SQLSTATE[HY000]: General error: 1030 Got error 28 from storage engine in magento site

we are facing following problem at GMT 5 pm from last 2 days.

SQLSTATE[HY000]: General error: 1030 Got error 28 from storage engine.

we have a 100 gb disk space with server on amazon web service [VPS]

there is magento site running with 20 gb data.

at night when we check the site , we got following error : http://ift.tt/1BOu8EX

when we check the disk space, it was full. it was 95%.

than when we restarted mysql, it came to normal 20 gb.

why the disk space suddenly increased from 20 gb to 95 gb in night ?

please give some solution for this.