zfs discuss - Jan 2012 - How many "rollback" TXGs in a ring for 4k drives?

Hello all, I found this dialog on the zfs-devel at 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...

Thanks,
//Jim


http://groups.google.com/a/zfsonlinux.org/group/zfs-devel/browse_thread/thread/182c92911950ccd6/aa2ad1fdaf7d7a07?pli=1

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.

On Jan 11, 2012, at 5:01 AM, Jim Klimov wrote:
> Hello all, I found this dialog on the zfs-devel at 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?
Yes.
 -- richard
> That''s something I did not think of
> previously, indeed...
> 
> Thanks,
> //Jim
> 
> 
>
http://groups.google.com/a/zfsonlinux.org/group/zfs-devel/browse_thread/thread/182c92911950ccd6/aa2ad1fdaf7d7a07?pli=1
> 
> 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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> http://mail.opensolaris.org/mailman/listinfo/zfs-discuss
-- 

ZFS and performance consulting
http://www.RichardElling.com
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