stat.
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| Posted by mrzippy, 09-25-2012, 07:23 AM |
| iostat just shows lots of php and mysql processes.. all with 99% iowait.
There doesn't seem to be any obvious process or reason for the high load.
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| Posted by ovais, 09-25-2012, 07:28 AM |
| Can you paste IOTOP result?
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| Posted by mrzippy, 09-25-2012, 07:38 AM |
| Next time the problem occurs I will paste it. Load just dropped down to normal 1.x range with no iowait.
It seems this problem occurs for about 1.5 hours every day around the same time...
It's really odd because we've basically removed every possible cron job that could be causing it.
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| Posted by PCS-Chris, 09-25-2012, 07:44 AM |
| If its primarily php and MySQL there are a few things you can look at
Are you using any php caching e.g. APC / eaccelerator?
How much memory have you give MySQL to cache table data?
The more you can get in memory, the less strain there will be on your disks.
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| Posted by BestServerSupport, 09-25-2012, 08:11 AM |
| Please check your mail queue, Apache, MySQL logs at the time when you find high i/o wait on your server.
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| Posted by funkywizard, 09-26-2012, 01:24 AM |
| Sounds like a backup is running that is using up all your disk i/o. A faster drive setup would likely cause the same problem but for a shorter period of time each day : P
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| Posted by mrzippy, 09-26-2012, 04:54 AM |
| Nope. We've scheduled backups to occur at a different time. So the load problem is definately not from backups.
BTW - The cpanel backup process takes about 3 hours to run on this certain server, and load never goes higher then 2.5 when it runs with iowait of about 25%.
I will post output from iotop the next time the load problem happens...
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| Posted by WebHostDog, 09-28-2012, 03:09 PM |
| Seems like attack, check the Apache status.
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