Date: Tue, 7 Jul 2009 10:10:28 -0700 (PDT) From: Matthew Dillon <dillon@apollo.backplane.com> To: freebsd-arch@FreeBSD.org Subject: Re: DFLTPHYS vs MAXPHYS Message-ID: <200907071710.n67HASb7088248@apollo.backplane.com> References: <20090707151901.GA63927@les.ath.cx> <200907071639.n67GdBD2087690@apollo.backplane.com>
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A more insideous problem here that I think is being missed is
the fact that newer filesystems are starting to use larger filesystem
block sizes. I myself hit serious issues when I tried to create a
UFS filesystem with a 64K basic filesystem block size a few years ago,
and I hit similar issues with HAMMER which uses 64K buffers for bulk
data which I had to fix by reincorporating code into ATA that had existed
originally to break-up large single-transfer requests that exceeded the
chipset's DMA capability. In the case of ATA, numerous older chips
can't even do 64K due to bugs in the DMA hardware. Their maximum is
actually 65024 bytes.
Traditionally the cluster code enforced such limits but assumed that
the basic filesystem block size would be small enough not to hit the
limits. It becomes a real problem when the filesystem itself wants to
use a large basic block size.
In that respect hardware which is limited to 64K has serious consequences
which cascade through to the VFS layers.
-Matt
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