[nycbug-talk] FreeBSD software RAID?

Miles Nordin carton at Ivy.NET
Thu Mar 8 17:47:50 EST 2007

>>>>> "pw" == Peter Wright <pete at nomadlogic.org> writes:

    pw> well that's why any hardware raid controller worth purchasing
    pw> supports BBU's and a write-back cache.

IIRC most Dell PERC are sold without

    pw>   otherwise you would need to use synchronous writes when
    pw> mounting the filesystem, which for some people may be
    pw> acceptable.

I disagree.

The NVRAM may allow you to turn on -o sync without losing as much
performance w.r.t. softdep as you would with a real disk.

but if you use -o softdep like a normal mount, (if it weren't for the
problems of NVRAMless RAID5) you would have the same level of
integrity protection with or without the BBU---that level which is
defined by softdep.

And on a non-NVRAM system, mounting the filesystem '-o sync' will
*not* help with the RAID5 write hole.  only NVRAM will help that.

    pw> this is an important distinction: if a RAID implementation is
    pw> tied to the OS, then any interruption to the OS increases risk
    pw> a data corruption.  by offloading this to a ASIC with a BBU
    pw> one mitigates this risk by allowing data in caches to be
    pw> sync'd to disk regardless of the sate of the OS.

but softdep does provide integrity guarantees on power loss.  and
using RAID3 instead of RAID5 closes the RAID5 write hole by always
doing full stripe writes (ZFS does the same thing).

    pw> i guess i'm just not sure what you mean by "RAID-on-a-card".

I mean so-called ``hardware RAID'' that's implemented by a tiny
computer on a PCI card or a motherboard.  This is what most people
seem to run to after they use software RAID5 and lose a bunch of data,
or get tired of waiting for mirror rebuilds with software RAID1.  and
it is IMHO junk.

    pw> i still don't see your point,

If you smoke your RAID card, and it's an old RAID card, and you can't
get another one, you can lose the contents whole array.  Sometimes,
even if you can get another one, some quirk of the multiple
BIOS/DOS/Windows configuraturs they give you makes it impossible to
get the old array working with your new card.  I've heard multiple
stories like this from friends, and the Interweb is full of them as
well with headings like ``Dell PERC OH GOD NO''.

Software RAID doesn't have this problem.  (nor does SAN under a
service contract.)  It has other problems, but not this one.
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