Software raid rules!

jason andrade jason at dstc.edu.au
Thu Apr 18 05:11:00 CDT 2002


On Thu, 18 Apr 2002, Sigbjorn Strommen wrote:

> In other words, the same raid config as before, but with software
> raid instead of hardware raid.

> The performance is so much better I can hardly believe it!

hi Sigbjorn,

i guess i wouldn't be too suprised by this.  i am assuming
that the raw speed of the OS on the 1650 with the copious
amount of cpu available far exceeds the cpu cycles available
on the raid controller.

this wouldn't be too suprising (e.g the clariion 4700 uses
two P3/733 cpus and NT4 embedded kernel to provide the high
speed controller for the fibrechanell stuff).

however, some things to consider - 

is your machine going to be lightly loaded ?  your raid ops will
be competing with application ops for the asme cpu cycles, so
observed behaviour might be "database access is heavy, disk i/o
drops - starves the database of i/o".  or the alternate, there
is a large amount of i/o, so this reduces cpu cycles available
to the database.  

how does your testing work for when you have a disk failure ?  i
have no doubts it should work in degraded mode, but have you tested
the i/o there, since that should be using a lot more cpu as it has
to xor the data from the disks to recreate it.   have you also
tested how the system behaves on a reboot ?  if you have no hardware
raid at all, you have a disk failure on the OS disk and you have
a panic/reboot, will it automatically boot from the failover disk
in software raid ?

> For the fun of it I did a stress test, running 8 bonnie sessions in
> parallel (1 session for each of the partitions on all the raid 5 volumes),
> and even then I get better block read speed for each process then
> single runs with hw raid...

what size of bonnie sessions are you doing ?  i am slightly sceptical
of those speeds (they are amazing) just in case the machine has a lot
of ram and these are being cached and read out of ram.  

also, i would do a lot more intensive write tests, as software raid
has historically been much worse on writes than reads.

> Another big advantage is that deactivating the aacraid card (using
> it in scsi mode) - kernel upgrades runs much more smoothly, no more
> struggling to get the system partitions recognized at boot time.

is this an issue ? i thought it was just a matter of passing the correct
arguments to the kernel at boot and it should detect ok.

> So...finally it seems I can put this server in full production. (I only
> hope that it also will be stable with sw raid, and with this kernel!)

best of luck with the software raid - it's very interesting to see other
people's experiences with this.  i will be sticking with hardware raid
though, as i have a higher comfort factor with it (and also, the aacraid
only has to have enough performance to keep up with the OS - we do all
the serious i/o using fibrechannel cards/raid servers/disks)

regards,

-jason




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