[mythtv-users] RAID 6 benchmarks

[Robin Hill]

On Tue Jun 15, 2010 at 11:44:07PM -0700, Yeechang Lee wrote:

> The biggest single impact on performance turned out to be something
> not discussed above. mdadm permits the addition to an existing array
> of a write-intent bitmap for the array to make the process of
> replacing a disk almost instantaneous. As the man page warns, however,
> an internal bitmap "can have a drastic impact on performance." In my
> case, I found that 250MB/s write speeds collapsed to 13MB/s (!) with
> an internal bitmap. The man page warns against external bitmaps for
> non-ext2/3 filesystems and I wouldn't want to depend on an external
> disk for such a purpose anyway. So, if you have an internal bitmap in
> your RAID array and want more speed, consider taking it off (adding
> and subtraction are both almost instantaneous) and seeing what
> happens.
> 
The default RAID bitmap is probably too granular - try changing the
bitmap chunk size.  The current mdadm version defaults to 64M (I think
earlier versions used 1M), but 128M has been suggested as a better
size.  The larger the chunk size, the less frequently the bitmap needs
updating, but the more data will need synching in the event of an
unclean shutdown.

Cheers,
    Robin
-- 
     ___        
    ( ' }     |       Robin Hill        <myth at robinhill.me.uk>  |
   / / )      | Little Jim says ....                            |
  // !!       |      "He fallen in de water !!"                 |
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