[BBLISA] Moving from RAID 0 to LVM RAID?

[Theo Van Dinter]

On Fri, Mar 07, 2008 at 08:29:51AM -0500, Edward Ned Harvey wrote:
> The three reasons I steer clear of software raid are:
> 
> #1  If a disk goes bad, I expect hotswappable drives, with a red blinking
> light.

Why would this require HW RAID?

> #3  Suppose you find a way to enable the above mentioned write caching in
> software kernel.  Sometimes the kernel is then telling applications data's
> written that's not yet written.  When a kernel crashes (rare but nonzero)
> there is more corruption than there would otherwise have been, if the cache
> was controlled by hardware which continues to write-out the contents of
> cache.  I don't know realistically how important this feature is, but it has
> only one potential; it must be equal or better than software write caching.

This is pretty BS.  Yes, if you cache writes to RAM, you potentially lose
data, which is why no one does this. ;)

It's also why you don't want to enable write caching on most HW RAID
controllers -- one power outage and oops, corrupt array.  The 3ware folks,
at least, document this, BTW.

The good HW RAID controllers, and proper implementations of SW RAID, use
something like NVRAM to solve this issue.  Power outage, kernel panic,
whatever, the log of writes/requests/etc is stored in a battery-backed
area so that at the next boot the system can replay the log and your
array/filesystems/etc are safe -- writes only get acked once the
information is stored in NVRAM.  (the newer 3ware cards, BTW, have a
battery backup unit (BBU) which you can attach to the card, thereby
making the write cache RAM safe to use)


SW RAID gives you some benefits you don't get in most HW RAID setups, BTW,
such as handling data integrity issues, and specifically data corruption
on/from disk.  Implementations from NetApp, Sun (ZFS), etc, do SW RAID w/
block checksums, which let you detect when a block on disk is not what
it should be.  Since it's part of a RAID setup, it lets you attempt to
correct the data otherwise throwing an error instead of just returning
invalid data.

-- 
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