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




Please respond to "ADSM: Dist Stor Manager" <ADSM-L AT VM.MARIST DOT EDU>
Sent by:        "ADSM: Dist Stor Manager" <ADSM-L AT VM.MARIST DOT EDU>
To:     ADSM-L AT VM.MARIST DOT EDU
cc: 

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