Linus Torvalds wrote:
> 
> On Thu, 2 Apr 2009, Jeff Garzik 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.
> 
> Are you sure that isn't an effect of double and triple indirect blocks 
> etc? The metadata updates get more complex for the deeper indirect blocks.
> 
> Or just our page cache lookup? Maybe our radix tree thing hits something 
> stupid. Although it sure shouldn't be _that_ noticeable.
> 
>> There is a similar performance fall-off for the Seagate, but much less
>> pronounced:
>> 	After 1GB:	52 MB/s
>> 	After 2GB:	44 MB/s
>> 	After 3GB:	steady state
> 
> That would seem to indicate that it's something else than the disk speed. 

Attached are some additional tests using sync_file_range, dd, an SSD and 
a normal SATA disk.  The test program -- overwrite.c -- is unchanged 
from my last posting, basically the same as Linus's except with 
posix_fadvise()

Observations:

* the no-name SSD does seem to burst the first ~1GB of writes rapidly, 
but degrades to a much lower sustained level, as observed before. 
Repeated tests do not produce ~80 MB/s, only the first test, which lends 
credence to the theory about background activity.

* For the SSD, overwrite is noticeably faster than dd.

* For the Seagate NCQ hard drive, dd is noticeably faster than overwrite.

* fadvise() appears to help, but mostly the results are either 
inconclusive or lost in the noise:  A slight increase in throughput, and 
a slight increase in system time.

The test sequence for both SATA devices was the following:

	3 x dd
	3 x overwrite
	3 x overwrite w/ fadvise(don't need)

System setup: Intel Nahalem(sp?) x86-64, ICH10, Fedora 10, ext3 
filesystem (mounted defaults + noatime), 2.6.29 vanilla kernel.

Regards,

	Jeff