Re: Write intent bitmap algorithm

[NeilBrown <nfbrown@novell.com>]

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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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