On Jan 11, 2012, at 5:01 AM, Jim Klimov wrote:
> Hello all, I found this dialog on the zfs-de...@zfsonlinux.org list,
> and I'd like someone to confirm-or-reject the discussed statement.
> Paraphrasing in my words and understanding:
> "Labels, including Uberblock rings, are fixed 256KB in size each,
> of which 128KB is the UB ring. Normally there is 1KB of data in
> one UB, which gives 128 TXGs to rollback to. When ashift=12 is
> used for 4k-sector disks, each UB is allocated a 4KB block, of
> which 3KB is padding. And now we only have 32 TXGs of rollback."
> Is this understanding correct?
> That's something I did not think of
> previously, indeed...
> On Aug 1, 2011, at 2:17 PM, Brian Behlendorf wrote:
> > On Mon, 2011-08-01 at 10:21 -0700, Zachary Bedell wrote:
> >> Given that uberblocks are 1k in size normally
> >> and that they're supposed to be written out in single atomic
> >> operations, does setting ashift=12 cause ZFS to pad out the uberblock
> >> to 4k so that only one block is in each atomic write? Assuming it
> >> does so, the label size must still be the normal 256k which would
> >> leave fewer uberblock slots in the ring (32 instead of 128)?
> > Exactly right. When ashift=12 then the uberblock size is padded out
> > to 4k. That means only 32 uberblocks fit in the on-disk space
> > reserved for the ring. It's one of the lesser known side effects
> > of increasing the ashift.
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