On Fri, May 20 2016, Ankur Bose wrote: > Hi All, > > We are doing performance analysis of MD raid1 and wanted to know the flow/algorithm of the writes with write intent bitmap. > As per "man md" below lines gives the algo > > "Before any write request is honored, md will make sure that the corresponding bit in the log is set. After a period of time with no writes to an area of the array, the corresponding bit will be cleared." > > Here we wanted to know after what period of time and what is the logic > to calculate this period of time which MD does ? When you create an array with a bitmap, or use --grow to add a bitmap, you can provide the --delay option to give a delay in seconds (which seems to not be documented in mdadm.8 :-( Patches welcomem. Alternately when the array is running you can inspected and change /sys/block/mdXXX/md/bitmap_time_base Every this-many seconds a task runs in the kernel which marks any bits with a count that recently reach zero as "should be written", updates the on-disk bitmap to clear any bits that were marked "should be written". So a bit with an active-write count of zero will be cleared in the non-disk bitmap between 1 and 2 delays after the count reaches zero. > > I see a function "void bitmap_daemon_work(struct mddev *mddev)" in bitmap.c file but trying to figure out the logic how it is used. Yes, this is the task that is run every 'delay' or 'time_base' seconds. The counter is offset bit 2 normally so when there are no active writes bitmap_daemon_work will find a counter which is '2' and will change it to '1'. If it finds a counter that is '1' it will change it to '0' and immediately clear the bit in the in-memory image of the bitmap. Then it will write out any pages of the bitmap which have changed. NeilBrown
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