Re: Convert ext4 lvm to normal ext4 partition

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On Saturday, November 13, 2010 08:59:48 pm Dean S. Messing wrote:
> Regarding your disk speed tests with hdparm,
> you may want to look at the "--direct" switch.

Oh, I like those numbers:
[[email protected] ~]# hdparm -t --direct /dev/sdb3

 Timing O_DIRECT disk reads:  2054 MB in  3.02 seconds = 679.13 MB/sec
[[email protected] ~]# hdparm -t --direct /dev/bench2/7g

 Timing O_DIRECT disk reads:  2052 MB in  3.00 seconds = 683.36 MB/sec
[[email protected] ~]#

There's hardware cache (about 4GB) involved in the array controller that I'm just not able to bypass.  Although I'm going to admit that those numbers seem fishy... 

And I do know why they look fishy.  There's another cache involved.  But I can't reproduce with /dev/sda on that box, and /dev/sda is on the same array, it's on the same HBA, and, while it's on a different LUN, it's on the same RAID group on the array:
[[email protected] ~]# hdparm -t --direct /dev/sda

 Timing O_DIRECT disk reads:  994 MB in  3.00 seconds = 331.18 MB/sec
[[email protected] ~]#

Which is more in line with 4G/s FC performance.  I'll have to dig into that anomaly.

Now, on my development F14 box hooked up to the same array, just a different LUN, and a 32-bit machine (dual Xeon 2.8, 4GB RAM), and using a 2G/s FC HBA (which will impact, and probably bottleneck, performance):
[[email protected] ~]# hdparm -t --direct /dev/sdw

 Timing O_DIRECT disk reads:  534 MB in  3.01 seconds = 177.46 MB/sec
[[email protected] ~]#

On another CentOS 5 box (dual 3.4GHz Xeon's, 2GB RAM, x86_64), hooked up to a different array with 4G FC:
[[email protected] ~]# hdparm -t --direct /dev/sdad

 Timing O_DIRECT disk reads:  864 MB in  3.00 seconds = 287.87 MB/sec
[[email protected] ~]#

(yeah, /dev/sdad, that's not a typo)
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