Redundant NFS storage setup (part 3) : ThedisappointingPERC5/E(solved?)

[Harald_Jensas at Dell.com]

> -----Original Message-----
> From: linux-poweredge-bounces at dell.com [mailto:linux-poweredge-
> bounces at dell.com] On Behalf Of Matthias Saou
> Sent: 04 January 2008 11:42
> To: linux-poweredge-Lists
> Subject: Re: Redundant NFS storage setup (part 3) :
> ThedisappointingPERC5/E(solved?)
> 
> Harald_Jensas at dell.com wrote :
> 
> > Kevin, Did you ever try to create your Hardware RAID striped
> partitions aligned?
> > This will cause less stripe crossings and thus less parity to
> calculate for RAID 5 writes. It should also improve read performance.
> >
> > In this article they report up to 30% performance increase, in their
> tests, with properly aligned partitions.
> > http://insights.oetiker.ch/linux/raidoptimization.html
> >
> > Assuming block size is 512 bytes.
> > If Stripe Size is (64KB/512 byte = 128 Blocks) align the partition
to
> block 128.
> > If Stripe Size is (128KB/512 byte = 256 Blocks) align the partition
> to block 256.
> >
> > 1. Enter fdisk /dev/sd<x> where <x> is the device suffix.
> > 2. Determine if any partitions already exist.
> > 3. Type n to create a new partition.
> > 4. Type p to create a primary partition.
> > 5. Type 1 to create partition No. 1.
> > 6. Select the defaults to use the complete disk.
> > 7. Type t to set the partition's system ID.
> > 8. Type in the code for the partition type you want.
> > 9. Type x to go into expert mode.
> > 10. Type b to adjust the starting block number.
> > 11. Type 1 to choose partition 1.
> > 12. Type 128 to set it to 128 (the array's stripe element size).
> > 13. Type w to write label and partition information to disk.
> 
> This is quite interesting, but I'm a little confused as to how to
> achieve this when using a gpt partition table.
> 
> Here's what parted shows me :
> 
> Model: DELL PERC 5/E Adapter (scsi)
> Disk /dev/sdb: 13.0TB
> Sector size (logical/physical): 512B/512B
> Partition Table: gpt
> 
> Number  Start   End     Size    File system  Name  Flags
>  1      17.4kB  13.0TB  13.0TB  xfs          MD1
> 
> The partition was created with "mkpart MD1 0 100%", and I don't see
how
> to access any "expert" features with parted. Here's what fdisk shows
> me (FWIW, since it doesn't support gpt) :
> 
> Disk /dev/sdb: 12995.4 GB, 12995497295872 bytes
> 255 heads, 63 sectors/track, 1579945 cylinders
> Units = cylinders of 16065 * 512 = 8225280 bytes
> 
>    Device Boot      Start         End      Blocks   Id  System
> /dev/sdb1               1      267350  2147483647+  ee  EFI GPT
> 
> As you can see, I have the "63 sectors/track" value which the article
> considers sub-optimal. Is there any way to change this?
> 
> Another thing I'll try is to use LVM on the block device directly,
> basically replacing the partition, an see how that performs.
> 
> Matthias
> 
> --

It is not the 63 sectors/track value that is sub-optimal, it is the fact
that the partition is normally aligned based on the 63 sector/track
value. Back in the day the ## sectors/track value actually told you
something about the physical layout of the disk. Thus back in the day it
made sense to align based on the sectors/track value. Now days it is all
hidden from you. It is better to think of the disk as a sequence of
blocks. And in a striped RAID those blocks are striped over several
drives. Thus we want to align striped RAID partition so that a single
I/O operation not spread across multiple stripes.


AFAIK GPT partitions do not have this problem.


--
Harald