Re: ext4_fallocate

[Theodore Ts'o <tytso@mit.edu>]

On Wed, Jun 27, 2012 at 07:02:45PM -0400, Eric Sandeen wrote:
> 
> fallocate 1g, do 16m of 4k random IOs, sync after each:
> 
> # for I in a b c; do rm -f testfile; echo 3 > /proc/sys/vm/drop_caches; fio tytso.fio | grep 2>&1 WRITE; done
> 
>   WRITE: io=16384KB, aggrb=154KB/s, minb=158KB/s, maxb=158KB/s, mint=105989msec, maxt=105989msec
>   WRITE: io=16384KB, aggrb=163KB/s, minb=167KB/s, maxb=167KB/s, mint=99906msec, maxt=99906msec
>   WRITE: io=16384KB, aggrb=176KB/s, minb=180KB/s, maxb=180KB/s, mint=92791msec, maxt=92791msec
> 
> same, but overwrite pre-written 1g file (same as the expose-my-data option ;)
> 
> # dd if=/dev/zero of=testfile bs=1M count=1024
> # for I in a b c; do echo 3 > /proc/sys/vm/drop_caches; fio tytso.fio | grep 2>&1 WRITE; done
> 
>   WRITE: io=16384KB, aggrb=164KB/s, minb=168KB/s, maxb=168KB/s, mint=99515msec, maxt=99515msec
>   WRITE: io=16384KB, aggrb=164KB/s, minb=168KB/s, maxb=168KB/s, mint=99371msec, maxt=99371msec
>   WRITE: io=16384KB, aggrb=164KB/s, minb=168KB/s, maxb=168KB/s, mint=99677msec, maxt=99677msec
> 

There's a pretty large range comparing the first set versus the second
set of numbers.  With the second set of numbers, the times are much
more stable.

I wonder if one of the things that's going on is file placement; if
you're constantly deleting and recreating the file, are we getting the
same block numbers or not?  (And if we're not, is that something we
need to look at vis-a-vis the block allocator --- especially with
SSD's?)

						- Ted