Re: [RFC PATCH 1/2] bdi: Create a flag to indicate that a backing device needs stable page writes
From: NeilBrown <hidden>
Date: 2012-10-30 00:34:41
Also in:
linux-fsdevel
On Tue, 30 Oct 2012 01:10:08 +0100 Jan Kara [off-list ref] wrote:
On Tue 30-10-12 10:48:37, NeilBrown wrote:quoted
On Mon, 29 Oct 2012 19:30:51 +0100 Jan Kara [off-list ref] wrote:quoted
On Mon 29-10-12 19:13:58, Jan Kara wrote:quoted
On Fri 26-10-12 18:35:24, Darrick J. Wong wrote:quoted
This creates BDI_CAP_STABLE_WRITES, which indicates that a device requires stable page writes. It also plumbs in a sysfs attribute so that admins can check the device status. Signed-off-by: Darrick J. Wong <redacted>I guess Jens Axboe [off-list ref] would be the best target for this patch (so that he can merge it). The patch looks OK to me. You can add: Reviewed-by: Jan Kara [off-list ref]One more thing popped up in my mind: What about NFS, Ceph or md RAID5? These could (at least theoretically) care about stable writes as well. I'm not sure if they really started to use them but it would be good to at least let them know.What exactly are the semantics of BDI_CAP_STABLE_WRITES ? If I set it for md/RAID5, do I get a cast-iron guarantee that no byte in any page submitted for write will ever change until after I call bio_endio()?Yes.quoted
If so, is this true for all filesystems? - I would expect a bigger patch would be needed for that.Actually the code is in kernel for quite some time already. The problem is it is always enabled causing unnecessary performance issues for some workloads. So these patches try to be more selective in when the code gets enabled. Regarding "all filesystems" question: If we update filemap_page_mkwrite() to call wait_on_page_writeback() then it should be for all filesystems.
Cool. I didn't realise it had progressed that far. I guess it is time to look at the possibility of removing the 'copy-into-cache' step for full-page, well-aligned bi_iovecs. I assume this applies to swap-out as well ?? It has been a minor source of frustration that when you swap-out to RAID1, you can occasionally get different data on the two devices because memory changed between the two DMA events. NeilBrown
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