Re: Linux 2.6.29
From: Mark Lord
Date: Mon Mar 30 2009 - 11:21:21 EST
Ric Wheeler wrote:
I am confused as to why you think that barriers (flush barriers
specifically) are not equivalent to drive write cache. We disable
barriers when the write cache is off, use them only to insure that our
ordering for fs transactions survives any power loss. No one should be
enabling barriers on linux file systems if your write cache is disabled
or if you have a battery backed write cache (say on an enterprise class
disk array).
Chris' test of barriers (with write cache enabled) did show for desktop
class boxes that you would get file system corruption (i.e., need to
fsck the disk) a huge percentage of the time.
..
Sure, no doubt there. But it's due to the kernel crash,
not due to the write cache on the drive.
Anything in the drive's write cache very probably made it to the media
within a second or two of arriving there.
So with or without a write cache, the same result should happen
for those tests. Of course, if you disable barriers *and* write cache,
then you are no longer testing the same kernel code.
I'm not arguing against battery backup or UPSs,
or *for* blindly trusting write caches without reliable power.
Just pointing out that they're not the evil that some folks
seem to believe they are.
Cheers
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