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Re: Issues with delalloc->real extent allocation

To: Dave Chinner <david@xxxxxxxxxxxxx>
Subject: Re: Issues with delalloc->real extent allocation
From: Geoffrey Wehrman <gwehrman@xxxxxxx>
Date: Tue, 18 Jan 2011 08:30:00 -0600
Cc: xfs@xxxxxxxxxxx
In-reply-to: <20110118002437.GS28803@dastard>
References: <20110114002900.GF16267@dastard> <20110114164016.GB30134@xxxxxxx> <20110114225907.GH16267@dastard> <20110115041629.GC11968@xxxxxxx> <20110117051827.GL16267@dastard> <20110117143708.GE11968@xxxxxxx> <20110118002437.GS28803@dastard>
User-agent: Mutt/1.5.14 (2007-02-12)
On Tue, Jan 18, 2011 at 11:24:37AM +1100, Dave Chinner wrote:
| On Mon, Jan 17, 2011 at 08:37:08AM -0600, Geoffrey Wehrman wrote:
| > On Mon, Jan 17, 2011 at 04:18:28PM +1100, Dave Chinner wrote:
| > | However, this does not solve the extsize allocation issues where we
| > | don't have dirty pages in the page cache covering parts of the
| > | delayed allocation extent so we still need a solution for that. I'm
| > | tending towards zeroing in .aio_write as the simplest solution
| > | because it doesn't cause buffer head/extent tree mapping mismatches,
| > | and it would use the above intent/done operations for crash
| > | resilience so there's no additional, rarely used code path to test
| > | through .writepage. Does that sound reasonable?
| > 
| > Zeroing in .aio_write will create zeroed pages covering the entire
| > allocation, correct?
| 
| Yes, though it only needs to zero the regions that the write does
| not cover itself - no need to zero what we're about to put data
| into. ;)

Glad you were able to understand what I meant.  Something I didn't think
of earlier though:  What happens when I try to use an 8 GB on a system
with only 4 GB of memory?  I'm not really worried about this pathological
case, but I do wonder what the effects will be of allocating what could
be significant quantities of memory in .aio_write.


-- 
Geoffrey Wehrman  651-683-5496  gwehrman@xxxxxxx

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