zfs discuss - Dec 2008 - How ZFS decides if write to the slog or directly to the POOL

Hello all,
 Somedays ago i was looking at the code and did see some variable that seems to
make a correlation between the size of the data, and if the data is written to
the slog or directly to the pool. But i did not find it anymore, and i think is
way more complex than that.
  For example, if we have a pool of just two disks, it''s fine to write
to the slog (SSD). But if we have a 20 disks pool, write to the slog will not be
a good idea, don''t you agree? But if someone has that configuration (20
disks and a slog), the ZFS code would not identify that, and write directly to
the pool?
 I''m asking this because i did some tests and seems like the SSD became
a bottleneck... and i guess that even if the admin did make such mistake, the
ZFS had the logic to avoid write to the intent log.

 Thanks a lot for your time!
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This message posted from opensolaris.org

Marcelo Leal writes:
 > Hello all,
 >  Somedays ago i was looking at the code and did see some variable that
 > seems to make a correlation between the size of the data, and if the
 > data is written to the slog or directly to the pool. But i did not
 > find it anymore, and i think is way more complex than that. 
 >   For example, if we have a pool of just two disks, it''s fine to
write
 > to the slog (SSD). But if we have a 20 disks pool, write to the slog
 > will not be a good idea, don''t you agree? But if someone has that
 > configuration (20 disks and a slog), the ZFS code would not identify
 > that, and write directly to the pool? 
 >  I''m asking this because i did some tests and seems like the SSD
 > became a bottleneck... and i guess that even if the admin did make
 > such mistake, the ZFS had the logic to avoid write to the intent log. 
 > 
 >  Thanks a lot for your time!
 > -- 
 > This message posted from opensolaris.org
 > _______________________________________________
 > zfs-discuss mailing list
 > zfs-discuss at opensolaris.org
 > http://mail.opensolaris.org/mailman/listinfo/zfs-discuss


Hi Marcelo, you are right on and this is being tracked as :

	6706578 a single zil writer should not abuse the slog

http://bugs.opensolaris.org/bugdatabase/view_bug.do?bug_id=6706578
Sun Storage 7000 line does already have a fix for this.

-r

http://blogs.sun.com/mws/entry/introducing_the_sun_storage_7000

> 
> Marcelo Leal writes:
>  > Hello all,
> >  Somedays ago i was looking at the code and did see
> some variable that
> > seems to make a correlation between the size of
>  the data, and if the
> > data is written to the slog or directly to the
> pool. But i did not
> > find it anymore, and i think is way more complex
>  than that. 
> >   For example, if we have a pool of just two disks,
> it''s fine to write
> > to the slog (SSD). But if we have a 20 disks pool,
>  write to the slog
> > will not be a good idea, don''t you agree? But if
> someone has that
> > configuration (20 disks and a slog), the ZFS code
>  would not identify
> > that, and write directly to the pool? 
> >  I''m asking this because i did some tests and
>  seems like the SSD
> > became a bottleneck... and i guess that even if the
> admin did make
> > such mistake, the ZFS had the logic to avoid write
>  to the intent log. 
> > 
>  >  Thanks a lot for your time!
> > -- 
>  > This message posted from opensolaris.org
> > _______________________________________________
>  > zfs-discuss mailing list
> > zfs-discuss at opensolaris.org
> >
>  http://mail.opensolaris.org/mailman/listinfo/zfs-disc
> ss
> 
> 
> Hi Marcelo, you are right on and this is being
> tracked as :
> 
> 6706578 a single zil writer should not abuse the
> e slog
> 
> http://bugs.opensolaris.org/bugdatabase/view_bug.do?bu
> g_id=6706578
 Thanks, i did put a link here:

http://www.eall.com.br/blog/?p=842
> Sun Storage 7000 line does already have a fix for
> this.
 ;-)

 Leal
[http://www.eall.com.br/blog]
> 
> -r
> 
> http://blogs.sun.com/mws/entry/introducing_the_sun_sto
> rage_7000
> 
> _______________________________________________
> zfs-discuss mailing list
> zfs-discuss at opensolaris.org
> http://mail.opensolaris.org/mailman/listinfo/zfs-discu
> ss
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