[linux-lvm] RFE? Really power of 2? extents, chunks and raid alignment

Mike Snitzer snitzer at redhat.com
Wed Jul 25 21:04:34 UTC 2012

On Sun, Jul 22 2012 at  2:22pm -0400,
Linda Walsh <lvm at tlinx.org> wrote:

> I was trying to figure out why I didn't get performance out of my RAID,
> except when operating on really large files where it's many stripes
> v. 1 stripe.
> I have 12 data disks in a RAID 50 (3 RAID5's in a RAID0) and use a suggested
> stripesize of 64k, so a stripe-width of 768k.
> Some issues that have been nagging me though are getting my allocations
> lined up on 768k boundaries.  xfs is no prob -- tell it 64k and 12
> and it does it.
> But I just realized that lvm doesn't really tell me where it is
> aligning things
> and worse, only lets me align both chunksizes for lv's and extents for vg's
> in powers of 2.   Um... Not a multiple of 4k? or 64k?
> Am I wrong in thinking this would tend to give me both pv's and lv's that
> are very likely NOT to be stripe-width aligned, but, worse, not stripewidth
> alignable, at all.
> This would mean that a high performance file system aware of RAID
> stripe with
> that tries to allocate chunks starting on a 768k boundary are likely
> to just
> get it completely wrong?  No?
> Or what am I missing?

The DM update for the 3.6 merge window adds non power of 2 support in
the kernel (for the stripe and thin-pool targets).

So the lvm2 constraints that require a power of 2 chunksize will be
relaxed very shortly.

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