[CentOS] Centos 5 on Large Disks

Morten Torstensen morten at mortent.org
Wed Oct 24 18:26:18 UTC 2007

Anup Shukla wrote:
> Format and run the dd command again.
> The speed is 130MB/s now.

It can vary a quite a bit depending on where you hit the disk. Remember, 
what you are testing is just how fast dd can read from /dev/zero and 
write to the file in a filesystem with 1k blocks. How that will map to 
real performance is another matter.

> Its a bit confusing.
> Does LVM slow down things? Or i did something that is not really of any 
> relevance to check IO speed.

LVM adds very little overhead. The file and placement on disk can have 
more to do with raw I/O bandwidth than anything else in this particular 

> I used mkfs.ext3 -m0 -E stride=96 -O dir_index /dev/sdb1 ...
> I have a RAID5 volume consisting of 6 disk with stripe size = 64k
> I hope the stride=96 is optimal.

Depends what you want... With 4k blocks in ext3 (default) a stride of 16 
(16 times 4 is 64) would read one stripe from a disk. With 6 disk 16 
times 6 is 96, so for every I/O you will hit each disk once and read one 
stripe. Ditto for writes. In general that is a good start.

> Should i stick with LVM, or go back to the older way?

I would never really consider not using LVM. The flexibility it adds for 
disk management is essential for managing your disks.


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