[lug] SW Raid question

[Sean Reifschneider]

On Mon, Jul 23, 2007 at 11:48:12PM -0600, dio2002 at indra.com wrote:
>however, i've heard they're slower than sw raid and prone to boot failures

You've heard wrong about performance.  For RAID-1 this is almost certainly
untrue.  In fact, the 3ware hardware RAID cards have worse performance in my
experience, unless you enable the most aggressive caching.  But, then you
get a nice warning about how it will eat your data, even with a battery
backup unit.

Using software RAID, because it is tightly coupled to the kernel, as far as
I know it interacts with the buffer cache properly and when the cache tells
it to sync data to disc before removing data from the journal, it does.  No
need to worry about a dumber controller faking the response and then never
getting it to disc because of a power-cycle or reset or the like.

It's really much more difficult than "hardware RAID is faster than
software".

The boot failure issue may be present on software RAID.  Older Red Hat
installs with LILO got this right, but newer ones require you to manually
go in and install the boot sector on all drives in the array.

I will say that while I've long been a proponent of software RAID, I'm even
more so right now.  Around a month ago the hardware RAID array behind
mirrors.tummy.com caused massive corruption of the file-system.  Do you
know how long it takes to archive several TB of data (particularly when the
file-system is now showing most of the small files as hundreds of megabytes
to several gigabytes), restoring them, and then re-syncing the files that
were wedged?  Around 3 weeks in this case...

>2) I can disable bios raid and go with sw raid like initially intended

2.5) Get another cheap SATA non-RAID card and replace the on-board with
this.

I presume you're talking about the 5015M-MT+ system?  I don't have
experience with this, but our other Supermicro systems seem to perform just
fine on the discs.  Though, ISTR that they are limited to 100-ish MB/sec
which makes me think they must be on a slower PCI bus.  I'll be honest
though, I haven't really farted around in the BIOS with the disc settings,
mostly I use the stock settings.  So I can't provide additional details on
the options there.

Sean
-- 
 Electricity travels a foot in a nanosecond.
                 -- Commodore Grace Murray Hopper
Sean Reifschneider, Member of Technical Staff <jafo at tummy.com>
tummy.com, ltd. - Linux Consulting since 1995: Ask me about High Availability