Re: Poor raid 1 performance?

Antoine Leca wrote:

In news:pefnf.455$El.94835@xxxxxxxxxxxxxxxxxxxxx, Peter va escriure:


However, in the common case where the limiting factor is media
access, RAID-1 will result in about the same figures as non-RAID
when seen from outside (as you explained long and wide, thanks.)


That is wrong. Mostly seen in random read performance,



We were not speaking about random access, but about sequential.



but also in sequential read performance; under some, more
sophisticated RAID1 controllers.



Sorry, I cannot make sense of your point. So I shall ask plain and clear:
what is relative performance of RAID-1 versus non-RAID for (long) sequential
reads?


And the answer is -- it depends on the implementation, and on the workload.

Clearly, RAID1 has more hardware (seek mechanisms and on-disk R/W channels)
to deploy than a single HD, but there is no RAID1 standard which mandates
how that additional hardware will be used. A N-HD RAID1 implementation could
be up to ~N times as fast as a single HD in read performance, or it could be
slightly slower; it could be nearly as fast as a single HD in write performance,
or it could be as bad as ~N times slower.

One workload dependency is related to R-W ratio, since reads might be much
faster with RAID1 and since writes might be much slower with RAID1.  Another
workload dependency is sequentiality, since seeks hurt RAID and non-RAID.
Another workload dependency is locality of reference, since caching matters.

Still another workload dependency for long sequential reads is how reads which
are long as seen by the app are seen by the RAID controller and by the HDs:
long reads could be either passed through intact by the OS, or they could be
split and handed off to the RAID controller approximately in parallel, or they
could be split and handed off to the RAID controller (completely or partially)
serially; the RAID controller, in turn, has the same choices of whether and how
to split its commands for handing off to the HDs; finally, for HDs with NCQ/TCQ,
the HD has some choices for how to deal with multiple outstanding commands.

All of the entities (OS/drivers, RAID controllers, HD controllers) between the
app and the magnetic stuff have finite resources; and all were designed by folks
with limited time, imagination, and budget. Implementation does matter, and
it does vary.

Sorry, but life is not simple.

--
Cheers, Bob
.

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