After researching write-intent bitmaps for a while, my understanding is
that they are used only to speed up "re-adding" drives by avoiding a
full resync, and to enable --write-mostly --write-behind. However, it
does introduce some pretty heavy load on whatever device it's on,
especially if it's an internal bitmap, because the head would have to
fly all the way to the superblock twice per each write. If it's an
external bitmap, then the device it's on would be too busy just serving
it to do anything else.
So if I were to place it on a tmpfs, I could eliminate this problem only
at the expense of being unable to re-add drives after a reboot, right?..
I've read somewhere that bitmaps only work correctly on ext2 or ext3
filesystems, but that probably means that it's not a good idea to put it
on a filesystem with delayed allocation like ext4 of zfs, otherwise I
don't understand why - and so I don't know if there would be any problem
with it running on tmpfs. Is there?..
By the way, Phil, you are a hero! :) I remember that it was you who
taught me about the "timeouts and scrubbing" problem a year ago, and you
always explain things so well! You must have a lot of patience and love
for all people! :)
--
darkpenguin
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