On 31/05/2013 09:52, Dominic Raferd wrote:
On 31/05/2013 08:54, Roman Mamedov wrote:
On Fri, 31 May 2013 08:47:00 +0100
Dominic Raferd <dominic@xxxxxxxxxxxxxxx> wrote:
This is my idea too (see my OP), but I am concerned about optimisation
(--write-behind, --bitmap and --bitmap-chunk settings) especially for
writes.
--write-behind=16384
I think this will not work, you will have to use 16383.
Oh, OK, so 16383 is the maximum then?
--bitmap=/mnt/sda1/write-intent-bitmap.file
Save yourself lots of maintenance headache, just use --bitmap=internal
--bitmap-chunk=256M
Looks OK.
Thanks Roman, but the problem with using --bitmap=internal is that, as
Neil Brown posted here on another topic a while ago, this requires a
synch write to both devices, and the use-case for which write-behind was
developed involved an external bitmap. Hence my plan to use external
bitmap file on a fast (SSD-based) separate partition - minimises any
slow-down caused by having to maintain the write-intent bitmap file.
I would be very grateful if someone could confirm whether, if I set up
RAID1 and with one of the drives specify --write-mostly
--write-behind=n, that maximum 'n' is 16383, and also whether it is
permitted in this configuration to set --bitmap=none and thus avoid the
overhead of maintaining a write-intent bitmap file? (My thinking is that
for my needs the extra safety provided by the bitmap file is overkill
and the slowing effect (and life-shortening of my SSD) might be more
significant.) If I have to have a bitmap file, it is presumably faster
to have a larger chunk size, is the maximum permitted 256M?
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