RHEL3 and SAS5/iR: very bad performance

Eric Rostetter rostetter at mail.utexas.edu
Wed Jan 24 11:06:26 CST 2007


Quoting Giulio Orsero <giulioo at gmail.com>:

> I thought RAID would write in parallel, so that at most you could get
> 1/2% penalty for synchronization between the 2 disks, not 40%.

If your RAID-1 implementation supports parallel writes, and it is configured
to do so, and you are not limiting the controller in any way, and the
filesystem or application is not limiting it in any way, AND there is no
disk contention (nothing else happening on the RAID 1 set), then yes it
should write in parallel and be about as fast as a single disk write.

> However, I found out that it's the most obviuos thing, the
> write-through/back cache setting. Enabling the write-back cache I get
> similar results to a single disk.

Sounds good.  Safe to use if you have battery backup on the raid card,
and the battery doesn't die.

> I thought that:
> - the RAID controller write-cache setting would only influence the
> controller cache

Yes, it should only influence the controller cache.

> - the RAID controller write-cache was primarily used for controllers  
>  doing RAID5

No.  It may be "more important" there, but it is helpful in any raid
mode.

It influences whether the controller can ack a write request once it
hits the cache, or only after it is written to all disks, which has
a tremendous performance impact at load.

> - the performance of a write-through RAID1 should be equal to a single
> SATA disk (since the HD cache was the same)

It should be nearly the same, but just slightly slower, if everything is
optimal.

> But it's seems this is not the case, does anybody know if disabling
> write-cache on a SAS5/iR (factory default) disable the hard disk cache
> too?

No, but I see no reason it would do so.

> So, given the 2 cases:
> 1) running with a single SATA disk on motherboard
> 2) running with a  SAS5/iR with write-back enabled
> In case of a non protected power failure, is one safer than the other?

If the SAS5/iR has batter backup, then it is safer.  If not, then they
are both unsafe.

How safe they are really depends not only on the controller, but filesystem
used, type of journaling if any, how often the linux memory caches are
flushed to disk, if the program uses sync calls or not, etc.

> Because to get the same performance of a single disk on motherboard I
> have to do write-back on the SAS5/iR with it seems switch on both the
> HD cache (like case #1) and the controller cache, so 2 caches instead
> of one to have problems with.

If you want to disable the drive cache, do so, but it will impact performance,
and isn't a very big risk to leave on.  The controller write-back caching
is safe if you have battery backup.  Not sure why you are worried, unless
the SAS5/iR has no battery backup, in which case you're SOL.

> Thanks
>
> --
> giulioo at pobox.com

-- 
Eric Rostetter
The Department of Physics
The University of Texas at Austin

Go Longhorns!



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