RHEL3 and SAS5/iR: very bad performance

Giulio Orsero giulioo at gmail.com
Wed Jan 24 09:30:31 CST 2007


On 1/24/07, Eric Rostetter <rostetter at mail.utexas.edu> wrote:
> Quoting Giulio Orsero <giulioo at gmail.com>:
>
> > Case #1 (simple SATA disk):
> > - bonnie (file=RAMx2): about 70MB/s writes, about 80MB/s reads
> > Case #2 (SAS5/iR DRV 2.06.32 FW-00.06.50.00-IE BIOS-6.06.00.02):
> > - bonnie (file=RAMx2): about 40MB/s writes, about 120MB/s reads
>
> This isn't bad.  RAID-1 will have slower writes (since it has to
> write twice), but faster reads (since it can do two reads at once)

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%.

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.

But then I have another question:
- With a single disk on motherboard I have
        SATA controller -> SATA CABLE-> CACHE+DISK
- With RAID I have
        SATA RAID controler+CACHE -> SATA CABLE -> CACHE+DISK

I thought that:
- the RAID controller write-cache setting would only influence the
controller cache
- the RAID controller write-cache was primarily used for controllers doing RAID5
- the performance of a write-through RAID1 should be equal to a single
SATA disk (since the HD cache was the same)

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?

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?
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.

Thanks

-- 
giulioo at pobox.com



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