[Nottingham] LVM + software RAID

[Peter Chang]

Martin,

> OK, so I was convinced to use LVM on top of software RAID.
>
> Any gotchas for such a setup?

Apart from what's been mentioned recently with certain filesystems, 
I don't think there's any gotchas.

> For performance, I'll be hoping for near x2 read speed for a simple RAID
> mirror on SATA II HDDs.
>
> But what happens for _write_ ?
> How clever is the software RAID for performance and recovery?

You're doing RAID-1 so that's anywhere between 0 to 100% (with two disks) 
improvement for reads but nothing for writes (or worse as md has to wait 
for the writes to finish on all the devices). The md driver attempts to 
distribute reads across devices.

Read errors may be recovered automatically if the blocks can be 
overwritten. Otherwise to recover, you bring the system down, replace the 
failed disk, boot to init level 1, replicate/rsync your data, remount and 
telinit to whatever. (Note md can work with RAID-1 on one partition.)

Read the md and mdadm man pages for more details.

> And what does the motherboard RAID hardware offer?

Propertiary striping formats, probably zero performance enhancement.

> Am I best staying with the Linux software RAID?

Yep, the disks will then be portable to other systems if you have to swap 
them out (for example, if your MB fails).

hth
  Peter


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