Re: [PATCH, 3.7-rc7, RESEND] fs: revert commit bbdd6808 to fallocate UAPI

Ric Wheeler
2012-12-07T22:02:46Z

On 12/07/2012 04:57 PM, Theodore Ts'o wrote:
> On Fri, Dec 07, 2012 at 04:42:06PM -0500, Ric Wheeler wrote:
>> The other things that I think we should try would be to convert over
>> larger chunks as we discussed on the list back in the summer (just
>> because the user writes 4KB does not mean that we cannot flip over
>> 1MB and zero that).
> Writing a megabyte is not free. If you assume that your HDD has a
> sustained write throughput of 100-125 MB/s, writing a megabyte will
> take 8-10ms. It might be a win if you amortize it over a large number
> of writes, but it doesn't help your 99.9 percentile latency numbers.
> (99.9 percentile latency numbers matters because eventually you'll
> have a user request which hits multiple serial long latency
> operations, and then the delay looks **really** user visible.)
>
> - Ted

Writing 4KB at a time to a disk cost XX units of time.

Writing to the same sector (especially for a HDD), cost XX units + a small amount.

I suggest that we try it out.

For SSD's, much better to use specific HW offload commands if possible like
WRITE_SAME (zeroed) or UNMAP/TRIM to get that performance boost since no actual
data is moved...

ric