Re: Dirty bits and sync writes

[Christoph Hellwig <hch@infradead.org>]

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@huawei.com/

> 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.