On Wed, 2012-07-04 at 15:11 +0200, Jan Kara wrote: > On Wed 04-07-12 15:21:52, Artem Bityutskiy wrote: > > From: Artem Bityutskiy <artem.bityutskiy@xxxxxxxxxxxxxxx> > > > > This patch changes the '__ext4_handle_dirty_super()' function which is used > > by ext4 to update the superblock via the journal in the following cases: > > > > 1. When creating the first large file on a file system without > > EXT4_FEATURE_RO_COMPAT_LARGE_FILE feature. > > 2. When re-sizing the file-system. > > 3. When creating an xattr on a file-system without the > > EXT4_FEATURE_COMPAT_EXT_ATTR feature. > > 4. When adding or deleting an orphan (because we update the 's_last_orphan' > > superblock field). > > > > This function, however, falls back to just marking the superblock as dirty > > if the file-system has no journal. This means that we delay the actual > > superblock I/O submission by 5 seconds (roughly speaking). Namely, the > > 'sync_supers()' kernel thread will call 'ext4_write_super()' later, where > > we actually will submit the superblock down to the media. > > > > However: > > 1. For cases 1-3 it does not add any value to delay the I/O submission. These > > events are rare and we may just commit submit the superblock for > > asynchronous I/O right away. > > 2. For case 4 - similarly, not terribly frequent event in most of workloads. > > It should be good enough to just submit asynchronous superblock write-out. > Well, it happens for every inode being truncated / deleted to it can be > rather frequent. That's why I wanted to have now == 1 case everywhere - > i.e. just recompute the checksum and do mark_buffer_dirty(). I'd just > remove the 'now' test in this patch and then in patch 5 remove the now > argument from the function and callers as you did. I am a bit confused. It seems you consider that 'ext4_commit_super()' is a considerably slower than just marking the buffer as dirty right away. But I do not really understand why - all it does - it just updates a couple of superblock fields and then marks the buffer as dirty (I assume sync == 0). So from my POW they are almost the same. And when csum is enabled - re-calculating csum will probably be the longest part. More important is that we dirty the superblock on every deletion - this mean that with my change we will re-calculate checsum on every deletion and I am not sure it is nice. Ideally, we should be able to calculate the checksum just before sending the buffer to the IO queue... I'll prepare a new patch-set and send it to you. Thanks! -- Best Regards, Artem Bityutskiy
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