On 2018/8/21 下午10:24, David Sterba wrote:
> On Tue, Aug 21, 2018 at 02:43:35PM +0100, David Howells wrote:
>> Qu Wenruo <quwenruo.bt...@gmx.com> wrote:
>>
>>> But to be more clear, NOSSD shouldn't be a special case.
>>> In fact currently NOSSD only affects whether we will output the message
>>> "enabling ssd optimization", no real effect if I didn't miss anything.
>
> There is a real effect.
>
>> That's not quite true. In:
>>
>> if (!btrfs_test_opt(fs_info, NOSSD) &&
>> !fs_info->fs_devices->rotating) {
>> btrfs_set_and_info(fs_info, SSD, "enabling ssd optimizations");
>> }
>>
>> the call to btrfs_set_and_info() will turn on SSD.Oh, I just missed the btrfs_set part. (even it's myself introduced that function). What a shame... Thanks, Qu >> >> What this seems to me is that, normally, SSD will be turned on automatically >> unless at least one of the devices is a rotating medium - but this appears to >> be explicitly suppressed by the NOSSD option. > > Right. So expected behaviour: > > - nothing: auto-detect non-rotating devices, enable SSD mount option in turn > - nossd: disable auto-detection of non-rotating devices > - ssd: enable SSD optimizations uconditionally > - ssd_spread: implies SSD and affects some allocator decisions regarding > new extent alignments >
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