[CFT] Improved ZFS metaslab code (faster write speed)

[Olivier Smedts]

2010/8/22 Martin Matuska <mm at freebsd.org>:
> Dear FreeBSD community,
>
> many of our [2] (and Solaris [3]) users today are complaining about slow
> ZFS writes. One of the causes for these writes is the selection of the
> proper allocation method for allocation of new blocks [3] [4]. Another
> issue a write slowdown during TXG sync times.
>
> Solaris 10 (and OpenSolaris up to november 2009) have the
> following scenario:
>
> - pool has more than 30% free space: use first fit method [1]
> - pool has less than 30% free space: use best fit method [1]
>
> This causes a major slowdown of the writes if we go below 30% of free
> space. On large pools, 30% may be terabytes of free space.
>
> OpenSolaris has changed this in November 2009 and the Oracle Storage
> Appliances also included the new code in Q1/2010 [1].
>
> The source [1] states, that with this change they archieved a speedup
> of: "50% Improved OLTP Performance, 70% Reduced Variability, 200%
> Improvement on MS Exchange"
>
> I would like to issue a Call For Testing for the following 9-CURRENT patch:
> http://people.freebsd.org/~mm/patches/zfs/zfs_metaslab.patch
>
> To apply the patch against 8-STABLE, you need to apply the v15 update first:
> http://people.freebsd.org/~mm/patches/zfs/v15/stable-8-v15.patch

This one doesn't apply cleanly since few minutes :
# svn log -l 1 sys/cddl/contrib/opensolaris/uts/common/fs/zfs/arc.c
------------------------------------------------------------------------
r211599 | avg | 2010-08-22 10:18:32 +0200 (Dim 22 aoû 2010) | 7 lignes

Fix a mismerge in r211581, MFC of r210427

This is a direct commit.

Reported by:    many
Pointyhat to:   avg

------------------------------------------------------------------------

But it does not seem hard to correct. Do you want me to submit an
updated patch for 8-stable ?

> The patch includes the following OpenSolaris onnv revisions:
> 10921 (partial), 11146, 11728, 12047
>
> And covers the following Bug IDs:
> 6826241 Sync write IOPS drops dramatically during TXG sync
> 6869229 zfs should switch to shiny new metaslabs more frequently
> 6917066 zfs block picking can be improved
> 6918420 zdb -m has issues printing metaslab statistics
>
> References:
> [1] http://blogs.sun.com/roch/entry/doubling_exchange_performance
> [2] http://forums.freebsd.org/showthread.php?t=8270
> [3]
> http://blogs.everycity.co.uk/alasdair/2010/07/zfs-runs-really-slowly-when-free-disk-usage-goes-above-80/
> [4] http://blogs.sun.com/bonwick/entry/zfs_block_allocation
> [5] http://blogs.sun.com/bonwick/entry/space_maps
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-- 
Olivier Smedts                                                 _
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