Messages in this thread | | | Date | Fri, 07 Dec 2012 17:02:32 -0500 | From | Ric Wheeler <> | Subject | Re: [PATCH, 3.7-rc7, RESEND] fs: revert commit bbdd6808 to fallocate UAPI |
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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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