why does shuffle in spark write shuffle data to disk by default?

[huan zhang]

Hi All,
    I'm wonderring why does shuffle in spark write shuffle data to disk by
default?
    In Stackoverflow, someone said it's used by FTS, but node down is the
most common reason of fault, and write to disk cannot do FTS in this case
either.
    So why not use ramdisk as default instread of SDD or HDD only?

Thanks
Hubert Zhang