[CentOS] 4.4/64-bit Supermicro/ Nvidia RAID [thanks]

Tue Dec 12 08:29:46 UTC 2006
Feizhou <feizhou at graffiti.net>

John R Pierce wrote:
> Feizhou wrote:
>> not so. I used to run boxes that handled 600 to 1000 smtp connections 
>> each. Creating and deleting thousands of small files was the 
>> environment I worked in.
> and if the power failed in the middle of this, how many messages were lost?

Heh, which boxes, the 3ware ones (750x series no battery backed 
cache...in fact no cache at all!) or the ide disks only ones or the scsi 
only ones or the compaq hardware raid scsi ones? Ans: Only on two 
occasions have I got corrupted queue files and that was because I used 
the XFS filesystem which is a disaster in a case of power loss. That was 
on a 3ware box. Had no problems with the rest.

>> I must say that the 3ware cards on those boxes that had them were not 
>> 955x series and therefore had no cache. Perhaps things would have been 
>> different if it were 955x cards in there but at that time, the 9xxx 
>> 3ware cards were not even out yet.
> We typically use 15000 rpm scsi or fiberchannel storage for our 
> databases, not SATA.

OOH, nice hardware you have for your uber databases. The outfit I worked 
for did well with mysql + software raid/3ware + ide disks. No need for FC.

>> Since you have clearly pointed out the performance benefit really 
>> comes from the cache (if you have enough) on the board I do not see 
>> why using software raid and a battery-backed RAM card like the umem or 
>> even the gigabyte i-ram for the journal of the filesystem will be any 
>> less slow if at all.
> its not the file system journal I'm talking about, its an application 
> specific journal file, which contains the indicies and state of the 
> queue files, of which there's a very large number constantly being 
> written.   We need to flush the queue files AND the journal files for it 
> to be safe.   These run around 10GB total as I understand it (not each 
> flush, but the aggregate queues can be this big).

Now you are telling me that somehow you have code that makes your 
database stuff its journal on your RAID controller's cache. Cool, mind 
sharing it with the rest of us?

Let me just say that I know that the code in the kernel for RAID 
controllers that have cache will, as you say, give the OK once the data 
that needs to be written hits the cache.

In the case of a RAM card, I am pointing out that that effect can be 
achieved by putting the journal of a journaling filesystem like ext3 on 
the RAM card especially since ext3 supports data journaling too.

If the aggregate queues are up to 10GB, I really wonder wonder how much 
faster your hardware raid makes things unless of course your cache is 
much larger than 2GB. Just on the basis of the inadequate size of your 
cache I would give software raid + RAM card the benefit of the doubt.

> if the server has a writeback enabled controller like a HP Smart Array 
> 5i/532, it all works great.  if it doesn't, it all grinds to a halt.   
> quite simple, really.    We have absolutely no desire to start 
> architecting around 3rd party ram/battery disks, they won't be supported 
> by our production system vendors, and they will make what is currently a 
> fairly simple and robust system a lot more convoluted..

Yada yada. The compaqs that had hardware raid with scsi disks were the 
slowest performers of all those boxes I managed not to mention that lack 
of any tools under 2.6 for monitoring or whatever too. I am not telling 
you to what hardware to use. What I am doing is contesting your claims 
of hardware raid with battery-backed cache being hugely faster than 
software raid. I will concede that there will be cases where it will 
indeed be hugely faster but not always.