On 07/01/2025 07:03, Darrick J. Wong wrote:
On Tue, Jan 07, 2025 at 07:10:12AM +0100, Christoph Hellwig wrote:
On Mon, Jan 06, 2025 at 06:37:06PM +0000, John Garry wrote:
+ /*
+ * On COW inodes we are forced to always rewrite an entire file system
+ * block or RT extent.
+ *
+ * Because applications assume they can do sector sized direct writes
+ * on XFS we fall back to buffered I/O for sub-block direct I/O in that
+ * case. Because that needs to copy the entire block into the buffer
+ * cache it is highly inefficient and can easily lead to page cache
+ * invalidation races.
+ *
+ * Tell applications to avoid this case by reporting the natively
+ * supported direct I/O read alignment.
Maybe I mis-read the complete comment, but did you really mean "natively
supported direct I/O write alignment"? You have been talking about writes
only, but then finally mention read alignment.
No, this is indeed intended to talk about the different (smaller) read
alignment we are now reporting. But I guess the wording is confusing
enough that I should improve it?
I know what you are saying, but I found the last paragraph odd. All
along we talk about writes, but then conclude by mentioning reads (and
not writes at all).
How about:
/*
* For COW inodes, we can only perform out of place writes of entire
* file allocation units (clusters). For a sub-cluster directio write,
* we must fall back to buffered I/O to perform the RMW. At best this
* is highly inefficient; at worst it leads to page cache invalidation
* races. Tell applications to avoid this by reporting separately the
* read and (larger) write alignments.
yeah, that ending is better, but maybe Christoph wants to keep the more
wordy original previous paragraphs.
*/
--D
