Michael Evans wrote:
On Wed, Apr 21, 2010 at 6:32 AM, Bill Davidsen <davidsen@xxxxxxx> wrote:
Michael Evans wrote:
On Sun, Apr 18, 2010 at 8:46 PM, Kaushal Shriyan
<kaushalshriyan@xxxxxxxxx> wrote:
Hi
I am a newbie to RAID. is strip size and block size same. How is it
calculated. is it 64Kb by default. what should be the strip size ?
I have referred to
http://en.wikipedia.org/wiki/Raid5#RAID_5_parity_handling. How is
parity handled in case of RAID 5.
Please explain me with an example.
Thanks and Regards,
Kaushal
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You already have one good resource.
I wrote this a while ago, and the preface may answer some questions
you have about the terminology used.
http://wiki.tldp.org/LVM-on-RAID
However the question you're asking is more or less borderline
off-topic for this mailing list. If the linked information is
insufficient I suggest using the Wikipedia article's links to learn
more.
I have some recent experience with this gained the hard way, by looking for
a problem rather than curiousity. My experience with LVM on RAID is that, at
least for RAID-5, write performance sucks. I created two partitions on each
of three drives, and two raid-5 arrays using those partitions. Same block
size, same tuning for stripe-cache, etc. I dropped an ext4 on on array, and
LVM on the other, put ext4 on the LVM drive, and copied 500GB to each. LVM
had a 50% performance penalty, took twice as long. Repeated with four drives
(all I could spare) and found that the speed right on an array was roughly
3x slower with LVM.
I did not look into it further, I know why the performance is bad, I don't
have the hardware to change things right now, so I live with it. When I get
back from a trip I will change that.
This issues sounds very likely to be write barrier related. Were you
using an external journal on a write-barrier honoring device?
Not at all, just taking 60G of free space of the drives, creating two
partitions (on 64 sector boundaries) and using them for raid-5. Tried
various chunk sizes, better for some things, not so much for others.
--
Bill Davidsen <davidsen@xxxxxxx>
"We can't solve today's problems by using the same thinking we
used in creating them." - Einstein
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