--- Michael Tokarev <mjt@tls.msk.ru> wrote:

Jan Engelhardt wrote:
[]

quoted

The other thing is, the bitmap is supposed to be

written out at intervals,

quoted

not at every write, so the extra head movement for

bitmap updates should

quoted

be really low, and not making the tar -xjf process

slower by half a minute.

quoted

Is there a way to tweak the write-bitmap-to-disk

interval? Perhaps 

quoted

something in /sys or ye olde /proc. Maybe

linux-raid@ knows 8)

Hmm.  Bitmap is supposed to be written before actual
data write, to mark
the to-be-written areas of the array as "being
written", so that those
areas can be detected and recovered in case of power
failure during
actual write.

So in case of writing to a clean array, head
movement always takes place -
first got to bitmap area, and second to the actual
data area.

That "written at intervals" is about clearing the
bitmaps after some idle
time.

In other words, dirtying bitmap bits occurs right
before actual write,
and clearing bits occurs at intervals.

Sure, if you write to (or near) the same place again
and again, without
giving a chance to md subsystem to actually clean
the bitmap, there will
be no additional head movement.  And that means, for
example, tar -xjf
sometimes, since filesystem will place the files
being extracted close to
each other, thus hitting the same bit in the bitmap,
hence md will skip
repeated bitmap updates in this case.

/mjt

I assume that if a block is already dirty then that is
cached somewhere in memory so you aren't writing to
the bitmap unless you're changing it for clean to
dirty? If that's the case then I would think that
writing to the map wouldn't be that expensive?



 
____________________________________________________________________________________
Now that's room service!  Choose from over 150,000 hotels
in 45,000 destinations on Yahoo! Travel to find your fit.
http://farechase.yahoo.com/promo-generic-14795097