Re: Linux 2.6.29

Jeff Garzik
2009-04-03T02:55:21Z

Trenton D. Adams wrote:
> On Thu, Apr 2, 2009 at 8:01 PM, Jeff Garzik <jeff@garzik.org> wrote:
>> Linus Torvalds wrote:
>> The most interesting thing I found: the SSD does 80 MB/s for the first ~1
>> GB or so, then slows down dramatically. After ~2GB, it is down to 32 MB/s.
>> After ~4GB, it reaches a steady speed around 23 MB/s.
>
> Isn't that the kernel IO queue, and the dd averaging of transfer
> speed? For example, once you hit the dirty ratio limit, that is when
> it starts writing to disk. So, the first bit you'll see really fast
> speeds, as it goes to memory, but it averages out over time to a
> slower speed. As an example...

overwrite.c is a special program that does this, in a loop:

write(buffer-N) data to pagecache
start buffer-N write-out to storage
wait for buffer-(N-1) write-out to complete

It uses the sync_file_range() system call, which is like fsync() on
steroids, wearing cool sunglasses.

Regards,

Jeff