[CentOS] Disk Elevator

Mon Jan 8 18:15:17 UTC 2007
Ross S. W. Walker <rwalker at medallion.com>

> -----Original Message-----
> From: centos-bounces at centos.org 
> [mailto:centos-bounces at centos.org] On Behalf Of Aleksandar Milivojevic
> Sent: Monday, January 08, 2007 1:00 PM
> To: centos at centos.org
> Subject: RE: [CentOS] Disk Elevator
> 
> Quoting "Ross S. W. Walker" <rwalker at medallion.com>:
> 
> > The biggest performance gain you can achieve on a raid 
> array is to make
> > sure you format the volume aligned to your raid stripe 
> size. For example
> > if you have a 4 drive raid 5 and it is using 64K chunks, your stripe
> > size will be 256K. Given a 4K filesystem block size you 
> would then have
> > a stride of 64 (256/4), so when you format your volume:
> >
> > Mke2fs -E stride=64 (other needed options -j for ext3, -N 
> <# of inodes>
> > for extended # of i-nodes, -O dir_index speeds up directory 
> searches for
> > large # of files) /dev/XXXX
> 
> Shouldn't the argument for stride option be how many file system  
> blocks there is per stripe?  After all, there's no way for OS 
> to guess  
> what RAID level you are using.  For 4 disk RAID5 with 64k chunks and  
> 4k file system blocks you have only 48 file system blocks per stripe  
> ((4-1)x64k/4k=48).  So it should be -E stride=48 in this particular  
> case.  If it was 4 disk RAID0 array, than it would be 64  
> (4x64k/4k=64).  If it was 4 disk RAID10 array, than it would be 32  
> ((4/2)*64k/4k=32).  Or at least that's the way I understood it by  
> reading the man page.

You are correct, leave one of the chunks off for the parity, so for 4
disk raid5 stride=48. I had just computed all 4 chunks as part of the
stride.

BTW that parity chunk still needs to be in memory to avoid the read on
it, no? In that case wouldn't a stride of 64 help in that case? And if
the stride leaves out the parity chunk then will not successive
read-aheads cause a continuous wrap of the stripe which will negate the
effect of the stride by not having the complete stripe cached?

-Ross

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