On 08/25/2009 06:58 PM, Neil Brown wrote:
On Monday August 24, tytso@xxxxxxx wrote:
On Mon, Aug 24, 2009 at 11:25:19PM +0200, Pavel Machek wrote:
I have to admit that I have not paid enough attention to this specifics
of your ext3 + flash card issue - is it the ftl stuff doing out of order
IO's?
The problem is that flash cards destroy whole erase block on unplug,
and ext3 can't cope with that.
Sure --- but name **any** filesystem that can deal with the fact that
128k or 256k worth of data might disappear when you pull out the flash
card while it is writing a single sector?
A Log structured filesystem could certainly be written to deal with
such a situation, providing by 'deal with' you mean 'only loses data
that has not yet been acknowledged to the application'. Of course the
filesystem would need clear visibility into exactly how these blocks
are positioned.
I've been playing with just such a filesystem for some time (never
really finding enough time) with the goal of making it work over RAID5
with no data risk due to power loss. One day it will be functional
enough for others to try....
It is entirely possible that NILFS could be made to meet that
requirement, but I haven't made time to explore NILFS so I cannot be
sure.
NeilBrown
I am not sure that log structure will protect you from this scenario since once
you clean the log, the non-logged data is assumed to be correct.
If your cheap flash storage device can nuke random regions of that clean
storage, you will lose data....
ric
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