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SubjectRe: limits on raid
dean gaudet wrote:
> On Sun, 17 Jun 2007, Wakko Warner wrote:
>
> > > i use an external write-intent bitmap on a raid1 to avoid this... you
> > > could use internal bitmap but that slows down i/o too much for my tastes.
> > > i also use an external xfs journal for the same reason. 2 disk raid1 for
> > > root/journal/bitmap, N disk raid5 for bulk storage. no spindles in
> > > common.
> >
> > I must remember this if I have to rebuild the array. Although I'm
> > considering moving to a hardware raid solution when I upgrade my storage.
>
> you can do it without a rebuild -- that's in fact how i did it the first
> time.
>
> to add an external bitmap:
>
> mdadm --grow --bitmap /bitmapfile /dev/mdX
>
> plus add "bitmap=/bitmapfile" to mdadm.conf... as in:
>
> ARRAY /dev/md4 bitmap=/bitmap.md4 UUID=dbc3be0b:b5853930:a02e038c:13ba8cdc

I used evms to setup mine. I have used mdadm in the past. I use lvm ontop
of it which evms makes it a little easier to maintain. I have 3 arrays
total (only the raid5 was configured by evms, the other 2 raid1s were done
by hand)

> you can also easily move an ext3 journal to an external journal with
> tune2fs (see man page).

I only have 2 ext3 file systems (One of which is mounted R/O since it's
full), all my others are reiserfs (v3).

What benefit would I gain by using an external journel and how big would it
need to be?

> if you use XFS it's a bit more of a challenge to convert from internal to
> external, but see this thread:

I specifically didn't use XFS (or JFS) since neither one at the time could
be shrinked.

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