questions using zfs on raid controllers without jbod option

[Gergely CZUCZY]

On Thu, 3 Dec 2009 09:38:09 +0100
Kai Gallasch <gallasch at free.de> wrote:

> 
> Hi list.
Hello,

> 
> What's the best way to deploy zfs on a server with builtin raid
> controller and missing JBOD functionality?
> 
> I am currently testing a hp/compaq proliant server with Battery Backed
> SmartArray P400 controller (ciss) and 5 sas disks which I use for a
> raidz1 pool.
> 
> What I did was to create a raid0 array on the controller for each
> disk, with raid0 chunksize set to 32K (Those raid0 drives show up as
> da2-da6 in FreeBSD) and used them for a raidz1 pool.
> 
> Following zpool iostat I can see, that there are almost all of the
> time no continous writes, but most of the copied data is written in
> spikes of write operations. My guess is, that this behaviour is
> caching related and that it might be caused by zfs-arc and
> raid-controller cache not playing too well together.
I'm also having such a controller, and I have also created a raid0 for
each of my disks, so i've got 14 raid0s on the smartarray.

Thinking of it, the stripe size of a raid0 speaks for itself. It's the
size in which data is striped among drives. When you've got a single
drive in a raid0 the data is striped among that single drive. That is,
that data is continuous on the drive, no effective striping is done. In
my opinion this loses the point of this parameter of tha raid0 array.

> 
> questions:
> 
> "raid0 drives":
> 
> - What's the best chunksize for a single raid0 drive that is used as a
>   device for a pool ? (I use 32K)
I don't really think this makes any differences in this configuration,
as I have noted above.

> 
> - Should the write cache on the physical disks that are used as raid0
>   drives for zfs be enabled, if the raid controller has a battery
>   backup unit ? ( I enabled the disk write cache for all disks)
Both caches are working. There is a hungarian guy who did some testing
on also such a controller, and next to ARC, the controller's own cache
also had some nice effect on performance.

> 
> raid controller cache:
> 
> My current settings for the raid controller cache are: "cache 50%
> reads and 50% writes"
> 
> - Does it make sense to have caching of read- and write-ops enabled
>   with this setup? I wonder: Shouldn't it be the job of the zfs arc to
>   do the caching?
> 
> - Does zfs prefetch make any sense If your raid controller already
>   caches read operations?
> 
> 
> Cheers,
> Kai.
> 
> 
> 
> 
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-- 
Sincerely,
Gergely CZUCZY
Harmless Digital Bt

+36-30-9702963