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RE: [linux-lvm] lvm over raid5 - sizes



Well, I have a similar problem/desire.

I want to have RAID5 for redundancy/reliability.  But I want to be able to
resize the file system stored on the RAID5 array, as well as being able to
add and remove hard disks to either increase/decrease available storage.

What would be your recommendation for a configuration?

At the moment, I'm considering using EVMS with its RAID5 regions, and put
LVM2 on top of it.

Thanks in advance,
    Erik.

> -----Original Message-----
> From: linux-lvm-bounces redhat com 
> [mailto:linux-lvm-bounces redhat com] On Behalf Of Sam Vilain
> Sent: Monday, February 28, 2005 12:11 AM
> To: LVM general discussion and development
> Subject: Re: [linux-lvm] lvm over raid5 - sizes
> 
> 
> Scott Serr wrote:
> > I have alot of ideas going through my head.  Here is my situation:  
> >  
> > I've had 5 IDEs with 4 partitions each.  Each partition is 
> in a RAID5  
> > md:stripped.  
> > md0 = hda5, hdb5, hdc5, hdd5, hde5  
> > md1 = hda6, hdb6, hdc6, hdd6, hde6  
> > md2 = hda7, hdb7, hdc7, hdd7  
> > md3 = hda8, hdb8, hdc8, hdd8  
> >  
> > md0-3 are in an LVM2 volume group.  This makes it really nice when  
> > expanding or adding another disk because I just pull say 
> md3 out of the  
> > volume group... rebuild md3 bigger and then add it back.  
> > I just upgraded, replacing hde with a bigger disk and while 
> the pvmoves  
> > are taking FOREVER...  I thought what is the right size for 
> the RAID5  
> > chunk-size?  
> >  
> > I have chunk-size = 128k right now.  
> >  
> > The pv extents size is (always?) 4MB.  
> >  
> > Am I just shooting myself in the foot by not making my RAID 
> chunk size  
> > atleast 1MB?  Can someone give me a reason to increase it?
> 
> 
> You're shooting yourself in the foot with RAID 5 and a crazy 
> setup like this in the first place.  I sure hope this isn't 
> for a production box :)
> 
> Just get more disks and use RAID 1.  It's faster, simpler, 
> and more robust.  You can still do all of the tricks that 
> you're looking for.
> 
> Hey, maybe one of the disks has failed.  Standard RAID 5 
> behaviour as designed in this case is to slow to a crawl.  
> Alternatively maybe you're just experiencing what is known in 
> the 'biz as "The Butterfly Effect".
> 
> With most striping, keep the chunk size as large as possible. 
>  It's a real waste to spend 10ms waiting for the disk platter 
> to rotate, and then only read 128kB of data.  Set it as high 
> as possible.
> -- 
> Sam Vilain, sam /\T vilain |><>T net, PGP key ID: 0x05B52F13 
> (include my PGP key ID in personal replies to avoid spam filtering)
> 
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