On Mon, Aug 09, 2021 at 03:48:56PM +0100, Christoph Hellwig wrote: > On Tue, Aug 03, 2021 at 04:28:14PM +0100, Matthew Wilcox wrote: > > Solution 1: Add an array of dirty bits to the iomap_page > > data structure. This patch already exists; would need > > to be adjusted slightly to apply to the current tree. > > https://lore.kernel.org/linux-xfs/7fb4bb5a-adc7-5914-3aae-179dd8f3adb1@xxxxxxxxxx/ > > > Solution 2a: Replace the array of uptodate bits with an array of > > dirty bits. It is not often useful to know which parts of the page are > > uptodate; usually the entire page is uptodate. We can actually use the > > dirty bits for the same purpose as uptodate bits; if a block is dirty, it > > is definitely uptodate. If a block is !dirty, and the page is !uptodate, > > the block may or may not be uptodate, but it can be safely re-read from > > storage without losing any data. > > 1 or 2a seems like something we should do once we have lage folio > support. > > > > Solution 2b: Lose the concept of partially uptodate pages. If we're > > going to write to a partial page, just bring the entire page uptodate > > first, then write to it. It's not clear to me that partially-uptodate > > pages are really useful. I don't know of any network filesystems that > > support partially-uptodate pages, for example. It seems to have been > > something we did for buffer_head based filesystems "because we could" > > rather than finding a workload that actually cares. > > The uptodate bit is important for the use case of a smaller than page > size buffered write into a page that hasn't been read in already, which > is fairly common for things like log writes. So I'd hate to lose this > optimization. > > > (it occurs to me that solution 3 actually allows us to do IOs at storage > > block size instead of filesystem block size, potentially reducing write > > amplification even more, although we will need to be a bit careful if > > we're doing a CoW.) > > number 3 might be nice optimization. The even better version would > be a disk format change to just log those updates in the log and > otherwise use the normal dirty mechanism. I once had a crude prototype > for that. That's a bit beyond my scope at this point. I'm currently working on write-through. Once I have that working, I think the next step is: - Replace the ->uptodate array with a ->dirty array - If the entire page is Uptodate, drop the iomap_page. That means that writebacks will write back the entire folio, not just the dirty pieces. - If doing a partial page write - If the write is block-aligned (offset & length), leave the page !Uptodate and mark the dirty blocks - Otherwise bring the entire page Uptodate first, then mark it dirty To take an example of a 512-byte block size file accepting a 520 byte write at offset 500, we currently submit two reads, one for bytes 0-511 and the second for 1024-1535. We're better off submitting a read for bytes 0-4095 and then overwriting the entire thing. But it's still better to do no reads at all if someone submits a write for bytes 512-1023, or 512-N where N is past EOF. And I'd preserve that behaviour.
