[rescue] Re: [SunHELP] Help with RAID

[Charles Shannon Hendrix]

On Thu, Jun 05, 2003 at 12:42:05PM -0400, Curtis H. Wilbar Jr. wrote:

> >The killer is having enough bandwidth for the I/O overhead.
> 
> I haven't put much thought into it yet, but how is the I/O overhead
> that much more than having the same number of individual drives on a bus ?

Because raid is almost always going to be multiple I/O operations for
each logical I/O operation, while with individual drives it's a 1:1
ratio.

> How does RAID 5 impact the bus ?  (Obviously one has to write the parity,
> and without diving into SCSI protocol, I'm sure less information is required
> to setup 10 Meg of transfer off one drive than 10 meg of transfer off a RAID
> 5 setup... but how significant is it ?

raid5 requires multiple I/O operations for each write.  You must read
parity and data from each stripe, compute new parity with the block you
are writing, and write the parity and the new block.

That's 4 I/O operations for each write.

If you are using a journaling raid which writes to the journal before
commiting to the raid, you have 2 more, a write and a read from the
journal.

That's why raid5 makes writing slower.

If you have tons of bandwidth, or hardware support, its not so bad.

> I have the oposite of optimum... I have 12 9 gig drives on ODS doing
> software RAID 5 + journaling along with two mirrored 4 G drives for the
> OS and a CDROM drive (all 15 targets !).

A journaled filesystem, or journaled raid?

Journaled filesystems often make raid work better.

You also want to be careful about choosing filesystem block sizes.  I
try to make the filesystem blocks at least the size of one raid stripe,
but my system is so slow it hardly matters.

At work I could play more because when I did raid on the job, we had
more hardware power.

> This is just a home fileserver, so space and data protection were more
> concerns than speed, so RAID5 was chosen to maximize the space while
> providing some protection for the data.

I'm in the same boat, creating a home raid5, and while I know my
controllers will take a pounding and performance will be low, I've given
up on being able to afford a tape drive that can handle what I have
stored.

I might instead use a 2+2 striped mirror setup, not sure yet.


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