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review: change default realtime extsize

To: xfs@xxxxxxxxxxx
Subject: review: change default realtime extsize
From: Nathan Scott <nathans@xxxxxxx>
Date: Mon, 28 Aug 2006 13:31:32 +1000
Sender: xfs-bounce@xxxxxxxxxxx
User-agent: Mutt/1.2.5i
Hi,

While testing something else recently, I noticed we were always
creating a large number of unwritten extents during page cache
writout to the realtime subvolume.  It turned out to be because
of the way we are typically called with page sized allocation
requests, but we always allocate much more than a page.  Back
when realtime subvols could only use direct I/O this was fine
(apps typically used larger IO sizes) but now its not so good.

These larger allocations are ending up meaning we need to do many
additional unwritten extent conversions (i.e. more transactions,
and more log traffic), which we can easily avoid.  Since we have
a default 4K filesystem blocksize on the data device, there would
seem to be no harm in matching that on the realtime subvolume, by
default.  This simple mkfs tweak does just that...

cheers.

-- 
Nathan


Index: xfsprogs/include/xfs_rtalloc.h
===================================================================
--- xfsprogs.orig/include/xfs_rtalloc.h 2006-08-18 11:50:35.916857000 +1000
+++ xfsprogs/include/xfs_rtalloc.h      2006-08-18 11:55:44.820162250 +1000
@@ -25,7 +25,7 @@ struct xfs_trans;
 
 /* Min and max rt extent sizes, specified in bytes */
 #define        XFS_MAX_RTEXTSIZE       (1024 * 1024 * 1024)    /* 1GB */
-#define        XFS_DFL_RTEXTSIZE       (64 * 1024)             /* 64KB */
+#define        XFS_DFL_RTEXTSIZE       (4 * 1024)              /* 4KB */
 #define        XFS_MIN_RTEXTSIZE       (4 * 1024)              /* 4KB */
 
 /*


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