zfs discuss - May 2007 - does every fsync() require O(log n) platter-writes?

After reading through the ZFS slides, it appears to be the case that
if ZFS wants to modify a single data block, if must rewrite every
block between that modified block and the uberblock (root of the tree).

Is this really the case?  If so, does this mean that every commit
operation (ie every fsync()) in ZFS requires O(log n) platter writes?

Thanks,

  - a

-- 
PGP/GPG: 5C9F F366 C9CF 2145 E770  B1B8 EFB1 462D A146 C380

Adam Megacz wrote:
> After reading through the ZFS slides, it appears to be the case that
> if ZFS wants to modify a single data block, if must rewrite every
> block between that modified block and the uberblock (root of the tree).
 > Is this really the case?

That is true when commiting the transaction grouptp the main pool
every 5 seconds. However, this isn''t so bad as a lot of transactions
are
commited which likely have common roots and writes are aggregated
and striped across the pool etc...
> If so, does this mean that every commit
> operation (ie every fsync()) in ZFS requires O(log n) platter writes?
The ZIL does not modify the main pool. It only writes system call
transactions related to the file being fsynced and any other transactions that
might related to that file (eg mkdir, rename). Writes for these transactions are
also aggregated and written use a block size tailored to fit the data. Typically
for a single system call just one write occurs. On a system crash or power fail
those ZIL transactions are replayed.

See also:
http://blogs.sun.com/perrin

Neil.
> 
> Thanks,
> 
>   - a
>

Ah, okay.  The slides I read said that in ZFS there is "no journal --
not needed" (slide #9):

  http://www.opensolaris.org/os/community/zfs/docs/zfs_last.pdf

I guess the slides are out of date in light of the "ZFS Intent Log"
journal?

Anyways, it all makes sense now.  Without a journal, you''d need to
perform the operation on slide #11 for every fsync(), which would be a
major performance problem.  With a journal, you don''t need to do this.

Great work, guys...

  - a

-- 
PGP/GPG: 5C9F F366 C9CF 2145 E770  B1B8 EFB1 462D A146 C380

Adam Megacz wrote:
> Ah, okay.  The slides I read said that in ZFS there is "no journal --
> not needed" (slide #9):
> 
>   http://www.opensolaris.org/os/community/zfs/docs/zfs_last.pdf
> 
> I guess the slides are out of date in light of the "ZFS Intent
Log"
> journal?
Yes , I can understand your confusion. Technically the intent log is not a
journal.
A journal has to be replayed to get meta data consistency of the fs.
UFS logging, EXT3 and VXFS all use journals. For perf reasons user data is
typically not logged leading to user data inconsistency.

On the other hand, the zfs pool is always consistent whether or not the
intent log is replayed.
> 
> Anyways, it all makes sense now.  Without a journal, you''d need to
> perform the operation on slide #11 for every fsync(), which would be a
> major performance problem.  With a journal, you don''t need to do
this.
> 
> Great work, guys...
- Thanks Adam.
> 
>   - a
>