svn commit: r289405 - head/sys/ufs/ffs

[Bruce Evans]

On Fri, 16 Oct 2015, Hans Petter Selasky wrote:

> On 10/16/15 08:21, Bruce Evans wrote:
>> [Bruce Evans didn't write:]
>> In addition, making the file contiguous in LBA space doesn't
>>   improve the access times from flash devices because they have no seek
>> time.
>
> This is not exactly true, like Bruce pointed out too. Maybe there should be a 
> check, that if the block is too small reallocate it, else leave it for the 
> sake of the flash. Doing 1K accesses versus 64K accesses will typically show 
> up in the performance benchmark regardless of how fast the underlying medium 
> is.

Now I don't unerstand the whole point of the change.  Anything that reduces
i/o's is good, but AFAIK ffs_doreallocblks() is all in software.  Writes
should be delayed so that it doesn't have to do extra i/o's to back out of
committed writes.  Often it reduces the number of writes and increases
their size by making blocks contiguous so that the write can be clustered.
Increasing the write size is especially good for flash devices, but maybe
ffs's default block size is already large enough.

Bruce