Re: raid is dangerous but that's secret (was Re: [patch] ext2/3:document conditions when reliable operation is possible)

From: david
Date: Mon Aug 31 2009 - 15:12:47 EST

On Mon, 31 Aug 2009, Christoph Hellwig wrote:

On Mon, Aug 31, 2009 at 08:50:53AM -0700, david@xxxxxxx wrote:
It would also be very useful to have all of our top tier file systems
enable barriers by default, provide consistent barrier on/off mount
options and log a nice warning when not enabled....

most people are not willing to live with unbuffered write performance.

I'm not sure what you mean with unbuffered write support, the only
common use of that term is for userspace I/O using the read/write
sysctem calls directly in comparism to buffered I/O which uses
the stdio library.

But be ensure that the use of barriers and cache flushes in fsync does not
completely disable caching (or "buffering"), it just does flush flushes
the disk write cache in case we either commit a log buffer than need to
be on disk, or performan an fsync where we really do want to have data
on disk instead of lying to the application about the status of the
I/O completion. Which btw could be interpreted as a violation of the
Posix rules.

as I understood it, the proposal that I responded to was to change the kernel to detect if barriers are enabled for the entire stack or not, and if not disable the write caches on the drives.

there are definantly times when that is the correct thing to do, but I am not sure that it is the correct thing to do by default.

David Lang
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