[PLUG] megaraid_sas write caching?

Aaron Burt aaron at bavariati.org
Sat Dec 15 18:24:58 UTC 2007


On Sat, Dec 15, 2007 at 02:34:32AM -0800, Aaron Ten Clay wrote:
> Hey all,
> 
> Just got Gentoo installed on a shiny new Dell PowerEdge 2950 server  
> with an LSI Logic MegaRAID SAS RAID controller. Everything is great  
> except this:
> 
> [   81.028864] sd 0:2:0:0: [sda] 1951399936 512-byte hardware sectors  
> (999117 MB)
> [   81.028991] sd 0:2:0:0: [sda] Write Protect is off
> [   81.029059] sd 0:2:0:0: [sda] Mode Sense: 1f 00 10 08
> [   81.029121] sd 0:2:0:0: [sda] Write cache: disabled, read cache:  
> disabled, supports DPO and FUA
> 
> As the kernel reports, both write- and read-caching are disabled. This  
> is grossly obvious during file copies of a few GB or more - the entire  
> system grinds to a halt for 5+ minutes per GB. Even the console is  
> unresponsive.

One usually disables write-caching with RAID.  It makes it much less
likely that you'll end up with an inconsistent RAID volume if there's a
power-cut.  

That said, the little caches on the drives won't help you much, anyway.
Depending on what technique is used for the copying, you're running into
a problem with some implementations of the block caching, readahead
and/or scheduler systems.  Basically, it tries to read as many files
as it can into RAM, then swaps madly, while writing files out, while
it's trying to read more, and trips over its own feet.  I don't know
what kernel config ad optimizations you're using in Gentoo, but poor
choices there could make it worse.

The cp command, I understand, does things in little chunks to prevent
this.




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