Compaq Smart Array 3200 Controller as a SCSI Controller

[Peter Corlett]

On Wed, Jul 15, 2020 at 01:09:21PM -0700, Ali via cctalk wrote:
[Hardware RAID controllers]
>> There is no good use case for them in 2020, which is why they're all
>> suddenly quite cheap.
> Why do you say that? Not disagreeing per se but just wondering the reasoning
> behind it.

On the "no good use case" front:

I avoid hardware RAID controllers for a variety of reasons, which mostly boil
down to the use of proprietary firmware and naïve RAID implementations. These
also apply to many software RAID implementations which blindly copied them.

The biggie is that proprietary RAID means proprietary on-disk formats. If the
controller fails, you need to find a replacement which understands the old
on-disk format. Good luck with that. Related is the generally shoddy nature of
firmware, and it's usually hard-to-impossible to e.g. query the SMART status of
individual disks.

The next-biggie is the RAID Write Hole. A traditional RAID implementation will
rewrite data you might consider to be at rest because it shares a stripe with
newly-written data, and on failure can corrupt said at-rest data. This is a
fundamental problem which hardware RAID controllers try to mitigate by having a
battery backup unit to deal with power failures, and can (potentially) also
work independently of an OS which crashed mid-write, but it doesn't really
solve it. What if your power stays out longer than the battery lasts?

Software RAID which implements traditional RAID cannot even apply this
mitigation and this is one of the reasons it has a bad reputation. The obvious
solution is to not implement tradional RAID, which is where ZFS and similar
copy-on-write journalled filesystem-cum-volume-managers come in.

The last bastion of hardware RAID controllers was if one was using a toy
operating system such as Windows where the software RAID options were woeful or
nonexistent, but it now has Storage Spaces.

On the "suddenly quite cheap" front: Plain SAS controllers based on e.g. the
LSI 9207 or 9211 are north of €100, whereas MegaRAID controllers based on the
LSI 9260 have plummeted to €30. The former either supports pass-through mode
out of the box or after reflashing with "IT" firmware, but the latter does not.

ZFS does work atop RAID (of either flavour), but is more robust if it can
manage the raw disks directly. A workaround with hardware RAID cards which
won't do pass-through is to configure them with single-disk RAID0 volumes, but
this is somewhat untidy and still has the problem of proprietary on-disk
formats and general inscrutability.