I agree with what Zlatko and Juraj are saying. I just want to add that in
disk subsystems such as ESS that do use RAID5 a lot of care has gone into
the introduction of cache destaging algorithms that mitigate the write
penalty. that is, writes are held in cache until they can be destaged
sinchronously thus calculating the parity once per stripe. it is a sort of
RAID3 style procedure. this does help, but the real performance problem is,
as I said in another post, trying to get all arrays and adapters to work
and spread the load across the machine. this does make a difference. it has
to be said, though, that in real situations a RAID5 based machine will
experience a performance loss of around 15-20 % compared to a RAID1 case.
this is not avoidable. the thing to understand is that we should not
compare RAID systems on a simple performance basis; the main reason for
choosing RAID5 is, quite simply, cost. an EMC machine will cost loads more
because it uses RAID1 (on top of the usual sky high EMC prices) when
compared to a RAID5 based machine. in many cases the performance loss is
acceptable so it's ok to go with RAID5; if performance is the issue, then
the only logical choice is RAID10. I don't feel confident in leaving data
with no protection, just as Juraj was saying.
TSM's ability to have many parallel sessions will have to be considered
together with the underlying hardware to come up with the best solution for
the particular case. that's the keyword I think; rules of thumb and
generalist approaches can only give us an order of magnitude for what we
are doing, but really every case needs to be analysed on its own.

Cordiali saluti
Gianluca Mariani
 Tech Support SSD
Via Sciangai 53, Roma
 phones : +39(0)659664598
                   +393351270554 (mobile)
gianluca_mariani AT it.ibm DOT com


                                                                                
                                                       
                      Zlatko Krastev                                            
                                                       
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DOT EDU                                                          
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                      Sent by: "ADSM:          Subject:  Re: allocating disk 
volumes on RAID5 array                                    
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                      05/29/2002 03:27                                          
                                                       
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Juraj, I completely agree with you. But this is argument against RAID 5.
The question is: if we accept presence of RAID 5 as a fact how to proceed
further.
Read-before-write penalty can be mitigated by tuning blocksize. OTOH TSM
is using big writes and this should not pose so big problem for it. To be
honest I personally avoid to put TSM volumes on RAID other than
EMC/Hitachi/IBM disk subsystems.

Zlatko Krastev
IT Consultant




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Subject:        Re: allocating disk volumes on RAID5 array

Zlatko,

> Write rate for RAID 5 is (note) again
> N times faster than single disk speed but decreases with penalty for
> recalculation if parity (highly controller dependent)

Often this calculation is not correct,
since raid-5 will be written in blocks which are typically
larger than the size of an i/o request,
which forces a read request ahead of write operation
in order to preserve remaining data.

For example on an raid-5 array consisting from 3 disks
a single write requset (from the OS perspective of view)
consists from
        1) read block from 2 disks, a half from the block from each disk
        2) update block´s data with those from i/o request and recalculate
parity of the block
        3) write block to 2 disks (half of to each disk) and write parity
to
1 disks
        SUM = 5 I/O requests in two serialized groups:
                2 Inputs in paralel plus 3 Outputs in paralel
This results in typical raid-5 drives beeing slow when writing.

Those interested may have look at some comparisions
at http://www.icp-vortex.com/index_e.html -> performance,
all controllers tested were much slower when writing Raid5.
Raid beginners will find nice picture explanation under
same address -> support -> raid level description.

regards
Juraj