IBM blade server abysmal disk write performances

[Matthew Jacob]

On 1/17/2013 8:03 PM, Dieter BSD wrote:
> I think it is time to ask the driver wizards why TCQ isn't working, so 
> I'm cc-ing the authors listed on the mpt man page. 

It is the MPT firmware that implements SATL, but there are probably 
tweaks that the FreeBSD driver doesn't do that the Linux driver does 
do.  The MPT driver was also worked on years ago and for a variety of 
reasons is unloved.

In general ATA drives have caching enabled, and in fact it is difficult 
to turn off.  There is no info in the email trail that says what the 
state of the SAS drive is wrt cache enable.

There is also no information in the original email as to which direction 
the I/O was being sent.

Let's also get a grip about linux vs. freebsd- using 'dd' is not 
necessarily and apple-apple comparison where writes are concerned 
because of the linux heavy write behind policy (plugging I/Os until it 
gets a large xfer built up and then releasing, which gets larger xfers, 
while freebsd will use the blocksize you tell it to (whether that's 
optimal or not).

I'll see if I can generate some A/B numbers using fio here and report back.