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disable preallocation

To: linux-xfs@xxxxxxxxxxx
Subject: disable preallocation
From: Ming Zhang <mingz@xxxxxxxxxxx>
Date: Tue, 18 Apr 2006 16:19:27 -0400
Reply-to: mingz@xxxxxxxxxxx
Sender: linux-xfs-bounce@xxxxxxxxxxx
Hi folks,

I checked the archive and found some discussion about how to do
pre-allocation. Interestingly, my questions is how to disable the
pre-allocation. What I want is no matter how much data I write to XFS,
as long as the size are multiple of file system block size, the fs will
not preallocate some blocks for me. Because if fs does not do
pre-allocation, later I read the block that no previous written data, it
can give me all 0, otherwise it might be some strange data.

I am dumping many huge sparse martrix to disk and if that whole block is
full of 0, i will not write it to disk at all, so have a hole there.
later when read back, i want to get all 0 as well. if fs do
preallocation, then i probably will get some random data.

I found that ext2/3 will do pre-allocation. will xfs do it or not?

Thanks!

Ming



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