Striping and Logical Volumes

[Jeremy Higdon]

In article <9101220711.AA12984 at crow.omni.co>, rpaul at crow.UUCP (Rodian Paul) writes:
> Perhaps someone can explain the following to me... 
> 
> I created a logical volume consisting of 3 * 1.2GB IPI2 drives, with a 
> stripe of 3.
> 
> Once the sucker's created (using partition 7 on each drive, i.e the whole
> drive, all partitions the same size) I get the following output from 'df':
> 
> (all figures in 512K blocks)
> 
> Filesystem                 Type  blocks     use   avail %use  Mounted on
> /dev/dsk/lv0                efs 4901760  669740 4232020  14%  /prod
> 
> Now if I take a look at a 1.2GB SMD drive (all I've got for reference but I
> assume a formatted SMD is close to a formatted IPI) I get the following
> amount of blocks on an efs file system: 1921920 blocks
>            Mutliply that * 3 and I get: 5765760 blocks
>      Subtract the difference of the lv: 4901760 blocks
>                              and I get:  864000 blocks
> 
> Or approx 400MB difference. Now I assume that there are a whole lot of
> tables created with a logical volume, even more if striped. I don't
> recall reading anything about how much space these tables will take up
> in the manuals (allthough a comment may in fact be there). So, could
> someone perhaps explain to me what kind of size differences to expect with 
> logical volumes and if striping uses more space.

Logical volumes do not take up any additional space.
The IPI disk drives have a useable capacity of 858 MB.  The SMD drives have
a useable capacity of 1033 MB (useable capacity does not include spare
sectors and tracks).  Thus, your numbers look about right.

> 
> If striping does in fact take up more space (I can't test this out at the 
> moment as the machine must stay up) can I expect more throughput? The manual 
> section I read said that ideally, you should arrange to connect the disks
> you are striping accross to be on different controllers. We're only using 
> 1 (Xylogics 7800 16-drive IPI disk controller 0: Firmware version 2.2.4)
> for 4 IPI2's. The machine is a 380s with 256MB RAM and doesn't have any 
> other drives on it as yet.
> 
> Can I expect much of a difference in performance if I remove the stripe?
> 
> Any comments will be greatly appreciated.

Actually, on IPI, you want to have different members of the stripe group
on different ports on the controller.  Since you have three drives in the
group, that probably means that two are on one port, and one is on the
other.  You would probably get the best performance if you made a two-way
stripe with each disk on a separate port of the controller.

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		Jeremy Higdon

SGI's opinions and mine might coincide.