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
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