Re: [PATCH - RFC] allow setting vm_dirty below 1% for large memory machines

[dean gaudet <dean@arctic.org>]

On Tue, 9 Jan 2007, Neil Brown wrote:

> Imagine a machine with lots of memory - say 100Gig.

i've had these problems on machines as "small" as 8GiB.  the real problem 
is that the kernel will let millions of potential (write) IO ops stack up 
for a device which can handle only mere 100s of IOs per second.  (and i'm 
not convinced it does the IOs in a sane order when it has millions to 
choose from)

replacing the percentage based dirty_ratio / dirty_background_ratio with 
sane kibibyte units is a good fix... but i'm not sure it's sufficient.

it seems like the "flow control" mechanism (i.e. dirty_ratio) should be on 
a device basis...

try running doug ledford'd memtest.sh on an 8GiB box with a single disk, 
let it go a few minutes then ^C and type "sync".  i've had to wait 10 
minutes (2.6.18 with default vm settings).

it makes it hard to guarantee a box can shutdown quickly -- nasty for 
setting up UPS on-battery timeouts for example.

-dean