RHEL3 and SAS5/iR: very bad performance

William Warren hescominsoon at emmanuelcomputerconsulting.com
Wed Jan 24 09:59:06 CST 2007


Single drive on the mobo means the OS is caching writes to ram which is 
going to be muuuuch larger than the disk cache at all.  When you are on 
the RAID controller with write back off you are effectively turning off 
any caching to the drives hence your lower numbers.  In the case of 
non-power protected failures it depends.  How big is the SAS card's 
cache?  if it's over 64 megabytes then it's safer for single drive to 
motherboard.  However..why not spend $100 for a ups that eliminates the 
worry and you now have protected caching.

Giulio Orsero wrote:
> 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
> 

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